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"Until it's recognized that the amplifier is AT LEAST 50% of the sound of the electric guitar, its full potential cannot be realized" Leo Fender
 
Below is information I've gathered to help in understanding tube amps. I give credit to Lee de Forest, Leo Fender, Everett Hull, Randall Smith, Jim Marshall, Aspen Pittman, Gerald Weber, Bob Pletka, R. G. Keen and other vacuum tube experts. 















Can I use a guitar cable as a speaker cable?

Not a good idea. It might work for a while, but it's risky. Especially if you are pushing high power through a cable with a small center conductor, you risk either an open circuit when the conductor dies, or more likely melted insulation, as the inner conductor gets hot, resulting in a short circuit. Neither is very good for your amp. Also, the additional capacitance in the cable will adversely affect the impedance your amp sees. It will degrade your tone at the least, and may make your tubes and other components wear out faster as they have to work harder.

 

Can I use a speaker cable as a guitar cable?

Nope. You probably won't try this more than once. The cable between your guitar and amp (or effects) has to be shielded to prevent picking up noise (electromagnetic radiation from lights, motors, radio, etc). If you use unshielded cable (such as 99.9999% of all speaker cable) this noise will get amplified and drown out your guitar signal in a horrible flood of sonic mayhem.

 

How often should I change my tubes?

Tubes are like lightbulbs - they could go out at any time. Generally, if you're playing hard, tube life can be anywhere from 6 months to a year, although I've seen tubes last much longer than that. Preamp tubes will usually last longer than output ones.

 

How do I know if I need to change my tubes?

If your amplifier makes strange noises (hisses or squeals), or if it experiences a lack of power or a lack of bottom end.

 

If I change my tubes, do I need to re-bias my amplifier?

For preamp tubes, which are self-biasing, the answer is no. Install the new tubes and you’re done. For output tubes, re-biasing is recommended as the bias will drift. If you are changing tube brand or type, a re-bias is required.

 

How do I determine if my preamp tubes or my output tubes need to be changed?

If you're experiencing a lack of bottom end and power, it's probably your output tubes. If you're hearing strange sounds coming from your amplifier, turn your gain section up in volume, and slowly turn the master volume down. If the noises persist after the master volume has been turned down, it's an output tube. If the noises go away after turning the master volume down, it's an input tube.

 

Should vacuum tubes glow?

Yes, they should glow. If they are not glowing then they cannot work properly or at all. The plate structure should not glow orange or have spots on it that are orange. The filament (or heater) which runs through the center of the tube and warms up the cathosde is the only part of the tube that should glow orange.

A purple or blue glow that pulses under a load is normal but a strong purple glow that shows up when a tube is not under load at idle means the tube is known as what' called "gassy" and is at towards the end of its usable life. This can be caused by an improper vacuum and when under lead shows a very strong purple glow.
 

Should tubes make noise?

Almost all 12AX7 type tubes make noise or, are to some extent, microphonic. The reality is that some are more noisy than others. A simple way that you can check how noisy your preamp tubes are is by turning the amp on, setting its controls as you normally would and then lightly tapping on each tube with a non conductive probe (I use a pencil, popicle stick or chop stick. You'll find that you'll be able to hear the tapping amplified through your speakers...this is normal. On the other hand, It is abnormal for a tube to ring loudly like a bell after it’s been tapped. If it does ring loudly then this is what's called "microphonic" and the tube should be replaced with a known good one.

With very high gain amps (ex. Peavey 5150 and Ultra, the Mesa’s and Marshalls) you'll find that by simply swapping the order of the preamp tubes you can find a combination which will minimize the amount of noise.

Conversely, power tubes are rarely microphonic although EL84 tubes seem to be the most susceptible to being microphonic and having filament rattle issues. An example of a worst scenario is a high gain combo amp using EL84 power tubes. In addition, speaker vibration can rattle tubes so much to cause issues as well. As stated above, while power tubes are rarely microphonic they can be checked just like preamp tubes.

 

What amp should I buy?

This is a frequently asked question. You can take a look at the "Buying Tips /  Good Values" tab for some suggestions OR...

First off, play as many amps as you can. Go to music stores, friends' houses, wherever. Find out what you like. If, however, this won't work (you're overwhelmed, live way in the boonies, don't have transportation, etc), you can skip this step for now and take in to account the following:

What type of guitar(s) you plan to use with the amp.

What type(s) of music you plan to play through the amp. More for lead, or rhythm, or both? Do you care about built-in effects and reverb?

The volume levels you need. Is this mainly for practice? If so, are you in an apartment with thin walls, where you can hear your neighbor's cockroaches breathing, or in a house with infants, or by yourself, or do you have a separate building to practice in? If it's for jamming or playing with others, what size rooms or venues will you be playing in? Will it be small coffee houses, small clubs, large clubs, small churches, large churches, auditoriums, parking lots, stadiums, or a mixture. Will you be playing alone, or with a band, and if a band, what other instruments and amplifiers are involved? Is the drummer extra loud? Can you mic the amp? Etc.

Any physical limitations involved. Do you have to move all your own equipment? Are you a huge guy, a small guy, a gal with little upper body strength, an Amazon, a 98-pound weakling who can barely hold the guitar up, or a jellyfish? Do you have any physical handicaps, such as being confined to a wheelchair? Does the amp have to fit into an Accord, or on the back of a Yamaha XS650E with universal luggage rack, or your 10 speed? Does it need to work under adverse conditions? Do you have to carry it up and down stairs? You get the idea.

How much you have to spend for the amp.

Go play all the recommended amps that sound interesting within your price range. Just remember that tone is subjective, and what works for someone else (even everyone else) may not work (at least best) for you. In the end, you're the one who has to like the sound!

 

Can I run my amp without a speaker?

If it's a tube amp, this is a worst-case scenario and will destroy something in your amp's output circuitry really fast - probably your output transformer (NOT cheap to replace), and maybe more. Never, ever do this. I don't care what anyone else ever tells you, don't do this!


I tried plugging my guitar into a Peavey tube head vtm 60 (no speakers, no dummy load) I then connected the Peavey via the effects send to the input jack of my Fender Blues DeVille. The Peavey sounds great as a preamp. From what I've been reading this is BAD. Is it?

Yes BAD is a good word to describe what you're doing. You see, a tube amp, unlike a solid state amp, needs to have a load on its output. The AC current and voltage on the primary and secondary sides of the output transformer are directly affected by the load on the secondary side. The optimum conditions exist when the secondary (or output) side of the transformer is properly loaded. Some tube amps use shorting jacks for the speaker outputs. When there is no load connected to the speaker jack, the output of the transformer is shorted to ground. Now while this is far from an ideal condition, it will, for a short duration, prevent damage from occurring in the output transformer. It will, however, cause damage to most transformers if this condition is sustained for any extended period of time because too much current is being drawn from the transformer. Since I believe your Peavey uses open jacks for the speaker outputs instead of shorting jacks, an opposite but potentially more dangerous condition exists. When there is NO load on an output transformer, it attempts to develop more voltage on the secondary side. This can cause the internal insulation of the transformer to break down and high voltage arcing can occur causing shorted turns or open windings.

There may be, however, a couple scenarios that may have spared your output transformer. First, is if the effects loop is a simple series loop, and the "send" jack actually breaks the signal path. This is not probable, as most amps use the "return" jack to break the signal, but I have seen a few designs that use the send, so it is at least possible. The next is if the effects loop in the amp is "pre-master", and you had the master volume turned off. This would prevent any signal from driving the output stage, so the tubes and transformer would not be attempting to drive a speaker load.

The best advice I can give you is to hook the amp up correctly and see if it still functions properly. If it does, and you still plan to use the amp in this manner, run right out (do not pass go, do not collect two hundred dollars) and purchase a proper load for the amp. THD makes a nice unit called the Hot Plate, and there are others made by Weber, Dr. Z, Tube Amp Doctor and Marshall. I believe most of these even have a line out on them that you can use instead of the effects send of the Peavey. That would give you the total sound of the amp including the output stage, where some nice things happen if you drive it hard enough. The tone and response of the output stage is a very integral part of the tone of our Budda Amps, which we, of course, recommend never be run unloaded.

Remember, a loaded amp is a healthy amp! Wait a minute, Let me re-phrase that. Ah, nevermind.

 

I just ordered 4 Eminence bp102's and a Piezo 3.5x7 horn in attempts to build my own cabinet. I'm a little confused about wiring them so it stays 8 ohms. I also don't think I'll need a crossover for the tweeter - it should just go straight in line with the 10's I plan on getting a Yorkville xs 400 head.

Assuming that the speakers you purchased are all 8 ohm, wiring the cabinet for a total impedance of 8 ohms should be no problem. You'll need to use either a series-parallel or parallel-series type of wiring. Here's the way we do it. Mount the speakers so that the speaker terminals on the top two speakers face each other. Do the same with the bottom two speakers. Now, take two leads and wire the plus terminal of the upper right speaker to the plus terminal of the bottom right speaker, and the minus terminal of the upper right speaker to the minus terminal of the bottom right speaker. Do the same with the left speakers. Now, take another wire and run from the bottom terminal on the right speaker to the bottom terminal on the left speaker. To finish up, take the two leads that go to the speaker jack and connect one to the upper terminal on the right speaker and one to the upper terminal of the left speaker, being careful to attach the lead coming from the "tip" of the jack to the positive speaker terminal and the lead from the "sleeve" of the jack to the negative speaker terminal. You should now have a cabinet wired in an 8 ohm parallel-series configuration. The piezo horn shouldn't really need a crossover, but you should probably install a cap in series with the positive terminal just to block the low frequencies. Try using a 1.0uf non-polarized cap. That should get your 4x10 rockin!

 

I was wondering if you guys could help me with a question. I have a Soldano 4x12 cab and I recently purchased a Mesa Dual Rectifier 2 channel amp. I am currently running out of the amp's 16ohm jack into the Soldano cab, which is a 16ohm cab. Could I, and would it be better/louder to rewire the cab to make it 4ohm or 8ohm? If you can help that would be great.

Since maximum power transfer in an amplification system occurs when the source impedance (your amp) matches the load impedance (your speaker cabinet), and since you're running your 16ohm cabinet from the 16ohm output of the amp, you're definitely achieving maximum power transfer. In other words, it's not gonna get any louder no matter how you rewire the cabinet. Besides that, a 16ohm 4x12 speaker cabinet contains 4-16ohm speakers and there are only two possible cabinet wiring options. Series-parallel for 16ohm, and all parallel for 4ohms. I personally don't think you'd find any improvement from running the amp at 4ohms. In fact, some schools of thought prefer running the highest impedance available since it uses the complete secondary winding of the output transformer. The only way that you may possibly increase the "perceived loudness" of your amp is to add another cabinet to move some more air! It doesn't always work, so you should probably look into borrowing or renting another 16ohm cabinet before you make an investment that will not achieve the results you want. Aw, what the heck... at least it'll look cool!

 

What is bias?

A vacuum tube's bias is the DC voltage applied to a tube’s control grid to control the electron flow from the heated cathode to the positively charged anode (aka plate) when the tube is idle.

 

How does the tone of 6L6 and EL34 tubes differ?

This is a commonly asked question and the following is a general reference as the results in various amps are about the same. 6L6 tubes have a full bottom end and clear highs with less emphasis on the mids. On the other hand, EL34 tubes have more of a mid range focus, breakup earlier and have a more complex, harmonic content. The differences between the two become more noticeable with volume. Part of Fender's signature clean, warm and clear sound comes from the 6L6 tubes and the satisfying Marshall crunch from EL34 tubes driven into distortion. So if you prefer low end thump and crystal clear highs, you should try 6L6 tubes. If your preference is for more of a classic rock tone with more complex mids you should try EL34 tubes.

 

What is my amp's phase inverter / splitter and why should it use a balanced one?

In your guitar tube amplifier, the phase inverter (aka splitter) is the preamp tube that splits the preamp signal into two, one that is in phase and another out of phase, that is, phase inverted. In a Class AB push pull amplifier, this keeps your power tubes running 180 degrees out of phase with each other. This is necessary because in a push pull amp one half of the phase inverter tube creates the upper half of the sine wave while the other tube creates the lower half. With a balanced phase inverter, both halves are putting out the level of signal amplification to the power tubes. With an unbalanced phase inverter some notes will sustain well while others will sound flat or lifeless. In almost all guitar amps the Phase Inverter / Splitter tube is the preamp tube closest to the power tubes. Likewise, most Class AB guitar amplifiers use a circuit that used both triodes in this tube.

 

Basic amp questions including what is headroom? And what is breakup?  

I get the question all the time, “what is headroom?” Headroom is often referred to as the volume your amp will achieve before distortion sets in. A more accurate description should include the explanation of dynamics and that most all music by its nature is very dynamic with peaks and valleys. A couple of exceptions would be hard core screamer rock and punk that simply comes on full tilt, smashes you up against the wall and then finally drops you at the end of the song. Another would be rap, which consists of a 60 to 100Hz pounding synthetic kick drum that never varies in volume and some guy trying to rhyme…

Now a good example of punk with great dynamics is the new Green Day "American Idiot" CD. While their politics are their own, I think this in one of the best recordings I've ever heard and the song writing is top notch. It has a permanent home in my CD changer. For good rap, old Run DMC is a good place to start.

I digress, so back to dynamics and headroom. A great example of this is evident on any Robin Ford or Ronnie Earl CD. These guys are Blues and Jazz based and you can hear passages that go from very subtle to really digging in.

A typical 50 watt guitar amp when cranked up to about half way on in the clean channel will be right at the point of breaking up when you dig in on a note or a chord. So imagine that you are recording a passage that is very dynamic and starts out softly and then builds in volume and that you want the tone to stay clean. Even with the amp half way up you are only using about 10 watts of power to play a light passage but when you dig into a note and the amp is called on to reproduce it then will be using most all of it’s power to do so and this is referred to as headroom. If your amp did not have the power to reproduce the clean tone you wanted at the peak of the passage you would be out of headroom!

I’m often asked, “What is breakup?” Breakup is simply distortion. When the typical 50 watt guitar amp is about half way up you will start to get preamp tube, power tube and speaker distortion (which is what I call the sweet spot). One of the most asked questions is “how do I get more power tube breakup?” The answer? TURN IT UP! The other alternative is an attenuator between your amps output and speakers which allows you to crank the amp up higher and then choke down the output before it gets to the speakers causing a lower overall volume.

 

My amp has multiple impedances available for the speakers. Is one of these the "best" impedance?

Use the [highest impedance] tap. There will be a slight difference in the tone (a tad more low end) but that's not the reason to do it; the 16-ohm tap uses the tranny's entire output winding to transfer power to the load, unlike the 4-ohm tap, which uses only a small portion of it. This minimizes thermal stress on the secondary. If you use your cabinet in stereo mode (two 8-ohm inputs) and run two wires from the amp, that's still a 4-ohm load, as the output jacks on your amp are in parallel. My rule for tube amps is: run the highest available impedance, if you have a choice.

 

Why are tube watts louder than transistor watts?

Watts are watts. But a tube amp with a certain wattage rating will likely be capable of producing louder, pleasing music than a solid state amp of equivalent wattage.

Here’s an example from a good friend of mine. ``When I was a kid, I saved up my money and bought a Silvertone 2x12 100W solid state amp to replace the little portable tube PA my Dad gave me to play through. The other guys in my band were using a silverface Bassman and Bandmaster. My Silvertone could get pretty loud, but when push came to shove, the lower-powered Fenders came through, but my Silvertone sounded thin and weak, even when cranked up. When I asked my Dad, who was a top electronics tech himself, his simplified explanation was that tubes and transistors amplified different sets of harmonics. I later learned that he was pretty much right on the money”. The "experts" can argue all they want, but I maintain that the primary difference in the sound quality between tubes and transistors is the simple fact that tubes maintain and deliver even-order harmonics far better than transistors. Even-order harmonics support the fundamental note, and therefore are richer and more musical. Odd-order harmonics do not support the fundamental, and are (for the most part) musically harsh.

``Putting a solid-state (transistor, digital, or what-have-you) amp up against tube amps is really an unfair match, as I learned with my Silvertone. Even with more than twice the RMS power, I couldn't compete with the rich tonality of the tube amps. The transistors just couldn't preserve and amplify the even-order harmonics''.

 

What do you mean by solid state, tube, valve and hybrid amps?

A tube is a vacuum tube. A valve is the British term for a vacuum tube. Solid state means transistor, or chips such as op amps that are built from transistors. (Diodes may also be either tube or solid state). We sometimes refer to solid state as "silicon" or "sand", but solid-state devices may be germanium based as well.

"Valvestate" is a Marshall marketing term to describe an amp design, which has a solid-state power section but the "characteristics" of a tube amp. Some of these actually include a tube or two in the preamp, but frankly, they don't come ANYWHERE close to the sound of an amp with a tube output stage.

Hybrid amps are available in two flavors: Tube output stage with solid state preamp (Peavey Deuce / Mace / early "Classics", MusicMan, etc) or ones with tube preamps and solid state output stages (some Marshalls, some Ampegs, Hartke, etc, etc) It's much cheaper (and more common) to build an amp with a tube pre than a tube power amp, however, IMO, you get a more "tubular" sound when you overdrive a tube output stage than if you saturate a preamp tube and amplify it in a Solid State power stage.

Some SS output stages are a bit more tubular in behavior than others. If the amp species "power MOSfets" in the output, it's a little (but not much) more “tubey” than if plain ol' bipolar transistors are used. Has to do with how the device responds when you get close to saturation (as well as differences in input impedance).

 

NOTE - Amps using solid-state diodes as rectifiers are not generally considered hybrid amps, as they do not use the solid-state devices in the signal path. Sunn amps using transistors in the tremolo oscillator circuit are usually not thought of as hybrids, for the same reason. Mesa/Boogies with 5 band EQ, on the other hand, are hybrid amps (if just barely) because the EQ circuit, using solid state components, is in the signal path.

 

Can I use an old PA amp as a guitar amp?

Sure. You may or may not like the sound, but there's no physical reason you can't do it. You may need different input and output connectors, or a Hi-Z/Lo-Z transformer to convert between your guitar cable and a mic input, and the speaker outputs may be a bit strange, but that's about it.

But some folks prefer to take the PA apart and build a custom guitar amp. Instructions are available at http://www.rru.com/~meo/Guitar/Amps/PA2Guitar/old2new.html .

 

Can I use my stereo as a guitar amp?

Not such a great idea, unless you use something besides your stereo speakers, and/or compress the signal. Plus, you may not like the sound of it, unless you use a guitar preamp.

Stereo speakers are not made to handle the dynamics of raw music. Distortion may appear as square waves, which are close enough to DC to fry your speakers. There are several other issues, which may also destroy your speakers.

A stereo is intended to faithfully reproduce the recorded material. A guitar amp is supposed to add tone. Recorded material is typically compressed quite a bit. Live guitar has far more dynamic content than most recorded material.

(The only way you are likely to really be happy with such a setup is using a guitar preamp for tone, and a compressor to save your speakers. At that point, you might as well buy a guitar amp!)

So while you can do it, it's generally considered a really bad idea to try to use your stereo as a guitar amp. That's not to say you can't, or that it might not work for you, or that it didn't work just fine for your third cousin in Montpelier. But I wouldn't do it and don't recommend it. If you do, and something fries, you were forewarned.

 

What is an amp/head/combo/cabinet/cab?

Technically, "amp" is short for amplifier - the electronics that amplify the guitar (or other) signal to the point it can drive speakers at useful levels (where "useful is in the ear and mind of the hearer 8^).

Cab is short for cabinet, which is a box containing the amp, speakers, or both. The cabinet is usually made of wood or a wood product, and may be finished with a natural finish, a fabric covering, or in rare cases, paint. (A new, tough, painted on "fabric" is also possible.)

You can have a speaker cabinet or a combo cabinet with amp and speakers. You can even have an amp head cabinet. You decide what is really meant from context. Usage probably breaks down like this:

 90 - 95%   : speaker cabinet

 5 - 9%     : combo cabinet

 .1 - 1%    : amp head cabinet

 

But when musicians use the term "amp", it may mean an amp head (just the amplifier in a box), or a combo (amp plus speakers in a box). It can't legitimately just mean a speaker cabinet (box with speakers in it). I'd guess that most guitar and bass players, when they refer to an amp, mean a combo - a cabinet with an amp and speakers in it.

 

What's the difference between tremolo and vibrato?

Vibrato is a modulated (up and down) change in pitch, such as you get when you wiggle your finger on a fretted string, or when you wiggle the whole neck or play with the whammy bar. Tremolo is a modulated change in volume, as if you were turning your amp's volume up and down quickly. Tremolo is common in surf music; vibrato is common in rock and blues.

NOTE - be careful when using these terms. Some people use them backwards to this, and some people use them interchangeably. In fact, Fender is responsible for a lot of the confusion, calling the whammy bar a tremolo bar, and calling the tremolo on their amps vibrato. To be fair, even dictionary definitions can be vague, although the primary definitions in most dictionaries will at least lean towards the definitions here.

This confusion is why many people refer specifically to "pitch vibrato". But then how do you refer to tremolo? "Volume tremolo" sounds stupid!

Oh, and please note the spelling. It's tremOlo, not tremElo

 

Can I build a tube amp cheaper than I can buy one?

For the most part, it's cheaper to buy. Especially if you buy used. Especially if you've never done this.

The cost of building a small amp you can get for $100 used is double that or more - not counting labor.

I don't know that the cost would be double at the 100 watt level, but I would be very surprised if you could build as cheaply as buying, unless you are very good and/or lucky at scrounging, and know what you are doing, or have someone willing to put in a lot of time educating you. Because just one wrong part could fry a bunch of others and set you back a bunch.

If you buy used, you may need to work on the amp. This will bring the price up. But unless the amp is in horrible shape, it will still be cheaper than building from scratch, or even most kits.

On the other hand, building an amp is a great way to learn about them. And when you've built one, you will know that amp inside out. So the benefits may outweigh the cost; only you can decide.

 

What do I do with the standby switch?

If your amp is equipped with a STANDBY switch, there is only one correct way to use it. DON'T flick both the POWER and STANDBY switches on (or off) at the same time, or leave the STANDBY switch permanently in the "operate" position while using just the POWER switch. When you turn your amp's POWER switch on, make sure that the STANDBY switch is in the "standby" position. A STANDBY switch interrupts the high voltage supply to the tubes; when the amp is on "standby," only the tubes' filaments have voltage applied to them. When you turn the amp on, leave it on "standby" for at least five minutes...ten would be even better. This allows the tubes to warm up gradually; the tubes can take the high-voltage surge when the STANDBY switch is moved to the "operate" position much better when they are warmed up first. After the STANDBY switch is placed into the "operate" (or "playing") position, it will still take the power tubes several more minutes to reach full operating heat. It won't hurt anything to play the amp while the tubes are still not all the way hot, but the amp won't sound as good as it can until the tubes reach full operating temperature.

If you're playing a gig, DON'T PUT THE AMP ON STANDBY DURING THE BREAKS! Once it's hot, LEAVE it hot! Putting the amp on "standby" every time you leave the stand just thermally cycles the tubes (hot/warm/hot/warm etc.) all night long; this causes expansion and contraction of the internal parts, and this is one of the ways that tubes wear out. If you're worried about your guitar feeding back while you're on a break, just turn the guitar's volume control all the way down before you lean it against the amp or place it in its stand. When the gig is over, put the amp on "standby" and let it cool down for five or ten minutes before you turn it all the way off.

 

But my tube amp doesn't have a standby switch!

Leave the controls zeroed (turned all the way down) while the amp warms up. Tubes can make some strange noises as they heat; popping, fizzing, crackling, creaking, etc. These sounds may be alarming, but they are quite normal. Let the amp warm up for five or ten minutes before you play anything through it.

The amp may have been designed not to need one. Many smaller amps with tube rectifiers have tube complements that work fine in these situations. If your tube amp has a solid state rectifier, though, you may wish to have a standby switch added, especially if the amp seems to eat tubes.

 

But my solid-state amp doesn't have a standby switch!

It doesn't need one. Turn it on. Play guitar. Turn it off when done. That's it.

 

When I turn my solid-state amp off, why does it chirp, screech or squeal?

As long as it's not a HUGE speaker-eating pop, it's ok and although not perfect, within the range of "normal".

This is just an educated guess, but I think what's happening is that when the power is turned off, you have a temporary imbalance in the positive and negative power supplies as the filter caps discharge unequally. In the few milliseconds where there still is some charge, the amp is imbalanced and instabilities can develop resulting in audible oscillations. Eventually the caps discharge to the point where the equipment no longer can amplify the oscillations. In your case, anywhere from 1/10 of a second to a full second or so. The result is an audible chirp or squeal.

 

When I turn my tube amp to standby or off, why does it go THUMP?

This is usually due to arcing across the switch. You may need a switch with a higher current rating, or with a standby switch, higher voltage rating. This is not unusual and generally won't hurt anything (other than the switch, which will probably wear out faster.)

 

Should I convert my older amp to a modern, three wire, grounded cord and plug?

Yes. If you're worried about the amp's value to the insane collectors who even worry about whether the dust is original, keep the old cord. But do the work or have it done. This can save your (or someone else's) life.

 

What are power attenuators?

Power attenuators are devices used to reduce the amount of power going from the amplifier to the speaker. As opposed to a volume control on an amp, attenuators are almost always external, outboard devices. Some examples are the THD Hotplates, Scholz Power Soak, Marshall Power Brake, Altair units, and Rocktron Juice Extractors, among others. People use power attenuators because they like the way an amp sounds when turned up loud, but don't need all that volume, and don't wish to buy another lower-power amp as a solution.

There are, however, a few modern, boutique amps (the Aiken Invader comes to mind) with built-in attenuator.

 

Are power attenuators dangerous to the health of my amp?

Here you need to recognize the purpose of an attenuator, which is to get that cranked amp sound at less than ear splitting levels. In actuality, your amp is working harder and that in itself puts more wear on your amp and its component parts. It’s somewhat similar to driving your car at 25 mph vs. 100. At 100 there is more strain on it and it will wear out faster. So with an attenuator, IT is not dangerous to your amp’s health. Using one will have you pushing your amp harder at that adds additional wear (ex. tubes will not last as long). I’ve heard very satisfying results with attenuators so realize what’s going on to get you that “golden” tube amp tone without going deaf in the process.

 

Which power attenuator is the best?

From my experience, THD Hotplates seemed to get the most consistently positive results, with the Power Soaks coming in second.

It seems as though the players reporting the best results with p.a.'s were those who only moderately attenuated the amps. Generally, the more you cut the volume back the more tone you lose. So trying to take a dimed head down to line/headphone level isn't likely to be your best-sounding setting. If you find that you really do need very low/line levels and all you have is that 100-watt head, then you might want to look at another solution like a smaller amp or a POD or whatever.

 

Do power attenuators color the tone of an amp?

Yes. They all do, to a greater or lesser extent. The net effect is not necessarily good or bad, it all depends on the amp / attenuator / speakers and how you're using them.

 

So it's safe to pull two of my four output tubes?

Yes, but make sure you pull one from each side of the output transformer. Otherwise you will just have a mess. Since this changes the output impedance, you'll need to adjust either your output impedance selector or your speaker load. Here's the deal on that:

When you have 4 tubes in a push/pull amp, that's two tubes in parallel per alternation of the sine wave. When you have 2 tubes whose plates are in parallel, the plate impedance (which is what the output transformer "sees") is halved compared to a single tube per alternation. (Just like speakers, when you have 2 8-ohm speakers in parallel, the total load is 4 ohms.)

If you don't have an impedance selector on your amp, you should adjust your load such that if you're running 4 ohms with 4 tubes, you should be running into 8 ohms with 2 tubes. The actual load under 50W conditions is twice that specified by the normal 100W output tranny.

If you don't change your speaker impedance, you need to adjust the head's impedance such that the speaker load is 2X what the 100W load would be. In the case of an 8-ohm cabinet, you need to select a 4-ohm output impedance on the amp. (8 ohms is twice what the amp thinks it should be running.)

 

I was wondering about pulling tubes on a Marshall amp. I have a JCM 2000 100W into a 1960 Cab (16 Ohm). Ive been thinking about pulling 2 power tubes, lowering the wattage to 50W, in order to push my amp harder and get even more distorted sounds.

I would like to know exactly how should I do this. I already know that I should set the amps impedence to 8 ohm, but someone told me I should pull tubes 1 and 4 or 2 and 3 (not 2 and 4), which confuses me, what is the tubes order? Counting from the left to the right? And the amp needs to be rebiased after pulling the tubes? What value (mV) it should be? 90mV anyway? Im using JJE34L power tubes.

 

In order to get less output and more crunch, pulling power tubes in most 4-output tube amps is OK, provided it's done correctly. It sounds like you already have the basics, but are just a little confused; so let me try to clarify this for you.

 

First, unlike reading a schematic and troubleshooting an amp, for this the tube order is really arbitrary. It doesn't matter if you start counting from left to right or right to left. All that matters is that you pull one tube from each half of the push-pull output stage. Either way you start counting, tubes 1 and 2 are going to be one half of the push-pull output and tubes 3 and 4 are going to be the other half. Simply pull one tube from each half and you'll be fine. So you see, it doesn't matter if you pull 1 and 4, or 2 and 3, or 1 and 3 or even 2 and 4, the result is going to be the same. One tube from each half will be removed, and that's the objective.

 

Now about biasing. Theoretically the bias will change because once the two tubes are pulled, there will be less current draw on the power supply and the B+ (or High-Voltage) will go up. Since the bias current of the tube is set by the bias voltage relative to the B+, once the B+ changes the bias voltage should theoretically be changed accordingly, however the change in bias current will probably only be a few milliamps, so it's not like the tubes are going to suffer a meltdown. In fact, there are plenty of amps out there that incorporate a "half-power" switch that simply disconnects two of the output tubes without ever addressing the bias voltage issue, so just go ahead and pull out two of the output tubes and see how it sounds. (don't forget to adjust your output impedance setting accordingly). Regarding your "correct bias" question, I usually like to run EL-34's somewhere between 35 and 40 mA per tube depending on the amp and particular type of output tubes.

 

There are a couple more things that you may want to be aware of. First, pulling 2 of the tubes will probably only reduce the power by about 40%. Second is that "half-power" doesn't mean "half-volume". Reducing the power by half (almost) will only reduce the output by 3db, and this is not an appreciable enough reduction in volume to be perceived by the ear as "half-volume", but it will, however, crunch up a lot better. Now you can crank it to "11"

 

How does power relate to loudness?

Though the rated power and the relative loudness of an amplifier are directly proportional to each other, one is not an exact indicator of the other for many reasons. The two we will cover here are which power rating is actually used, and how efficiently that power is delivered through a speaker.

Manufacturers have through the years resorted to different ways of expressing the power ratings of their amplifiers. From a “marketing” stand point, big numbers sell products.

For example, a Fender Twin Reverb could be classified in any of these ways:

 100 R.M.S. and/or Continuous watts

 150 E.I.A. watts

 220 Peak Music watts

 250 Pulsed Program watts

 500 Peak watts

 1000 Peak-to-Peak watts

 

Notice, these are all ways to put a "different" number in front of the word "watts", and yet they indicate the same thing with respect to the relative loudness of the Twin Reverb amplifier. So now you can see that not all "watts" are the same, unless "specifically" stated by the manufacturer. (In fact, they should also designate a frequency range and maxiumum distortion value and type. For most discussions, people are assuming the typcial guitar frequency range and negligible values of distortion. Some people allow lots of distortion, though, so it's still slightly nebulous)

To me, amplifier output performance stated in terms of R.M.S. watts is the only meaningful number to accurately represent the power level of a sine wave. That sine wave is the basic of all musical vibration. That basic can be found in the sound of the flute, the fundamental vibrating note of the guitar string, the note produced by the Hammond B3, the Rhodes piano, the acoustic grand piano, etc.... and is common to most all natural musical melodic instruments.

So now we have a standard for what comes out of the amplifier only. There is still the working relationship that the speaker plays in the reproduction of the sound from the amplifier. The efficiency/sensitivity of the speaker's ability to convert electrical energy in to sonic/sound energy in the air for the ear to hear is now the heart of the matter.

This sonic energy in the air is measured in terms of Sound Pressure Levels using an SPL meter. This is a calibrated meter that hears what you are hearing at the same time, and tells you exactly how loud the sound is in decibels (standard units for measuring sound levels).

The rated efficiency of a speaker is commonly determined by placing a 1,000 Hz sine wave at one watt on the speaker and measuring the speaker's loudness 1 meter away with an SPL meter.

Now, let's connect the dots, and put this all together. We have two Twin Reverbs that perform identically. One Twin Reverb has a set of speakers with a sensitivity rating of 101 db, and the other Twin Reverb has speakers with a sensitivity rating of 90 db.

The Twin Reverb with the 101 db rated will be much louder than the other Twin Reverb for the same amount of power output delivered to the speakers.

Now it can be seen that the efficiency/sensitivity of the speaker plays a major part in exactly how loud an amplifier is going to actually be.

 

Are all 6L6 tubes interchangeable?

Not quite! You can substitute freely between the 6L6, 6L6G, 6L6GA, and 6L6GB. You can use a 6L6GC in place of any of these. You should not use any of the lower rated tubes in an amp designed to use the 6L6GC tubes. Doing so will most likely fry the new tubes. At the very least, you can expect drastically shortened tube life. 6L6GCs are designed to handle better than 50% more power than the other 6L6 tubes. For newer tubes (see above) check with whomever you buy them from.

Make sure whatever tube you are going to use can handle the voltage in your amp. Some newer tubes will not last as long in amps that ran voltages beyond the tube ratings, whereas many NOS tubes would survive well beyond their rated maximums.

 

Which tubes should I use?

In a perfect world, you could walk into any music store and say, ``I want to sound like [whatever]'', and a light would come on in the air above the head of the person behind the counter, and they would rummage around and produce exactly the tubes you needed. Free. Because they would be independently wealthy, doing this as a labor of love.

But the reality is, that to get the right tubes, you either need to try lots of tubes yourself, or have a tech you can work with to get the sound you want. This is even true with NOS. And you generally have to pay for them.

 

Ideally, you want tubes that have been tested. Even NOS tubes should be tested, since what's left includes a lot of rejects that should have been thrown away, and despite some factories' excellent quality control, there were still some duds. Most people also want reasonably matched output tubes, if only because it makes biasing easier. I can't stress enough that until you know what you are doing, you should only buy from a reputable dealer.

 

Testing-wise, for guitar amp purposes, it's not sufficient to find an old tube tester and see if the tubes register "good". You want tubes that will hold up under your voltage and current conditions (i.e., those found in your amplifier), tubes that are not excessively microphonic, tubes that will last, tubes that sound good. This requires a variety of testing, and most top tube vendors go through a lot of tubes finding the best ones. They will also be able to help (to an extent) with the tone of the tubes.

 

Some folks will tell you that a certain brand of a certain tube will always sound or perform a certain way. This just isn't necessarily true. Certain tubes of certain brands will hopefully be similar, but while some fall into a narrow range of tones and specs, others are all over the map. Again, it helps to have a reputable tube vendor who really knows their stuff. And, yeah, you gotta pay for them.

 

Do I really need matched tubes?

In general, you probably want matched output tubes. (Hi-fi folks like matched preamp tubes, but most guitar amp folks don't care about that.) Matched tubes mean they will all draw the same current under the same conditions, at the very least. This yields the most uniform waveforms, makes biasing easier, and is generally easiest on your output transformer and tubes.

On the other hand, unmatched tubes can produce distortion sounds that some people like. If this is the case, great - just realize that to keep that sound, you'll need to match any new sets of tubes to your current set, instead of to each other, per se.

 

My amp hums - help!

If it's an older tube amp, or just one that hasn't been played in quite a while, it may need the power supply capacitors replaced.

 

Beyond that, here's a good, solid approach on debugging amp hum, which was it was in response to someone in a country using 50HZ power; you may need to think 60 instead of 50 and 120 instead of 100, depending on where you are.

 

Plug the amp in, turn it on and set all the controls on your amp and your guitar to normal playing position. Hum, right? OK, reach over to the amp and yank the guitar cable out of the input jack. Does the hum go away? If it does, it's your guitar, or your guitar's proximity to the amplifier, which throws a 50 Hz field out to a distance of several feet; if your pickups are located within this field, they will couple the 50 Hz signal into the amplifier's input. Does the volume or character of the hum change when you move the guitar relative to the amplifier? If so, the cure is distance. Move away from the amp. Many bedroom jammers sit next to or even on top of their amps; this will often cause the problem you're having, especially if your guitar has single-coil pickups.

 

If the hum doesn't go away when you unplug the guitar, you'll have to look inside the amp for the problem. Does the reverb control affect the volume of the hum? Try repositioning the spring tank; the recovery transducer may be picking up hum from the power transformer. Wrapping the tank in aluminum foil sometimes helps, too. Experiment with poking leads into different positions inside the chassis; you may find that a certain lead arrangement makes an improvement. Is the hum just as loud with the volume control all the way down as it is with the controls in playing position? This points to a power supply problem, although this particular kind of hum should be at 100 Hz, not 50. The extra smoothing capacitor you've fitted can't solve the problem if it's caused by all of the factory-fitted capacitors being defective. Check them. Also, make note of whether certain controls on the amplifier (equalization, reverb, volume, etc.) affect the volume or tonal character of the hum; this may provide a clue pointing to the portion of the circuit where it is originating. As a last resort, remove the mounting bolts from the power transformer and try re-positioning it; the field it throws may be coupling into the audio circuitry, and a change of orientation may help.

 

Will leaving my amp on a lot wear out the tubes faster? Should I rotate the tubes for better life, like tires?

Leaving it on all the time is just wasting power and yes, the life of the output tubes.

One thing to avoid is leaving the amp on standby for very long periods of time. This tends to poison the cathodes of today's tubes rather quickly.

Shut her down when you’re not playing. Your light bill will thank you.

No reason to rotate tubes, with one exception: If you are using an amp with 4 output tubes and it has a "half power" switch, OR you are pulling two of the four tubes for a half-power trick, then you might want to rotate the 4 output tubes only. Swap the inner and outer pairs once every couple of months or so.

If you don't want to bother with that, then don't worry about it.

 

What does a rectifier do?

The rectifier turns AC (alternating current) from the wall into DC (direct current, like a battery provides). Amp electronics need DC to work. Without rectifiers, you'd need batteries, and even a Champ would suddenly be a lot bigger and weigh a bunch more (and cost a lot more to operate). A Fender Twin or Marshall would be useful only to the very rich or the military.

 

Why is the high voltage called B+? What are A, B and C voltages?

A, B and C once referred to battery classes for circuits. (You'll notice that tube texts generally show batteries as power sources for the various circuit voltages - they meant it!) You had “A” batteries for the heaters, “B” batteries for the plate voltages and other (relatively) high voltage supplies such as screen grids, and “C” batteries were for control grid bias supplies. I say classes instead of sizes - you could buy a “B” battery in a variety of voltage and current ratings. The first piece of gear I ever disassembled was a portable, all-tube AM radio (about 8"x6"x2", and it weighed several pounds). It had a steel chassis with 1.5V filament tubes, and three batteries. The B cell was about the size of a model airplane starter battery, but bigger than most you see, and IIRC, was 67.5 volts for the B+. That radio, BTW, was a marvel of packaging. Considering the size of the components, it had a far greater component density than just about any non-digital stompbox you're ever seen. Things were downright cozy in there. I can still feel and smell the caps, the resistors. I took it apart with a woodburning tool. There was magic in that package, and I just had to learn that magic!

 

What is the difference is between even and odd order harmonics as it relates to amps

Harmonics are a by-product of distortion. Even when an amp sounds clean, there is generally some distortion present, and the nature of that distortion may produce predominately even, predominately odd, or similar amounts of both even and odd harmonic distortion.

 

The terms "even" and "odd" are very basic. Harmonic distortion is produced in multiples of the fundamental frequency. If the multiple is even, (2, 4, 6, etc.,) the harmonic distortion is even order. Likewise, if the multiple is odd, (3, 5, 7, etc.,) the harmonic distortion is odd order.

 

For example:

Lets say you play an open "A". The fundamental frequency is 110Hz. This is the open A string on a guitar. If your amp is producing predominately even order distortion, then the harmonics will be even multiples of 110Hz. The 2nd harmonic will be 110*2 or 220Hz, the 4th will 110*4 or 440Hz, and so on. Isn't that nice? 220Hz is also an "A", one octave above the fundamental, and 440Hz is "A", two octaves above the fundamental. The 6th will be 660Hz, which is a slightly sharp "E", (the 12th fret of the 1st string is 659.255Hz.)

As it turns out, except when distortion is severe, the amount of harmonic content, or the amplitude, diminishes as the order of the harmonic increases. Because of this, the out of tune 6th harmonic is not of any great consequence.

So, even order harmonic distortion is considered more "musical," as the 2nd and 4th harmonics are just octaves of the fundamental.

Odd order, on the other hand, is clearly not musical. The 3rd is 330Hz, which is a slightly sharp "E", the 5th is 550Hz, a quite flat C#, and so on. But this is what rock and roll is all about! Disonant energy!! An amp producing predominately 3rd harmonic distortion is nearly like playing in parallel fifths! A serious no-no in real music, but done all the time in rock.

And nearly all of the great rock and roll guitar amps produce predominately odd order harmonic distortion. The trick is to design an amp that produces the odds and evens in a magical proportion...

 

Caveat: I would say that odd order harmonics are not necessarily "unmusical" if the overtones are mixed with the fundamental in moderate amounts. Many guitarists prefer the sound of predominantly even order harmonics. But stepping outside the guitar world for a moment, a clarinet, particularly in its lower register, produces an overtone series that contains prodominantly odd-order harmonics. Odd-order harmonics, mixed in proportion to the fundamental tone, tend to produce a sound approximating a square wave -- that typical reedy sound (compare a clarinet sample with a pure square wave on a synth, if you have one). Of course, no real world instrument produces a true square wave, but a clarinet is a good example of a predominantly odd-order, "square-wavish" sound. Certainly not "unmusical," although it's also certainly not a sound that many guitarists are going for!

Mixed OUT of proportion, odd-order harmonics can become harsh and unpleasant (to my ears) very quickly....

A final point is - the magic (especially for guitarists) happens with the right mix of even and odd order harmonics. To take another example, a violin, cello, or other bowed, stringed instrument has a very complex mix of even and odd order harmonics, and if any amp manufacturer can ever nail that tone, sign me up!

 

What should I do to avoid getting shocked while playing?

First, make sure you have a properly wired three-wire power cord with a three-prong connector. Get a cheap neon tester. Keep it in your giggin' kit. Touch one probe tip to your finger and the other probe to the suspect chassis (or mic, etc). If it glows even a little bit, you have a problem. At the outlet, one probe to the small slot (120V) and the other to the round or "D" shaped (ground) slot or cover screw should light the neon. If the wide slot (neutral) to ground makes it glow, use a different receptacle!!

$2.00 can save your life (or make it less painful at least!!). The last time I was at Radio Shack I noticed they had plug-in receptacle testers for around $6. 

It very well could have been an incorrectly wired outlet at the venue that was the cause of your shock.

 

What do capacitors do in my amp?

There are three basic functions for capacitors in tube amps. First are the filter caps - the big electrolytic "cans"-  and they are there to reduce the amount of hum from power supply ripple. These caps do indeed "wear out" (dry up, develop leaks, develop weak spots in electrolyte if unused) and may need replacing. A good tech can check this and replace as needed. For very old amps, many folks just assume that they are all used up and replace every one of them.

 

The second function is a cathode bypass cap. I don't want to get too technical here, so let's just say that they help stabilize the bias on your preamp tubes (some amps also use them for the power tube bias). These are often smaller electrolytic caps, and can also wear out. If they're bad they often affect the overall gain, particularly in the low freqs.

 

Finally, there are coupling caps. These provide isolation between stages of the amp (or between the input and the first stage). They typically do not wear out, (except the older, paper/oil types) but replacing them will likely change the tone of your amp. The exact type of aural change is a matter of great debate in the high-end audio world with such words as detail, clarity, soundstage, imaging - you get the idea. You may want to play around with these, but the best way to do it is to replace them one at a time, and then play the amp to evaluate the effect. If it doesn't sound better, then put the old cap back (I know, I know, this technique may result in a local maxima that's sub-optimal, so shoot me). This approach takes some technical savvy (or a very patient tech), but it can help tweak the last bit of tone from an amp. (In general, larger values add bass and some mids, and may up overall volume a tad; smaller values do the opposite.)

 

The fourth of the three types of caps are the tone filter caps - they're kind of like coupling caps in the way they effect tone. Changing the type of caps used here can change the "voicing" of the amp. Try it and see.

 

Why do speakers die?

There are two causes of speaker failure - electrical and mechanical. Electric damage is usually due to excessive heat in the voice coil. This is why a 10 watt speaker can last for a long time when driven by a 50 watt amp under certain conditions (i.e., time to cool off as occurs with very clean sine wave signals). Mechanical failure occurs when the suspension is compromised by the stroke of the piston action (excursion) exceeding design limits. This condition is interactive with electrical as it takes a given quantity of power and frequency to cause a speaker to jump that far. Point is it is not as simple as matching "rated output" with "rated handling".

 

First of all it is very important to realize that wattage is not static. The output of your amp is not a steady tone and when it is driven into clipping at some point the rise time is so fast it is difficult to discern it from a square wave, representing many attributes of a much larger output. Simply put, wattage is at best a guideline and by itself is insufficient to describe what is happening in this case that is capable of blowing a speaker.

A speaker's impedance changes with frequency and its excursion (the stroke of the piston action) changes with both frequency (much greater with bass notes) and volume levels. An open back cabinet allows unrestricted excursion compared to a closed box. Additionally if the speaker is rear mounted (the front of the speaker is flush with the back or inside of the baffle) it is possible to skew the voice coil off axis by unmatched tightening of mounting bolts causing early failure. Front mounting is not failsafe but is generally less strenuous on the speaker basket depending on whether the basket is cast (strong) or stamped (weaker) and how thick and spongy the gasket material is.

If all this seems confusing, the best thing to do is to examine one or more blown speakers to determine what manner of failure is occurring. Is the surround or the spider ripped? If not use a razor to cut out the surround and the spider and remove the cone and voice coil as a unit. If there is evidence that the coil of wire has rubbed (you will see the clear enamel-like insulator has been scratched) then overtightening is likely. Both of the above are mainly mechanical failure.

 

If the coil is unscathed but is melted like a fuse then the problem is electrical failure. In a solid-state amp this can be caused by DC voltages leaking from outputs. In tube amps it is from being overdriven electrically, usually from excessive clipping, impedance mismatch, etc.

 

The solution, assuming you must play at these levels of volume, frequency, and overdrive is to limit the mechanical or electrical cause. In some cases closing or porting an open back cabinet can solve the problem. Adding a second speaker enclosure is a great solution as it adds electrical handling and halves the excursion for a given frequency/volume assuming the speakers and enclosures are at least similar. Otherwise the only choice is to choose a speaker, which has much greater power handling capacity.

This presents an additional problem because unless it is also as efficient as the lower powered speaker it will not be as loud at a given amp output. For example a speaker rated at 100db/1 watt/1 meter will be 1/2 as loud as a speaker rated at 103db/1watt/1meter. So unless you are acquainted with a speaker that handles a lot more power just as efficiently, and has the tone color you like, the best solution is multiples of the one you already like. If you play a really wide variety of venue sizes either build or buy a number of single speaker enclosures (modular), dual speaker cabs, or go the tried and true 4 x 12 route.

Remember, an amp doesn't know how many speakers it is connected to. All it sees is impedance. A 30 watt amp will happily drive say 100 speakers if the total system impedance is matched to the amps output requirements (or theoretically infinite speakers as long as Z matches). In fact it will be more efficient because each magnet and voice coil is effectively an electromechanical engine.... more engines, more efficiency.

 

What is Class A, Class AB, etc? What's the best?

These refer to how an amplifier stage is biased. With guitar amps, the only place they mean much is in the output stage, since pretty much everything up to that point is always Class A. Note that the following definitions apply to how the amp is biased to run at maximum clean volume without exceeding the tubes' rated maximum power dissipation.

 

  • A “Class A” amplifier is an amplifier in which the grid bias and alternating grid voltages are such that plate current in a specific tube flows at all times.
  • A “Class AB” amplifier is an amplifier in which the grid bias and alternating grid voltages are such that plate current in a specific tube flows for appreciably more than half but less then the entire electrical cycle.
  • A “Class B” amplifier is an amplifier in which the grid bias is approximately equal to the cutoff value, so that plate current is approximately zero when no exciting grid voltage is applied, and so that plate current in a specific tube flows for approximately one-half of each cycle when an alternating grid voltage is applied.

 

All single ended amplifiers (whether they use one output tube, or more than one in parallel) are biased Class A, unless they distort. (More on this below.) I'm not aware of any guitar amps biased Class B (much less C, D, H or any other class). Most guitar amps with push-pull output stages are Class AB.

``The suffix 1 may be applied to the letter or letters of the class identification to denote that grid current does not flow during any part of the input cycle. The suffix 2 may be used to denote that grid current flows during part of the cycle.''

``The uninformed teeming millions assume that the letters applied to different amp classes are some kind of grading system that indicates what the sonic goodness is, which of course is not the case at all (except that Class C is pretty useless for audio except as a distortion generator).''

 

Guitar amps can, for practical purposes, be biased as Class A, although technically, if the waveform can be driven into distortion where the negative peak is clipped at cutoff, meaning no plate current flows, then the amp is not Class A. A [perhaps] less technically correct, but more useful approach, is to consider that if any given tube is driven into clipping and cutoff at the same level, it's still biased for Class A. Yes, this may violate the "letter of the law". But it's a useful concept. And, in fact, the RCA manuals refer to things this way.

On the other hand, to speak of an amp that is biased for Class AB but operating in Class A, or vice versa, is just plain nonsense.

 

What does all this mean, in practical terms?

Power - A “Class A” amp can't exceed the static power dissipation of the tubes, because full power is applied to each tube at all times. In a Class AB amp, each tube is only conducting for part of each wave's cycle, so the plate has a chance to cool off a bit during the times it isn't conducting.

Distortion - this is really a matter of whether the amp is push-pull or single-ended. Push-pull amps tend to have much lower even-order harmonics. On the other hand, push-pull amplifiers have crossover distortion, which single-ended amplifiers don't have.

For a detailed explanation, check Randall Aiken's excellent presentation at http://www.aikenamps.com/, selecting "Tech Info", then "The Last Word on Class 'A'".

 

What difference does the glass tube shape make?

Not much really. A 6L6G might not fit in equipment designed for a 6L6GB. Theoretically, a 6L6G would dissipate heat a bit better due to its larger glass area.

 

Is there a way to design for increased 2nd harmonic distortion without getting much 3rd (which gives a congested sound)?

Push-pull amps have the inherent characteristic of even order harmonic distortion cancellation. The more carefully the amp is AC balanced, the lower the even order products. This means a couple options exist:

  • Deliberately unbalance the output section, either by an imbalance in the phase splitter, or by using output tubes that are mismatched (primarily in terms of transconductance, not plate or cathode current).
  • Use a single ended amp instead of a push-pull one.
  • I've watched on my spectrum analyzer as I adjusted the AC balance on an amp. What occurs is that if you "unbalance" the drive to the output tubes in the "right" direction (you need to be able to see on the analyzer to know which way is "right") the 3rd, 5th, 7th, etc. odd-order harmonics increase VERY slightly, but the 2nd, 4th, 6th , etc. even order harmonics increase significantly. The second increases the most.

Even in audio gear, the optimum distortion pattern seems to be a gradually descending amount as the order of the distortion increases. The distortion products decrease at a fairly constant rate as the order increases.

(Note: order simply means the multiple of the fundamental frequency. 2nd harmonic is 2x the fundamental tone, 3rd is 3x the fundamental, 8th is 8x the fundamental, etc.)

In hi-fi gear, even though low distortion is desirable, the types of distortion often dictate the sound quality. The pattern above is desirable, with as few higher order distortion components as possible (odd or even order). BTW, SS audio amps tend to produce odd order distortion; tube audio amps produce more even order distortion, thus the common preference for the more musical tube amps. Even order distortion is considered more musical, more pleasing to the ear than odd order distortion.

It would appear that in instrument amps almost the same thing holds true. While low distortion is NOT a design goal of most instrument amps, clearly a musical distortion pattern could be. Maybe that's why a Champ has such a good tone (in many players opinion anyway) - a single ended amp has no cancellation at all! You get all the distortion products rather than an exaggerated proportion of odd order that occurs in push-pull amps.

LV or somebody could comment on this, but it seems to me you could try/hear this for yourself by making your push-pull amp into an SE amp (temporarily). You could lift the coupling cap to one output tube (of a push-pull pair). Leave the DC flowing to avoid transformer problems, and you could have a single ended amp to try. With one coupling cap disconnected you only get single ended AC. There are some limitations to this, such as a big loss of power/volume, but it seems like something I'd try if I was a musician seeking a different tone. I know, the NFB loop will try to compensate, and when I drive the output past its class A output level to AB the distortion will be VERY large since I'll be cutting off a big part of the waveform, etc. Might be fun to try though. Anyway...

OK, if you've gotten this far then you probably have a better idea of what's going on with tube amps. This Q&A is only select info as there is much more info available but it should help in answering some frequently asked questions.
 
Good tone is always worth it. Now go forth and find yours.
 
And may your riffs be hot as fire,
Steve















Call Steve McKinley at 404-545-3581 or e-mail atlantatubeamp@mindspring.com