Class A tube life

[Lazarus1140]

What is the life expectancy of tubes in a Class A amp, and what are the factors that contribute to their longevity (or lack thereof).

In particular, I am asking about a Matchless, Dr. Z, or similar amp.

 

[Rich_S]

Re: Class A tube life

Sorry to start your thread with a mini-rant but you're not talking about Class A amps, you're talking about cathode-biased amps. Very few push-pull amps operate in Class A. The definititive paper on this issue was written by Randall Aiken, in the advanced section of the Tech Info page of his website. (The only true Class A guitar amps in common circulation are low powered single-ended amps like the Fender Champ).

So, talking about cathode-biased amps like the AC-30 and its offspring, because the cathode-biasing scheme naturally induces a bit of softness into the response, I would think that, all things being equal, cathode biased amps are a little easier on tubes.

However, there are more important contributors to shortened tube life. High B+ voltage is probably the worst; some people jack up B+ to get more power out of a given pair of tubes. For example, anybody getting a true 20 watts out of a pair of EL84s is likley using a high B+ which will wear out tubes fast.

There are non-electrical issues, too. Vibration and heat have a bearing on tube life, so mechanical design aspects such as head vs. combo and ventilation have an impact, too.

 

[richard parker]

Re: Class A tube life

I have 2 cathode biased amps - Laney VC30 and Cornford Roadhouse 30. I've had the Roadhouse for about 3 years, it runs on 2 X EL34s and I've just replaced them because one tube failed. The Laney has 4 x EL84s and has been running fine on the tubes it came with about 10 years ago. Both amps have done a fair amount of rehearsals and gigs.

 

[Lazarus1140]

Re: Class A tube life

Rich,

So can it be assumed that every amp with more than one tube is automatically a push/pull circuit. I have no problem with a mini-rant or a maxi-rant either. I know nothing about amp circuits and don't pretend to know. I have a small amp that I understand to be single ended class A and I don't doubt it as it runs really hot. I don't particularly like its sound but I can't blame that on its operating class when it was very inexpensive and obviously cheaply made.

I read something online that lead me to believe that the Matchless and Dr. Z amps were actually Class A (not to mention info on their own websites) ......... that's what lead to my question. I just don't want an expensive amp that runs hot.

As an aside, I don't understand why anyone would bother using the term when it doesn't apply. I am not disputing you in the least. I am just frustrated with the ubiquity of misleading information.

Richard, Thanks for the good report!

 

[Lazarus1140]

Re: Class A tube life

Obviously I read the article you linked before posting above. I realize, too, that I have read that at least once before. Even so, I am so thick headed I'm not sure I totally understand.

 

[crusty philtrum]

Re: Class A tube life

A lot of push-pull amps considered to be 'class A' are actually running class high A/B most of the time.

What is important is this .... running class high A/B or A, the power valves are running at high current all the time, thus working those tubes harder, even if you're playing quietly or not playing at all. Regular class A/B gets much better tube life because the tubes idle just like a car motor, and deliver more current as required, so that when you're playing softly or not playing at all, tube wear is minimal.

With amps that run in high AB or class A, one needs to be aware of the fact that the tubes are working hard even when they're not really doing much, and operate the amp accordingly. In other words, if you're going to walk away from your amp for an hour or two, switch it off. Use the standby switch more often. Make sure the amp is well-ventilated. Give the amp a few extra minutes after shut-down before moving it.

Yeah, there is a price to pay for that kind of sound. But if you're aware, you can extend tube life. Your tubes won't last as long as they would in most class AB amps, but a little care will help maximise their useable life, and your ear will probably tell you when they're due to be replaced.

 

[Rich_S]

Re: Class A tube life

The marketing people are mostly to blame - they persist is calling low-powered cathode-biased amps "Class A" even though it's flat-out wrong. It's become a tone buzzword: "Oooh - Class A... it must sound good". Even magazines like Premier Guitar and Guitar Player do it (consistently). It drives me nuts.

As for your question about two tubes - almost all amps with two output tubes are push/pull, and almost all of them are Class A/B. Class A/B is simply a good way to get a decent amount of power out of a pair (or quad) of tubes.

There are a very few (boutique) amps out there that run multiple tubes in parallel, even though they are single-ended (and therefore Class A).

Imagine a Champ with a single 6V6 output tube. Single-ended, Class A, good for about five watts. If you wanted a bit more power, your could put two 6V6s in parallel (and change transformers for more current) and get 2-tube single-ended output stage, still Class A, that's good for maybe 8 or 9 watts.

Put those two 6V6s in a Deluxe Reverb in Class A/B push/pull with a fixed bias and they'll give 22 watts. This is why there are so few pure Class A amps out there. In fairness, the Deluxe Reverb is fixed bias, which maximizes its output power. If you ran it cathode-biased, it might only do 18-20 watts, very close to the ubiquitous dual-EL84 cathode-biased amps we think of a "Voxy".

To recap, though; I believe tube life has more to do with mechanical concerns and high B+ voltages than with operating class. Also, how well the amp is treated is a factor: how often is it powered up & down, how careful are you when moving them amp between warm and cold environments, that kind of stuff.

Edit: Crusty brings up bias, which is another good point. Running a hot bias gives the kind of sound that a lot of people want to hear, at the expense of tube life. The flip side is a colder bias, which gives longer tube life but we generally think of a sterlie; so it's a trade off.

In cathode-biased amps, the decision on where to run bias is taken out of our hands; the designer/builder selects the bias point and puts in the resistor/cap pair required to obtain it. After that, we (the players) can't mess with it without replacing components. It's also self-correcting. If you stick in a hotter set of tubes, the cathode-bias circuit tends to push bias colder to automatically compensate.

But Crusty's right - amps running a hot bias can be rough on tubes - but I think that's more common with fixed-bias amps.

 

[Lazarus1140]

Re: Class A tube life

So in Class A/B .......... no matter how "hot" the tubes are biased to run, they're still cycling on and off as each handles half the wave?

What would you expect the power amp's output to be with a pair of EL34s in parallel in trus Class A, and what would be typical out put for a pair of EL34s in a Class A/B configuration? In other words, might it be possible to bypass the BS and buzzwords just based upon an amps output?

I mean, in my ignorance I used to think a pair of EL34 would pretty much result in 45 - 50 watts. Then, I started noticing some, like one Rivera for example, with 40 watts, and now I see Matchless amps at 30, 35, and 40W (each with a pair of EL34s).

 

[Koreth]

Re: Class A tube life

The differing amounts of power to be had from the same output tube configuration is a result of how the rest of the amplifier is configured, especially the power supply. Remember that power (Watts) is Volts*Amps. If the power supply runs the tubes at lower voltage, then the signal will not be able get as large of a voltage swing from peak to peak, and you'll have less power. If the power supply cannot supply enough current for the power amp to put out its max theoretical power, then then there will be less output power. Really, the output power in an amplifier comes from the power supply circuit. All the power tubes do is modulate the current from the power supply into wave that sounds rockin' when fed into a speaker.

Those lower power amps you see that still use a pair of EL34 tubes are probably running the tubes at lower voltage and feeding them a little less current.

 

[guitfiddle]

Re: Class A tube life

Both certain US military writings on the subject, and the RCA Tube Manual, define class a as

Operating condition in which the plate current for a given tube flows for the entire 360 degrees of the wave form.

That's it. Says nothing about whether or not there is more than one tube.


Here's a good read from the website Rich posted.

For audio amplification, a class A amplifier can be either single-ended or push-pull. Now, you might be thinking, how can a push-pull amplifier be class A? Doesn't one side amplify half the waveform and the other side amplify the other half? Isn't this why we use a phase splitter? These are common misconceptions. You can, indeed have a true class A amplifier that operates in push-pull mode. Amplifier class has absolutely nothing to do with output stage topology. If the output tubes on either side of a push-pull pair are biased in class A (halfway between cutoff and saturation), then the current in each side will still flow for the full 360 degrees of the input cycle, just in opposing directions. As one tube's current increases from the midpoint, or idle, bias current, the other tube's current is decreasing by an equal amount. The output transformer sums these oppositely-phased currents to produce the output waveform in the secondary winding. As one side reaches saturation, the other side reaches cutoff, just as they would in a single-ended class A amplifier. Neither side cuts off at the full, unclipped output power of the amplifier. The output power of a push-pull class A amplifier is exactly twice the output power of a single-ended class A amplifier operating under the same conditions of plate voltage, bias, and effective load impedance.

Another misconception is that of cathode biasing. The method of biasing has nothing to do with the class of operation. You can have a fixed-bias class A amplifier or a cathode-biased class AB amplifier, or vice-versa. The presence of a cathode bias resistor and bypass capacitor is not an indication of class A operation.

 

[Rich_S]

Re: Class A tube life

Laz, the varying amounts of power from a pair push/pull pair of EL34s could be from differnet biasing methods or from different B+ voltages. It's hard to tell without a schematic.

A single EL34 is good for about 11 watts in single-ended, so you might get 20 out of a parallel Class A pair, maybe more like 18. In Class A/B, here's the power at several B+ voltages:

Volts Watts
315 35
375 45
400 55
450 58
475 70
725 90

The EL34 rated to 800 volts for about 100 watts, but it would sound like **** up there.

 

[aleclee]

Re: Class A tube life

A lot of push-pull amps considered to be 'class A' are actually running class high A/B most of the time.

Actually, they're always operating in Class AB. Though the name might imply otherwise, operating class is a function of design, not operation. A Class AB amp is still Class AB, even if it's operating at a point where both tubes are always conducting. That's kind of the point, if you understand what Class B is.

As for the original question, it's pretty simple: the hotter you bias tubes, the less time they last. The harder you push tubes (in terms of output), the less time they last. If you bias a Class AB amp hotter, you lose tube life even in the same amp. If you crank it up, you lose tube life.

Regardless of amp design, tube life pretty much boils down to how many hours they're running and how hard they are working. Such is life when dealing with glowing bottles.