Guitar amp tube compliment vs. power

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mattdhall
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Guitar amp tube compliment vs. power

Post by mattdhall » Sat Dec 11, 2010 6:47 am

This is a question that's been bothering me for a while. How is the tube compliment of an amplifier related to its output power? I have a 15 watt 70s Princeton Reverb that has two 6L6s for the power section. I'm looking at getting a 40 watt Hot Rod Deluxe that also has two 6L6s. The 60 watt Deville also has two 6L6s. I guess what I'm wondering is, if it's not the number or type of tubes that determines output power, what is it that does? And would it be possible to modify a smaller amp to increase its wattage?

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Post by vvv » Sat Dec 11, 2010 8:06 am

Dude, I'm pretty sure yer Princeton has 2 6V6's for power tubes.

Power tubes give a general range of power.

I consider 'em, lower to higher, as 6V6, EL84, EL34 and 6L6 as the most common.

A pair of 6V6 is often around 15 watts, a pair of 6L6 is often around 50 watts - this is just off the top of my hungover head, mind you.
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Post by hogfish » Sat Dec 11, 2010 9:12 am

If the amp hasnt been modded, Princetons are 6V6s in the power section.

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Post by ??????? » Sat Dec 11, 2010 9:37 am

Look up the data sheets for any tubes in question and you can learn a lot about what single tubes or pairs of tubes can accomplish in a given configuration.

RATED output power depends on:

Number of output tubes
Tube type
Class of operation (A, AB1, etc)
B+ (plate) voltage
Bias scheme (and proper bias)
Amount of filtering in the power supply
Rectifier (SS rectifier or amount of inter-electrode resistance in tube rectifier)
The point at which the output transformer core-saturates
%THD specified in the testing procedure
Gain structure within the amp

and maybe a few others

In practice, it can also depend upon other factors, such as input signal.

Remember that the typical "rated" power is based upon a 1kHz sine wave into a dummy load. When it distorts to a given level (typically 1% THD or maybe another-- there's really no standard), that's deemed the "maximum output power."

In a guitar amp, this number means practically nothing, despite the fact that everyone seems to treat it as the end-all measure of loudness in an amp.

It gives no real idea of how loud a guitar amp will be in practice, except by pure coincidence.

First reason is that almost nobody uses guitar amps with ≤1% THD. If the amp is even compressing/fattening slightly, it's beyond that point. The specs tell you nothing of what happens after that-- some amps continue to get way louder into distortion, and some do not. This is part of the reason for the "tube watts are louder than solid-state watts" myth. Of course they're not-- it's just that tubes tend to fatten and compress gently, whereas solid state devices tend to clip all at once.

Second reason is that speaker efficiency is at least as important. Speaker sensitivities can vary by several dB even among speakers of the same size (not to mention speakers of different sizes), and doubling power only gives you a few dB. So changing from one brand of speaker to another might (or might not) be the same as doubling (or halving) the output power of the amp itself. This is very important.

Not to mention the impact of adding more speakers, which gives further sensitivity/efficiency increases...

Voicing of the amp can also contribute to apparent loudness due to fletcher-munson considerations, and finally, even cabinet design can maximize (or place at a serious disadvantage) the output of the driver. Ever heard how thin, small, and quiet a speaker sounds outside of its cabinet?

Sometimes there seems to be a loose correlation between output power and loudness, but this is mostly coincidence. Higher-power amps tend to be designed to be loud, and in the old days at least, tended to need larger number of speakers to handle this power without blowing. And higher power-handling speakers tend to be more expensive speakers, which (at least back then) tended to have higher sensitivities/efficiencies. Higher powered amps used to be more expensive, so greater care was taken to use quality components (transformers, rectifiers) and design.

So yeah, not to sound pedantic, but the number of "watts" in a guitar amp is supremely irrelevant. Turn the amp on. Play. Is it loud enough? Sold! Is it not loud enough? No sale!


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Re: Guitar amp tube compliment vs. power

Post by Zoltar » Sat Dec 11, 2010 10:13 am

mattdhall wrote:This is a question that's been bothering me for a while. How is the tube compliment of an amplifier related to its output power? I have a 15 watt 70s Princeton Reverb that has two 6L6s for the power section. I'm looking at getting a 40 watt Hot Rod Deluxe that also has two 6L6s. The 60 watt Deville also has two 6L6s. I guess what I'm wondering is, if it's not the number or type of tubes that determines output power, what is it that does? And would it be possible to modify a smaller amp to increase its wattage?
A lot has to do with the pre-amp section. If it has a higher gain section, it will amplify more. I don't know what the numbers are right now, but if you get double the gain in the pre amp stage, your output gain should be doubled also.

Another thing is the voltage used in the power stage. Tubes can (and should) be pushed beyond their linear range, giving more gain, but creating more distortion.

I suppose the tone section could also affect the output power, depending on the stage and the amount of attenuation. Also, some amps use positive/negative feedback which will also affect output power.

Combining all this, the rated output power is usually estimated. It is fair to say that getting more out of a tube will cost you somewhere else in the chain (tone, distortion, noise, etc.) Also, wattage does not always equal volume.

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Re: Guitar amp tube compliment vs. power

Post by ??????? » Sat Dec 11, 2010 10:32 am

Zoltar wrote:A lot has to do with the pre-amp section. If it has a higher gain section, it will amplify more. I don't know what the numbers are right now, but if you get double the gain in the pre amp stage, your output gain should be doubled also.

Another thing is the voltage used in the power stage. Tubes can (and should) be pushed beyond their linear range, giving more gain, but creating more distortion.

I suppose the tone section could also affect the output power, depending on the stage and the amount of attenuation. Also, some amps use positive/negative feedback which will also affect output power.

Combining all this, the rated output power is usually estimated. It is fair to say that getting more out of a tube will cost you somewhere else in the chain (tone, distortion, noise, etc.) Also, wattage does not always equal volume.
If I may give an opinion on some things here--

Preamp gain, any NFB and EQ filters/tone stack rarely ever impact the rated output power meaningfully. Unless either creates a situation where the power amp cannot get the signal up to ~1% THD (which I've never seen happen in an amp that was operating properly), they will have absolutely no real impact on the FINAL output power rating of the amp. They might impact apparent loudness of the amp once pushed into distortion (or they might not, depending on design), and they might impact the type of range the controls have, but they will certainly not affect the "numbers" in any real-world scenario.

I've never seen positive feedback employed in amplifier design. In any situation Im familiar with, this would create a runaway oscillation, which is a malfunction.

Finally, output power is NEVER "estimated," at least not in published specs. There's a very real procedure that employs a 1kHz sine wave and a dummy load, as well as a distortion analyzer. When it reaches 1% THD (or something else, if the manufacturer chooses another criterion), then the output power is realized.

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Re: Guitar amp tube compliment vs. power

Post by Zoltar » Sat Dec 11, 2010 11:03 am

??????? wrote:
Zoltar wrote:A lot has to do with the pre-amp section. If it has a higher gain section, it will amplify more. I don't know what the numbers are right now, but if you get double the gain in the pre amp stage, your output gain should be doubled also.

Another thing is the voltage used in the power stage. Tubes can (and should) be pushed beyond their linear range, giving more gain, but creating more distortion.

I suppose the tone section could also affect the output power, depending on the stage and the amount of attenuation. Also, some amps use positive/negative feedback which will also affect output power.

Combining all this, the rated output power is usually estimated. It is fair to say that getting more out of a tube will cost you somewhere else in the chain (tone, distortion, noise, etc.) Also, wattage does not always equal volume.
If I may give an opinion on some things here--

Preamp gain, any NFB and EQ filters/tone stack rarely ever impact the rated output power meaningfully. Unless either creates a situation where the power amp cannot get the signal up to ~1% THD (which I've never seen happen in an amp that was operating properly), they will have absolutely no real impact on the FINAL output power rating of the amp. They might impact apparent loudness of the amp once pushed into distortion (or they might not, depending on design), and they might impact the type of range the controls have, but they will certainly not affect the "numbers" in any real-world scenario.

I've never seen positive feedback employed in amplifier design. In any situation Im familiar with, this would create a runaway oscillation, which is a malfunction.

Finally, output power is NEVER "estimated," at least not in published specs. There's a very real procedure that employs a 1kHz sine wave and a dummy load, as well as a distortion analyzer. When it reaches 1% THD (or something else, if the manufacturer chooses another criterion), then the output power is realized.
I started typing before I read your original post, yours was much more accurate and had much better information. I guess what I was talking about was volume rather than rated output power. I have been reading a lot about lower powered amp and the wattage listed on some is calculated, not measured (at least I thought I read that) I am thinking about things like the Egnaters, THD, that sort of thing.

as for positive feedback, I might have been thinking about oscillator design, because you are right about that. I remember using positive feedback, I didn't know if guitar amp designers did the same thing. (as long as they don't go over unity, it might work?)

It is very rare that I can post something on Tapeop, refill my coffee, and have a response when I get back. If I had read your response, I wouldn't have posted mine. Thanks for clarifying this for me.

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Post by vvv » Sat Dec 11, 2010 1:57 pm

To address a little about what ??????? said in his very informative post, I would like to make a observation.

In my experience, a 6V6 amp is intended to be rather quieter than a EL34 which, while rated about the same, never seem to be quite as loud as some 6L6's, if only because 6L6's are often quartets.

That is, I believe obviously, design choice, butt I've never heard a really loud 6V6, or a really quiet 6L6 design.

That said, the little Pro,Jr. with its EL84's gets surprisingly loud.
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Post by Scodiddly » Sat Dec 11, 2010 2:55 pm

There's also power supply design. 6L6 tubes are typically able to produce about 25 watts each, but aren't always run that hot. Similar situation with 6V6 tubes, which is why the Princeton only does about 12-15 watts while the older Deluxe models (which still had 6V6 if I remember right) did more like 20 watts.

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Post by calaverasgrandes » Sat Dec 11, 2010 4:03 pm

EL34s can run way higher watts than they are usually configured for. By way of example the el34 powered Taynor YBA1a is getting about 45 watts per tube. 90 watts total. The YBA3 runs a little bit more conservatively. It gets about 140 watts out of a quad of el34's. So only 35 watts per tube.
But then a little farther south Ampeg was getting about 50 watts a tube in its SVT heads. Later versions would be 6550's but the first ones were some oddball tube. 6146b or something?
To me it is surprising how different power tubes sound in an amp. My bass amp (Mesa Bass 400) takes 6L6 or 6550 tubes. The 6L6 sound good, but it really sounds great on 6550 tubes.
But honestly 80% of the tone of most tube amps I have owned is in that first tube, V1. It does the lions share of gain in you amp being that it is applying 20db or more of gain to your guitar signal. Swapping out for a JJ tesla made my amp much more raw and aggresive (in a good way). Opting for an old RCA 12axya mad my amp sound like crushed black velvet. Smoooth.
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Post by ??????? » Sat Dec 11, 2010 4:31 pm

6V6 was originally designed as a car radio tube, but found their way into many consumer applications. Maximum power for a pair of them in AB1 is somewhere just north of 20w, but that's with a plate voltage that's kinda high. That's also in fixed-bias, like a Deluxe Reverb.

Cathode bias (like a Princeton or tweed Deluxe) lowers you down to about 15w per pair in AB1, probably.

A pair of EL84s is probably pretty similar, or maybe somewhere slightly less.

A pair of 6L6GCs in fixed bias AB1 is probably somewhere around 50w.

etc etc etc. Look up tons of info on the TDSL tube data page.

http://tdsl.duncanamps.com/tubesearch.php

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Post by farview » Sat Dec 11, 2010 7:14 pm

calaverasgrandes wrote:But then a little farther south Ampeg was getting about 50 watts a tube in its SVT heads. Later versions would be 6550's but the first ones were some oddball tube. 6146b or something?
7027a's are what used to be used.

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Post by Scodiddly » Sat Dec 11, 2010 7:55 pm

farview wrote:
calaverasgrandes wrote:But then a little farther south Ampeg was getting about 50 watts a tube in its SVT heads. Later versions would be 6550's but the first ones were some oddball tube. 6146b or something?
7027a's are what used to be used.
Ampeg used the 7027's in the V4 series and such. The originals were indeed the 6146, complete with a wire attached to the top of each power tube.

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Post by calaverasgrandes » Sun Dec 12, 2010 12:54 am

Kinda like the Traynor YBA3a with its 4 tubes putting out 250 watts. NO IDEA what those are. I only hage ever seen one YBA3a.
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Post by vvv » Sun Dec 12, 2010 7:28 am

calaverasgrandes wrote:Kinda like the Traynor YBA3a with its 4 tubes putting out 250 watts. NO IDEA what those are. I only hage ever seen one YBA3a.
"... tubes were three 12AX7As in the preamp and four 6CA7 outputs."

From here, what also says,

"Identical to the YBA-3 but double the power (max continuous output was well over 400 Watts), YBA-3A was the most powerful bass head on the market when introduced & probably also the heaviest at apx.100 lbs."

The 6CA7, of course, is the EL34!
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