How do supply voltages affect power tube performance?

[Magnatron]

I'm wondering what effect different voltages have on power tube performance and amp sound overall.

Say for a push/pull output using 6V6 tubes, a 680 volt signal is sent to a rectifier, resulting in a B+ of ~408 volts.

A second identical amp has a 540 volt signal sent to the rectifier, resulting in a B+ of ~324 volts.

What would be the functional difference between the two amps and how would their performance differ?

[martin manning]

This scenario you are describing is what people do with "power scaling," which is regulating the HV down to reduce output power. Since power goes like V^2, at 324V you would have approximately 63% of the output power at 408V, or -2 dB. This is not a huge reduction. Ideally, the response of the amp is the same, but the sound might be different to your ears, and I believe the difference will have at least as much to do with your perception of it at lower volume as it does with differences in frequency response and harmonic content. The speaker is another factor, and I don't know how its frequency response would vary.

[Magnatron]

Martin, VERY helpful. Thank you.

So I'm wondering a bit more. If I'm looking at a transformer to provide those two voltages for a Princeton Reverb clone build, which should I use for the best performance, signal, and functionality? The spec associated with this build calls out the 125P1B, which has an output of 660 volts. Unfortunately, I might be stuck using the 125P23B transformer, which has the 540 and 680 volt outputs.

Thanks again!

[martin manning]

What is your goal with this project? Two levels of power in one amp? I don't think 2 dB is worth the trip.

[Magnatron]

Oh no. I'm just looking to make the correct pick for voltage from the transformer. I'm not sure which of the two will give me the best performance.

I'm just learning tube amp theory as I go through this project. I'm reading Blencowe's book "Designing Tube Amps", so hopefully I'll be there sometime when my eyeballs stop spinning with the math!

[martin manning]

OK, If you have to use 125P23B I would consider using the 540V taps and replacing the Princeton's 5U4 with a 5AR4/GZ34. That should increase the B+ voltage 30V+.

[R.G.]

Martin's right. A quirk of the human hearing apparatus is that differences in loudness are highly compressed. A 2db difference in output power is going to be just noticeably different in loudness, and that only at full power without any output stage clipping.

As a gee-whiz comparison to sound "half as loud" you need an amp to put out about 1/10 of the power. A 100W amp is about "twice as loud" as a 10W amp. The difference in sound or tone or ... whatever it is ... will depend far more on the human psychoacoustics than on the actual power supply level.

"Best performance" is a heavily loaded term in audio. You have to first be able to say what "better" and "best" mean, because there is no definition that even most guitarists will agree on.

[Phil_S]

Say for a push/pull output using 6V6 tubes, a 680 volt signal is sent to a rectifier, resulting in a B+ of ~408 VAC.
A second identical amp has a 540 volt signal sent to the rectifier, resulting in a B+ of ~324 VAC.

I'm going to butt in on a bit of technical stuff.
1) B+ is VDC not VAC.
2) I don't think I'd want to run 6V6's at the higher voltage (408) stated. It is well above spec for this tube. Although 324VDC is also above spec, we see this in many amps and it seems to be OK.
3) To get from 540VAC to 324VDC depends on the choice of rectifier.

[Stevem]

I would first like to ask if you have a base line to judge by, because this would surly help to guide what folks here reply to you!

Have you every played thru a well set up and functioning PR?

If so then what where your impressions?

What did it do well for you and in what ways might it have been lacking?

[Magnatron]

I would first like to ask if you have a base line to judge by, because this would surly help to guide what folks here reply to you!

Have you every played thru a well set up and functioning PR?

If so then what where your impressions?

What did it do well for you and in what ways might it have been lacking?

I've played some on a PR, but I couldn't say how well it was set up. That said, I can't really point to any characteristics that were memorable other than a general like of the sound. I play a lot of blues and like everything from clean to crunchy, without heavy overdrive.

I'm far more interested in learning the tech and building these amps. Most of my playing is done through a Helix, so this is more of a side hobby for the fun of it. This is my second amp build. My first was a two stroke.

[Magnatron]

I'm going to butt in on a bit of technical stuff.
1) B+ is VDC not VAC.

Yes. That was a fat finger mistake.

Thanks for the interesting thoughts. I'm looking at Martin's recommendations to change out the rectifier. Might be fun to play with.

[Magnatron]

OK, If you have to use 125P23B I would consider using the 540V taps and replacing the Princeton's 5U4 with a 5AR4/GZ34. That should increase the B+ voltage 30V+.

Thanks Martin! I'll try this idea. Fun to play and learn. Thanks for the advice.

[Roe]

I'm wondering what effect different voltages have on power tube performance and amp sound overall.

Say for a push/pull output using 6V6 tubes, a 680 volt signal is sent to a rectifier, resulting in a B+ of ~408 volts.

A second identical amp has a 540 volt signal sent to the rectifier, resulting in a B+ of ~324 volts.

What would be the functional difference between the two amps and how would their performance differ?

lower voltage gives slower response, earlier breakup and more compression with more mids.

[Lynxtrap]

FWIW, 408V on the power tubes would be pretty spot on for a BF Princeton Reverb if you want it to sound "stock".

[R.G.]

lower voltage gives slower response, earlier breakup and more compression with more mids.

... and an after taste reminiscent of cloves...

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