B+ voltage effect on tone

[jazbo8]

Assuming all the other factors remain relatively constant, then for push-pull operation, on paper the following should occur:

higher B+
-> lower idle current (to keep Pdiss in check)
-> higher plate resistance
-> lower (than optimal) load with the same OPT
-> lower odd harmonics
-> mellower, warmer tone

Jaz

[martin manning]

Assuming all the other factors remain relatively constant, then for push-pull operation, on paper the following should occur:

higher B+
-> lower idle current (to keep Pdiss in check)
-> higher plate resistance
-> lower (than optimal) load with the same OPT
-> lower odd harmonics
-> mellower, warmer tone

Good point- idle current is another variable that hasn’t been addressed yet.

For a given power level, the idle point chosen will determine what part of the output cycle remains within Class A push-pull operation, where one would expect more even-order harmonics and a warmer tone than in Class AB operation. Considering this, I respectfully disagree with your conclusion above and tend to back Leo_Gnardo's subjective observations.

First, there is a small range over which the idle dissipation could be adjusted such that the percentage of Class A vs. Class AB at maximum power output could be held constant, but I think it makes more sense to consider what will happen at a constant power level that is less than or equal to the lower-voltage case.

If idle current is held constant as plate voltage is increased (and there is a range over which this can be done without exceeding the maximum plate dissipation), the absolute Class A power output remains unchanged. In that scenario, I don’t think there will be much difference in tone as long as the output power in use is within the range available at the lower plate voltage. Beyond that point the Class A power will become a smaller fraction of the output, and I would expect relatively more odd-order harmonics.

What most people will do in practice is one of two things:

1) Set the idle current to hit a given fraction of the maximum plate dissipation regardless of the plate voltage. In this case the available Class A power will be reduced, and more odd-order harmonics will be present at higher power levels.

2) Adjust the idle current for best sound quality, within limits, wherein they will probably be attracted to a hotter bias setting when running at higher plate voltages. This will tend to reduce the tonal differences between higher and lower plate voltages, but it is likely that the Class A power will be reduced, and the odd-order harmonics will increase.

Regarding the OT primary impedance, if it is held constant, the load line does fall slightly with respect to the “optimum” (whether that is maximum power, or minimum distortion, or whatever you like) as the plate voltage is increased, but I don’t think that this is significant. In the case of a 6V6, increasing from 300V to 400V plate voltage, the impedance change needed to put the load line back in the same relative position with respect to the (scaled) grid curves is only 13%. Re the idle current and plate dissipation, this same change in plate voltage, with idle current held constant, moves the idle plate dissipation from 60% to 80% of the maximum, with the available Class A power being the same.

[rdjones]

And then there's impedance...

OK, here's just one set of variable interactions :
a) Increase B+ to raise power output.
b) Increase B+ and OT primary impedance to maintain power level but change the tone.

[pdf64]

I've also noticed that high VB+ with 6L6 / EL34 ( eg >480V) tends to result in a harder, more forward tone.
And 6V6 tend to be too bright and fizzy with a VB+ much over 400V.
These comments are with regard to classic guitar amp schemes and are observational, not from any kind of controlled test, factors other than VB+ could have more influential.
Pete

[jazbo8]

Gents,

I knew something didn't seem quite right when I wrote it even when I said everything else should stay constant, I then changed the idle current, and even with that, I came to an in-correct conclusion duh!

Thanks for sorting me out

Jaz

[ampfab]

i'm working on an amp right now, the power trans has two sets of taps on the high voltage windings. it has a cathode biased, ultra linear kt77 power amp. I installed the two sets of windings on a switch so I can switch between the two. 470v and 400v B+. I have the bias adjusted a little hot when it's running at higher voltage, its a bit cold at the lower voltage. there is very little noticeable difference in tone, a tiny bit quieter, maybe softer tone (may be my imagination) when at higher volume. the difference in tone/volume is so small that i'm considering removing the switch and just running at the higher voltage.

[EtherealWidow]

Thank you a lot for that info. You know, this thread was started when I read someone else saying that EL84's getting a little ratty at higher B+. Got a little excited thinking about all the different potential characteristics of a tube at different voltages, but failed to take in all the other ridiculous amount of variables. Thanks y'all. Trying to get that excitement under control.

[jazbo8]

i'm working on an amp right now, the power trans has two sets of taps on the high voltage windings. it has a cathode biased, ultra linear kt77 power amp. I installed the two sets of windings on a switch so I can switch between the two. 470v and 400v B+. I have the bias adjusted a little hot when it's running at higher voltage, its a bit cold at the lower voltage. there is very little noticeable difference in tone, a tiny bit quieter, maybe softer tone (may be my imagination) when at higher volume. the difference in tone/volume is so small that i'm considering removing the switch and just running at the higher voltage.

Since you already have the ultra-linear taps, did you try the amp with the UL connection? I always wonder how UL effect the tone in a guitar amp, since I don't particularly like it in my hi-fi amp...

[ampfab]

Since you already have the ultra-linear taps, did you try the amp with the UL connection? I always wonder how UL effect the tone in a guitar amp, since I don't particularly like it in my hi-fi amp...[/quote]

I am using the UL taps, KT77s seem to be made to be run UL. I tried the KT77s in a marshall jcm800 clone that I have and didn't like them, seemed to be lacking some low end compared to the el34s. they sound great in this amp. I did not set the amp up to switch between UL and non-UL, so I don't know how the two configurations would compare.

[Colossal]

You know, this thread was started when I read someone else saying that EL84's getting a little ratty at higher B+. Got a little excited thinking about all the different potential characteristics of a tube at different voltages, but failed to take in all the other ridiculous amount of variables. Thanks y'all. Trying to get that excitement under control.

Sorry, but I have to completely disagree with 'someone else'. I've run EL84s at 360-400V and they DO NOT sound ratty. Ask Reeltarded, he's had an earful. Yes there are a LOT of variables and it can be hopelessly challenging to sort them all out... Don't forget the other very important variable: the guy on the other end of the guitar putting signal into the amp. The player makes a huge difference. You can take the same amp and guitar and put ten different players through it and they will sound different in how they 'play' the amp. A good tube amp is going to make the poor technique much louder and that can be demoralizing or inspiring, depending on how you want to take it. Don't blame the tubes and don't buy into broad generalizations

[M Fowler]

I have EL84 amps with high voltage too and I don't get ratty tone.

I have since started using lower plate voltage just to save on my JJ tubes
but I love a hot running EL84.

[EtherealWidow]

Alright, but like the above posts would suggest, that change in tone wouldn't necessarily come from the higher B+, but from the higher idle current, right? I know that the higher B+ is causing higher idle current, but the change in tone is mainly coming from that higher idle current, right?

[cbass]

Heres my diseducated opinion.Raising or lowering your preamp voltages will have much more affect on tone than 30 or 50 volts difference on the Outputs

[Firestorm]

Nothing happens in a vacuum

When you change one thing, you change other things, too. To hear the difference made solely by output tube B+, you have to hold everything else not constant absolutely, but constant relatively: the screens, the bias, the AF peak of the signal at g1. In theory, if you keep everything in the same relative place, a change in B+ should produce only a change in output power (provided you keep the tubes in the linear part of the curve).

[jazbo8]

I am using the UL taps, KT77s seem to be made to be run UL. I tried the KT77s in a marshall jcm800 clone that I have and didn't like them, seemed to be lacking some low end compared to the el34s. they sound great in this amp. I did not set the amp up to switch between UL and non-UL, so I don't know how the two configurations would compare.

Sorry I mis-read your post, so you are switching the B+ not the UL. Good to know still, thanks.