Voltage regulation for master volume?

[Shawnee]

Hello guys,
     I had an amp that had a voltage regulator as a master volume (mosfet, a 1M pot, and various resistors connected between the 32/32 cap can and the standby switch). So if this is lowering plate voltage, how is that ok if the plate voltage directly affects the reflected primaty impedance of the output transformer?

[pdf64]

Can you expand on what you perceive the issue to be?

[sluckey]

Hello guys,
     I had an amp that had a voltage regulator as a master volume (mosfet, a 1M pot, and various resistors connected between the 32/32 cap can and the standby switch). So if this is lowering plate voltage, how is that ok if the plate voltage directly affects the reflected primaty impedance of the output transformer?

The whole idea of that circuit is to lower the volume. A mismatch of impedance results in lower power transfer so that kinda plays right along with the whole volume reduction idea.

[st]

So if this is lowering plate voltage, how is that ok if the plate voltage directly affects the reflected primaty impedance of the output transformer?

Plate voltage doesn't directly affect primary opt impedance. What you're regering to is a variable voltage regulator (vvr).

[Shawnee]

Thanks for the replies.
I was kicking around the idea of building something like a Soldano Hot Rod 25 (400v on the plates of a pair of EL34's to get around 25 watts).  I read something about using an output transformer with a different primary impedance (I guess I should really say turns ratio) than what would normally be used for a 50 watter because of the lower plate voltage. Is that correct? So the issue is not an over current issue, it's more of a loss of power/bad tone? That also got me to thinking about the VVR and how that could be ok if you are lowering the plate voltage. How does the primary impedance of the output transformer change with the voltage drop? I try to google for answers but I didn't find a good explanation. Let me just say that understand ohms law but a tube is still a mystery for me because it's not linear.

[MakerDP]

Read this:
http://www.valvewizard.co.uk/se.html

Then this:
http://www.valvewizard.co.uk/pp.html

First one is about single-ended designs but it's a "prerequisite" to the push-pull read.

[Shawnee]

Will do. Thanks.

[PRR]

Pentode best-load impedance is always much lower than the tube internal impedance.

Working for MAXimum power, use the corrected plate curves, or just steal a condition from datasheet or known-good amplifiers.

Working for LESS power, don't worry about the fairly small change of "optimum".

Yes, the math suggests that a 4:1 reduction of voltage (say 400V to 100V) "should" take a 2:1 increase of load (say 4k to 8k) for best power. But you didn't scale-down 4:1 for "best power"! You scaled for LESS power. The small mis-match just means you get down to quite small power. Which is what you want.

[Shawnee]

Thank you. So if the math is linear, then going from 500v to 400v is 20% which would mean increase the impedance 10% right? So say 2 EL34's with 500v on the plates with a primary impedance of 3.6K would only bump up to a little under 4K (which doesn't really change anything) correct?

[PRR]

> doesn't really change anything

Correct.