Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: leroy on October 17, 2021, 10:45:37 pm

Title: Halving Plexi output power by dropping screen resistance
Post by: leroy on October 17, 2021, 10:45:37 pm

Hi All

I'm looking to produce a lower (half) power Plexi with EL34's.
Currently I have 440 on the plates, 438 screens and a 4k OT.
I have read that if I reduce the screen voltage (using fixed VVR) by sqrt2 (1.41) that power output will halve.
So changing the screens from 438 to 310 will effectively produce half power?
This would also reduce the headroom of the output section making it easier to overdrive?

Thanks
Leroy. 


 

Title: Re: Halving Plexi output power by dropping screen resistance
Post by: PRR on October 18, 2021, 02:33:59 pm

If the original design used all available cathode current, and if the speaker were a resistor, Vg2/1.4 would be in the ballpark.

Not quite because of Child's Law, but...

The resistive assumption is the bigger trip-up. At bass resonance, speaker Z is very high, choking your current won't cut the signal to the speaker much.

If you drop *both* Plate and Screen, power is sure to drop, for any load situation.

"Half power" is not much of a drop. I wear a thing with 2dB volume steps which is audible but not large; and 1.5 steps (3dB) is not much larger.

Half voltage (a bit less than 1/4 power) may be a more interesting goal.

I once worked a self-biased amp nearly to 1/10th voltage with no shocking flaws. 13W to 0.1 Watts. Pea-power does not shake your chest like 13W can, and it wanted more bass, but it was fine.