• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Screen resistor sag limiting zener.

bendyha

Member
2014-01-18 4:40 pm
Whilst thinking about switching possibilities
From...
Stiff supply with screen supply choke and fixed grid1 neg. bias.
To....
Softer supply with univeersal voltage dropping sag resistor, cathode self bias and a large voltage dropping, sagging, screen resistor.
But...
Maybe I find that when the tube is being driven hard, the desired bouncy compression caused by the screen grid current pulling the 0V. curve down, goes so far that it is clipping too hard.
So...
Why not parellel the resistor with a zener, so that when the screen resistor has enough volts dropping across it, the zener kicks in and says so far, and not further, and the supply is restiffened.
Am I missing something?
Dispite an extensive search, I have not seen this method mentioned anywhere ; because it is a poor idea? or just that no-one has found a need for it or that it could be a good idea.
There are similar things being done with cathode resistors and zeners to stop the current draw pulling the bias down so cold that problems start arising; such as in overdrive conditions where the cathode resistor bypass cap charge doesn't discharge and the bias cools even more.... cross over problems.. .class C...
 
I put screen limiting resistors in to keep the screen from melting in gross overload conditions. Seems like this would not protect as well against that as a resistor.

I have done a current-limited stiff supply before for the screen. I just did a Mosfet-follower regulator and I had a BJT that would make the screen supply sag by pulling the gate of the Fet down over a certain current limit. I set the limit to keep it higher than normal operation.

In testing, I overdrove the amp and the signal peaks had little volcano craters. Weird looking. My amp was designed to not clip in normal use, though. It is at ear-bleeding levels at clipping. Not sure if this would sound bad if you had it clipping and playing music.