• 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.

High Voltage Derived Bias Supply

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mr2racer

mr2racer

Member
Joined 2009
Hey Anyone,

I need a negative bias supply and unfortunately the power transformer doesn't have a bias tap. I tried using a separate transformer but it is a source of noise.

If I want to use the HT supply I understand how to set up the resistors to get the necessary voltage/adjustment. The question is what type of rectifier should I use?

Right now the HT from the main transformer is attached to a GZ34 rectifier tube. It seems I have two options? I could connect to one side to the HT and use a half wave. Or attach to both and use a full wave. It will be a SS rectifer so the bias comes on before the HT.

Someone sent me a basic schematic to do this but I've lost it. If I remeber correctly it had one reversed diode in series and one to ground.

Thanks, Kevin
 
TonyTecson

TonyTecson

Member
Joined 2003
from the web....

hwpsu2.gif


http://education.lenardaudio.com/en/14_valve_amps_6.html
a14_ps_ac-bias.gif
 
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seppoa

Member
Joined 2013
Yes, the circuit shown by mr2racer is wrong, The 470k resistor should be connected to the opposite side, to the 100 k resistor. And 100 uF caps to bias are way to big, as kevin pointed at. Better to use one of the earlier circuits, and 20k pots with about 1 uF of decoupling :)
 
Tubologic

Tubologic

Member
Joined 2004
As Seppoa said above: NEVER regulate the bias supply in a fixed bias mode amp, unless B+ is regulated too.
And also, you should check the bias supply RC time constant which must be shorter than the main B+ supply rise, or you might under-biasing the output tubes during start-up. (don't use a too big bias filter cap if your bias supply is derived from the H.V winding through a high value resistor and your main B+ rectification is solid state)
 
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seppoa

Member
Joined 2013
AJT, yes unregulated supplies are OK, the issue is that the plate supply and the grid bias should track, ie if B+ grows, the G1 bias have to get bigger, in the negative voltage. So need the G2 supply track the variations, in symmetry to B+...
Well, the heater supply are not so important, but also variations here influence the current thru tubes,,,
 
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