• 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.

negative voltage drop across a diode

siting in my car thinking what the answer of this would be....If I have -14 of bias voltage in an amp that has a built in bias voltage indicator and a separate bias current indicator, and need to adjust/tweak (because it is always off) the indicated bias voltage with out effecting indicated bias current, I was thinking I could do this by putting a diode in series with the bias voltage meter. this would change the bias voltage by the forward voltage drop of the diode ( about .5 to .7 volts) with out changing values to the indicated bias current meter . so the question, if I start with -14 volts and assume I have a diode with a .7 voltage drop, will the indicated bias meter voltage change to -13.3 or does it become -14.7. I'm thinking it will become -13.3 volts which is what i need it to do. I do not think it would increase the negative voltage to -14.7 since no increase in potential is actually happening. does this make sense?
Thanks, Mooly. I started to look for a schematic that would represent the real world problem that I am trying to solve....but in simple terms the best way of clarifing is that I have a bias circut with both a bias voltage meter and a bias current meter and the neither of the meters can be adjusted. the bias voltage meter is off about 5%. So I need to change indicated bias voltage with out changing indicated bias current. So the easy way in my mind is to put a diode in series with the bias voltage meter since this would have no impact on actual bias performance of the tube while not influencing indicated bias current on the seperate meter. I just was not sure with direction the change would be....but your verification is enough to do a test when I get back.


Paid Member
2007-09-15 8:14 am
OK :)

I'm imagining issues such as you altering the voltage the meter sees (using the diode), and that change altering the bias current (because the meter will be drawing slightly less current if its reading less). Interactions :)

Why not add a preset pot in series with the meter to adjust it exactly (you can always fit a fixed value after). Then if there were any change in bias current you could add another high value resistor across where the voltmeter and its new series resistor connect to draw the current lost by the meter pulling slightly less.