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24.205.128.2
In Reply to: RE: Adjusting Plate Voltage on Preamp Output Tubes posted by Maxamillion on June 30, 2009 at 18:23:59
When I change plate voltages, I then measure the change in idle bias current of tube (measure drop across cathode resistor & calculate the current). Normally, if the voltage change isn't too great on the anode plate, the idle current will compensate somewhat and you'll still be in an appropriate idle current operating range. If not, then start playing with the cathode resistor. I measure with no inputs connected and zero volume (if you have control).
Thanks, that's the info I was looking for! So I will monitor the idle current and adjust the cathode resistor value if needed.
Have you tried using a pot (rheostat) on the plate to adjust the voltage? I know the guitar guys like to do that sometimes. If so, what pot did you use (wattage, ohms, etc.).
Yes, I have on output tubes, when trying to dial-in Class "A" cathode biased amps. I usually put a 100 to 150-ohm rheostat inline with a fixed resistor, say 100 ohm, while fine tuning the bias to tone (eg: for an amp that calls for a 220-ohm resistor). The fixed resistor in there just in case the rheostat fails or someone (read: me) cranks the thing the wrong way.
Once I'm happy with tone and status of tube, I replace rheostat and fixed resistor with one unit. Check link for rheostats.
As for gain stage tubes, I haven't used a potentiometer. I play the adding parallel and/or series combos until I hit the point I like (I try for midway between saturation and cut-off). Then, measure the resistance of the mess and replace with one resistor of near equal value. I don't see why you couldn't use a pot, though.
Thanks for the info, FL!
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