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Adjustable Cathode Bias for SE Amp

10/26/2000 5:39 PM
Adjustable Cathode Bias for SE Amp
I am looking for a schematic for an adjustable cathode bias for a champ type amp. I have 365 volts at the plate and am using a 6l6 tube. The variety of currents shown by different tubes various by + or - 5ma which makes switching out resistors a pain.I have seen one design in the Torres book but the potentiometer appears to be a weak link.It follows a 350 ohm 10 to 20 watt cathode resistor. It seems that a standard pot would not handle the wattage. Any Ideas, or did I miss something ? Thanks
10/26/2000 5:50 PM
Carl B.

Why not use both cathode biasing *and* put a negative bias voltage on the grid? The grid is easilly adjusted, and you'd limit it to ranges that would give you +/- 5mA variability.  
The idea is that most of the characteristic cathode-biased type thing is achieved, yet a reliable adjustment to the bias is allowed.  
Your other choice is a 20 watt, 500 ohm ceramic resistor with a sliding center tap that's screwed down solid onto the windings. Very cheap and reliable. Ohmite makes 'em, and they're in the Mouser catalog.  
- Carl
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10/26/2000 5:55 PM

This might not be exactly what you had in mind, but the "Prince-o-wails" Princeton mod which appeared in Guitar Player a few years back featured some kind of an adjustable bias control. I can't remember the exact nature of the circuit. Does anybody have this schematic on the web?
10/26/2000 6:05 PM

flosopher :  
for that article/schematic .  
10/26/2000 11:50 PM

Third option--  
1: Choose a cathode reistor that is about 25-35% larger than the circuit would normally use (i.e., if it would normally have a 470 ohm cathode resistor, try a 610 ohm or thereabouts).  
2. Hook a voltage divider off the cathode with a pot in the lower half--use a 15k resistor and a 5k pot *wired as a variable resistance*.  
3. Hook the tube's grid resistor to the pot wiper (instead of ground), and you have instant variable cathode bias.....  
Like this (assuming a 470 ohm Rk):  
10/27/2000 12:04 AM
Tonal effect?
These are some pretty cool ideas; would one implement something like this for tonal effects, or would the variations be too subtle, and simply to adjust operating conditions?  
-- phil
10/27/2000 3:13 AM
Re: Adjustable Cathode Bias for SE Amp
Right on Dutch!  
Although I haven't noticed this used in a guitar amp recently, it's still an elegant, old reliable way to put a "little" variable positive voltage on the grid to bias a tube up that has a fixed cathode resistor.  
Good call.  
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