Bias not right on Princeton Reverb

[ElPedro]

Hey all!

Hope this isn't too dumb of a question.

The PR that I just built is running at -29 to -30VDC.  Fender spec calls for -34.  The amp sounds good, but wondering if I can/should add another resistor inline with the 100K (in series not parallel) to try to get the voltage close to -34? I know part of this may be personal sound preference, but from a technical point of view, would this be the proper way to achieve the correct voltage?

[Ed_Chambley]

Did you install a adjustment pot?  In reality the -34 should be as close to center of your pots rotation.  IOW if you have a 10K pot, you should get -24 to -44 end to end.

To achieve proper bias for a pair of tubes, the -34 is simply your starting point.  This will vary as different 6V6 tubes draw different current.

Did you install 1 ohm resistors from pin 8 to ground to measure Mv?

[sluckey]

You need to DECREASE the value of that 100K to increase the bias voltage.

[EL34]

You don't set the bias by looking at a schematic voltage


Your amp may be totally different than that


You set the bias by measuring the bias current through the power tubes

What is you bias current on each power tube?

[ElPedro]

You all make great points!  I didn't put any of the "mods" in that you mentioned, as I made the circuit a stock fender circuit.  I'll add those items and bias correctly tonight.


Thanks everyone!

[sluckey]

I'm guessing you got that -34V figure from a Fender schematic? Maybe this one...

     http://el34world.com/charts/Schematics/files/fender/Fender_princeton_rev_gz34_aa1164.pdf

If so, look at the notes at the top of the schematic, in particular, note 1.