Princeton Reverb re-tube and bias

[dognmoon]

In the last week or so, my '74 Princeton Reverb has been suffering from a slow but very noticeable rise and fall in output level. I'm assuming this is a power tube issue so I'm gonna retube.

Does anyone know if there's a relatively safe way to bias this amp (i.e. external bias pot)? I'm assuming I'll need the Weber Bias Rite...


also, Is there any technical issue with using an a/b/y pedal to switch betwen inputs on this amp so I could use the a/b switch (leave the Y alone) and use input 1 as a solo boost and input 2 as rhythm volume?

[donnyjaguar]

I would confirm its the power tubes before blowing money on new ones only to have the amplifier trash them. If the gain is changing it could be any of a number of things.

[schmit]

Im no amp tech by any means but I do know that there is not a bias pot in a Princeton.....I just had my SF gone thru and updated and the tech said biasing the tubes is done by soldering in a specific valued resister. Its done on a trial and error basis....If the bias is too low you desolder and put in a higher value resister....and repeat until its correct....

You can have a tech install a bias pot but I believe you lose your tremelo circuit......maybe a *real* amp tech can explain better than I

[Blue Strat]

Schmit is correct on the first part, incorrect on the 2nd.

By the time your tech finds the "right" resistor value, he could have installed a pot and probably saved you labor time. Worst case, having the pot with certainly save you labor cost in the future.

Your A/B scenario will only work effectively on a multichannel amp which the PR is not.