HNY! and a 70's Princeton question.

[SixStrRzr]

Hi,all,

New user here - been playing blues & rock,some jazz and country for almost 40 years,semi-pro.

I have a mid 70's Fender Princeton "no-verb" that I just replaced the 6V6's in. With a bias probe,one tube's pulling 24 ma,and the other 54...is this normal?

I also tried to find a bias trim pot - evidently these amps don't have one?

Is there a way to match the power tubes more closely to get this amp humming (no pun intended) at peak performance?

Thanks much,

Geoff

[Twist]

Which chassis do you have? (6G2 or AA964) The vibrato on those varies the bias voltage to produce a change in output levels. You can replace the 27K or 30K resistor paralled across the 25uf 50 volt cap on the little board behind the power light with a 50KL pot. I would lift the pot off ground with a 15K resistor to make sure while adjusting you dont completely zero out the negative voltage.

[stokes]

Are you using a matched pair of tubes?Some Silverface Fenders had a "bias balance" set up instead of the bias control,so mismatched tubes could be used,but I am pretty sure your Princeton has a fixed resistor,and no "balance" pot.The mod recommended by Twist will let you adjust the bias to both tubes,but wont help if your power tubes are not matched to begin with.

[SixStrRzr]

I doublechecked the tubes by switching sockets,etc..one particular tube always draws 54ma, while the other 24. So it's tube-dependant,and not socket-dependent. I suspect I have a defective tube. (JJ)

Are you using a matched pair of tubes?Some Silverface Fenders had a "bias balance" set up instead of the bias control,so mismatched tubes could be used,but I am pretty sure your Princeton has a fixed resistor,and no "balance" pot.The mod recommended by Twist will let you adjust the bias to both tubes,but wont help if your power tubes are not matched to begin with.

[stokes]

It would seem you just have an unmatched pair of tubes.Did you buy them as a "matched pair"?With that amp you need to use matched tubes.Some SF's had a bias balance but not that one.

[SixStrRzr]

Yes, bought as a matched pair. One must be defective.


It would seem you just have an unmatched pair of tubes.Did you buy them as a "matched pair"?With that amp you need to use matched tubes.Some SF's had a bias balance but not that one.

[stokes]

How long have you had them?Maybe you can get a refund or replacement pair.

[SixStrRzr]

I'm working with the vendor...I actually got the tubes in April, but just got around to retubing the amp,so their technically out of warranty. But the tubes have zero hours on them, so we'lll see.

How long have you had them?Maybe you can get a refund or replacement pair.

[stokes]

Could be that the tube isnt defective,just a case of bad matching on the vendors part.Maybe they will disregard the warranty issue if that is the case.Good luck.

[SixStrRzr]

Would bad matching result in 24 ma vs. 54 bias reading for the 2 tubes?


Could be that the tube isnt defective,just a case of bad matching on the vendors part.Maybe they will disregard the warranty issue if that is the case.Good luck.