Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bohema79 on April 20, 2014, 05:53:15 am

Title: Fender BF Princeton Reverb platage voltage drops
Post by: bohema79 on April 20, 2014, 05:53:15 am

Hi Guys.
I just put a new set of matched JJ 6v6s tubes in my 1966 Fender Princeton Reverb and have a problem with dropping plate voltage.

I start out with 435V plate voltage and the tubes biased at 23.5

After about 10-15min of warm up the plate voltage drops to 425V - bias stays unchanged.

After another 15min the plate voltage drops to 419V.
And so on... Last I checked it was 409V.

Can you please tell me what might be the problem?
Is it the set of tubes going bad?

Thanks!

Title: Re: Fender BF Princeton Reverb platage voltage drops
Post by: stratele52 on April 20, 2014, 09:11:01 am

To set properley bias on new tubes you need to warm up tube at least half hour and adjust bias if needed. New tubes may need many hours to stabilize sometimes.


Sure if voltage drop bias change . If you have 23.5 ma at 435 volts you will not have the same 23.5 ma at 409 volts plate voltage if you don't adjust bias pot

Title: Re: Fender BF Princeton Reverb platage voltage drops
Post by: HotBluePlates on April 20, 2014, 01:30:50 pm

Quote from: bohema79 on April 20, 2014, 05:53:15 am

I just put a new set of matched JJ 6v6s tubes in my 1966 Fender Princeton Reverb and have a problem with dropping plate voltage.
...
Can you please tell me what might be the problem?
Is it the set of tubes going bad?

There is no problem. New tubes will have a drift in their current (maybe for quite a while). When they drift to a little higher current, the plate voltage will drop some.


If you don't see your 6V6 plates glowing cherry red, don't worry & enjoy the amp!

Title: Re: Fender BF Princeton Reverb platage voltage drops
Post by: bohema79 on April 20, 2014, 03:24:50 pm

Hi,
Thank you guys for all the tips and advice. I will do as told.
Best wishes,
Piotr