Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: smackoj on December 27, 2012, 08:53:06 am

Title: cleaning up hum on a '83 Princeton Rev. II
Post by: smackoj on December 27, 2012, 08:53:06 am

A friend recently purchased a 1983 Princeton Rev. II from the orig. owner. Nice amp that has been well cared for. I did some general cleanup maintenance on it, sprayed all the pots, checked for anything looking burnt, bent or broken. I checked the B+ and got a nice 374 vdc on the plates. Heaters seem good at 6.2 vac. This model uses a solid state rectifier and it has 2 small value caps on V1 and V2 to prevent high squeal oscillations. I left those alone for now.

Main problem is hum at idle and pretty dang loud hum when the "Lead" switch is thrown. This model has 3 push-pull knobs for Lead, mid-boost and Bright switch. You can also use the floor switch that triggers the reverb and the Lead effect.

Questions: 1. Should I look for a replacement can cap or should I install a couple tag strips and swap out the power filter caps with Spraques or similar?. I looked for the same value can caps online and they don't seem to be out there anymore?  80-40-20 at 450v. I know most guys leave the old cans in place so it looks stock but is that the best way to go? There is plenty of open space on the bottom of the chassis if it makes sense to drill holes and build a dog house for the power caps?

2. The reverb is pretty lame as-is stock so any ideas on helping that would be nice?

3. The Lead effect is too noisy so I need help cutting thru the fog and getting down to business quieting it down so it is useable.


                                       http://www.el34world.com/charts/Schematics/files/fender/princeton_rev_II_boost.pdf (http://www.el34world.com/charts/Schematics/files/fender/princeton_rev_II_boost.pdf)

Thanks guys.....hope all have a good new yr.

Jack D   :icon_biggrin:

Title: Re: cleaning up hum on a '83 Princeton Rev. II
Post by: stratele52 on December 27, 2012, 10:02:36 am

Hum at idle ;

1- Check hum balance

2- Improper matched power tubes

3- Filter caps. Put some new cap in parallele to old one and if hum deasapear , replace old cap.

4- Defective shorting input jack.

Lead effect and reverb ; I don't know for now

Title: Re: cleaning up hum on a '83 Princeton Rev. II
Post by: smackoj on December 27, 2012, 10:05:31 am

thx strat tele     :icon_biggrin:

Title: Re: cleaning up hum on a '83 Princeton Rev. II
Post by: stratele52 on December 27, 2012, 10:27:26 am

You are welcome  ; )

Title: Re: cleaning up hum on a '83 Princeton Rev. II
Post by: PRR on December 27, 2012, 05:42:21 pm

The amp was pretty-OK when new. If it is offensive now, fix what has gone bad. After 29 years, main filter caps are a first guess, maybe the whole problem. Jacks are always suspects.

Title: Re: cleaning up hum on a '83 Princeton Rev. II
Post by: PRR on December 27, 2012, 05:49:30 pm

Power point B, triangle cap-tag, is critical to lead channel hum. Go ahead and tack 22uFd across B-Gnd and see how much it cleans up.

The reverb and Lead are tangled together to save a buck. The reverb driver is bright and will distort- hey, that's our Lead! The rev transformer has a resistor to tap-out signal through opto-switch back into the amp at a low level. IMHO, it is what it is. Messing with reverb will mess with Lead. Doing it right is more trouble than selling it on and finding another less-tricky amp.