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Amp Problem

PostPosted: Wed Apr 02, 2014 12:32 pm
by Greg Butler
I have a Fender Princeton Reverb amp (tube) from about 1972. Recently, the volume suddenly increases and after a short time returns to its normal setting. Any ideas about what may be going on? Is this something that could be fixed with a tube being replaced...or should I seek out a serviceman? Thanks!

Re: Amp Problem

PostPosted: Wed Apr 02, 2014 2:14 pm
by Terry Tolley
If I found the correct schematic, the 7025 tube (V1) has the volume and tone controls in it. That tube can create this symptom, but there are two capacitors in the cathode circuit of each half of the tube that has a higher likelihood of being the culprit of this symptom.

Somewhat less likely is the first half of the 12AX7A (V3), but there is a capacitor in the cathode circuit of the "A" section that could create this symptom also. Electrolytic capacitors tend to lose moisture over time and can change effective value with temperature and will cause a symptom like this.

Were it my amp, I would change all three of these capacitors. Parts cost is less than $5.00 from the sources I found. From the description, I would say the tubes are O.K., but without making some voltage measurements, that is just a guess.

Re: Amp Problem

PostPosted: Wed Apr 02, 2014 10:32 pm
by Greg Butler
Much thanks, Terry!