Missing gain in Princeton Reverb

[pete692]

After eliminating tubes, what are the usual culprits that could contribute to gain loss in the preamp section? My SFPR is putting out a strong 14.5 watts, but it recently has dropped a lot of volume and bass. The tone used to be very substantial and beefy, and now it is very whispy and the bass has gotten way farty. I can crank it to ten and still talk over it. It used to peel paint off the walls on 4. It feels like I've lost a stage of gain somewhere. The power tubes are biased up fine and I had the output checked at 14.5 watts. I had an amp tech test it and he says it's okay, but it just doesn't sound right or feel right.

[Enzo]

Oh, leaky coupling caps, leaky filter caps, leaky cathode bypass caps.

[pete692]

I forgot to add this issue first showed up with a loud oscillation/feedback that wouldn't go away. The reverb pan, all tubes and output tranny have all been changed. The feedback is gone now, but so is most of the gain.

[Enzo]

The trace a signal through the amp. Chances are something about the work you did is not right. A missed connection perhaps, and you are hearing crosstalk instead of the complete signal path.

[pete692]

O.k. so we replaced the cathode bypass caps, and driver caps, even though they checked fine, and it fixed it. The bass is back and so is the volume. Can a cap check good and still malfunction while in operation?

thanks for the advice

[Albert Kreuzer]

Anything is possible, but more likely it was a bad solder joint.

[Enzo]

COuld just be solder, sure. But those cathode caps can dry up easily over the time that amp has been around.

Yes, your cap can "test" OK and still be worthless. The cap function on a hand meter usually just reads value, and even that is inferred. it doesn't put real world conditions on a cap, and it doesn't check for ESR. In fact, in some gear I used to service, worn out caps often read high! SO that 22uf cap might measure close to that, but when it tried to function in a circuit it was unable to perform.

ALL of that is moot if your solder connection had failed of course, but that is how your part can check OK but not be OK. Like so many testers, if a hand meter tells you it is bad, it probably is bad, but if it says OK, you don;t really know.