Location: Denver, Colorado.
Symptoms: Amp rebuild.
When I listened to the amp, I didn’t really hear anything in particular that was wrong with it. But the owner was unhappy with it. Someone had previously done some work on it, replacing a few capacitors, including the main filter capacitor on the output of the 5U4 rectifier tube.
I brought the amp back and bench tested it. It failed.
The problem was primarily in the power supply as these waveforms were measured at the Aux Amp output, which is essentially the output of the preamp stage. The final output stage was loading the power supply and breaking into oscillation. Most often when an amplifier breaks into oscillation, there is a faulty capacitor someplace.
I decided to do my standard “re-cap” where I replace all of the electrolytic capacitors and some of the rolled capacitors of smaller values subjected to high voltages. I tested some of the old capacitors once they were out of the circuit (except in rare cases, capacitors can’t be tested while they are in the circuit). There are two large can capacitors that contain four capacitors each. All of the capacitors in one of the cans were dead (reading very low). All of the capacitors in the other can were better, but all were out of tolerance. Those cans probably were the culprits of the oscillation.
Both Wurlitzer and Seeburg amplifiers of this era use 6L6 tubes in the final stage of the amplifier. From a theoretical point, the two tubes should be gain-matched because one tube drives the upper half of the waveform, the other tube drives the lower half of the waveform (a.k.a. push-pull amplification). But these amplifiers seem to be pretty forgiving if the tubes are not matched. I once saw a Seeburg operating with one 6L6 tube missing.
The owner and I decided to go ahead and get new output tubes. The nice thing about 6L6 tubes is that they are still being made today due to their popularity for use in guitar amps.
After all the work was finished on the amp, it tests perfectly at all volume levels.