The latest bum amp in my workshop was a Fender Blackface Super Reverb, ca. 1964, which is quite possibly the heaviest amp I have ever carried. The Normal channel was fine, but the Vibrato channel had an intermittent volume drop, just enough to be noticeable.
|Fig. 1. The Beast.|
|Fig. 2. This is why The Beast weighs 70 to 80 pounds.|
A visual inspection revealed a missing solder joint on the bias capacitor (see Fig. 3 below), which was not the cause of the problem, but certainly worth soldering.
|Fig. 3. A conspicuous absence of solder.|
I didn't see anything else untoward, so I did the chopstick test (with the amp on, poking each component with a chopstick). Still nothing. New tubes did not solve the problem either, so I did another chopstick test to no avail. I recorded all the voltages, but nothing jumped out as weird.
I started testing continuity among all the components and I found a cold solder joint on the board going to pin 1 on V5. I resoldered it, and the problem was gone. I took voltage readings again, and THEN the first measurement looked weird, even though it didn't jump out at me at the time. I guess I was too busy not electrocuting myself to notice the first time.
Both problems appear to have been the result of a recent recap job (I didn't do it) that was mostly good, but with a couple of iffy solder joints.