Are Univox amps naturally "hummy"?

[Axtman]

I'm working on 2 Univox amps that have the same problem. They both have a hum at no input/volume. I have replaced filter capacitors and have tried to locate the source of hum to no avail.

[nosaj]

Build a signal probe and start tracing. You will need a secondary amp could even be one of those little bluetooth speakers with a 3.5 jack.
You will also need a signal source your phone playing music at the input will sufffice.
nosaj

[Leo_Gnardo]

I can think of a couple of things. Well 3 or 4 to start out with, easy enough to check or try before you go on a long hum hunting safari.

Sometimes hum derives from output tubes plate current being significantly different, one tube compared to the other. This sort of imbalance will leave you with a hum that persists even if you remove all other tubes from the amp. Check it out, if that's the case it may save you chasing phantom problems all over the place when this is the actual source.

Similarly, an insufficiently filtered negative bias supply will introduce hum. In some amps the factory installed filter isn't enough, in others the bias filter cap has deteriorated over decades. Or both.

Amps that have multisection filter caps might introduce hum into the preamp by having all filter caps employing the same ground. Can't avoid that with a can cap. Sometimes you can beat down hum by replacing the cap that serves the preamp stage with a single cap, and grounding that cap's negative pole to chassis somewhere in the preamp section, not at the same point the other higher-voltage filters are grounded.

Then there's the filament supply. Sometimes it can be an advantage to install a hum-dinger pot instead of relying on a filament winding center-tap or a pair of resistors to create a virtual center tap.

Hope this helps.

[Axtman]

Boy do I feel like a complete idiot. I should have listened to Ronald Reagan, "Trust, but verify". After countless hours of a wild goose chase trying to solve the hum problem I realized that the NOS can capacitor that I installed was faulty. I clipped in a new capacitor across the first section of the multi-section capacitor and the hum went away. I replaced the can capacitor with a new can cap (higher voltage and capacitance) and sure enough no hum.

I swear I tested the NOS capacitor with a multimeter and ESR meter before I installed it. I also have to eat my words on a previous post where I exclaimed that can caps rarely go bad.