No sound from this amp, only HUM!

[slidincharlie (Carlo P)]

I have a (mid-sixties?) Farfisa F-8 combo on my bench now: EZ81, 2xECC83, 1xEL84.
When I turn it on, as soon as it has warmed up it produces a rather loud hum, which is not affected by the volume.
If I plug a guitar in, NO sound comes out of the speakers (2x6").
I have tried to swap tubes with known good ones to no avail.
What should I check?
I suppose that, since the volume does not affect the level of hum, the trouble must be after the volume knob, right?

BTW: glad to meet you guys in this new version of the good old beloved Ampage

[Cuff]

Hmm...

Do you have a scope or just a multimeter? If you can get some sort of signal into then you can start following it with your scope. That is about the only way I know to troubleshoot after the idea of the tubes being bad is eliminated(darn you military training). Oh yeah, check the bias voltages on the tubes as well.

[slidincharlie (Carlo P)]

Thank you Cuff,
I hope I can do it later today.

[Don Symes]

it produces a rather loud hum, which is not affected by the volume. If I plug a guitar in, NO sound comes out of the speakers (2x6").

A couple of questions that seem obvious -

(This one is an issue with older amps in the USA, I don't know about Italia)
Does the amp have a 2-wire power cord, or a proper grounded one? If it's just the 2 wires, for your safety and that of your customer please persuade him to have you convert it to a 3-wire, grounded setup before you do anything else. Definitely do that before you let it out of the shop.

The hum, it comes from the speaker or maybe something else (one of the transformers)?

Plugging a guitar in kills the hum? Does just a cord do that?

With guitar plugged in, you getg NO sound? no hiss, no buzz, nothing at all?
No sound as you play with the controls (not even scratchy pots)?

I suppose that, since the volume does not affect the level of hum, the trouble must be after the volume knob, right?

Probably, but ... have you double-checked the ground connections?
Tightened the jacks and pots?

If the hum is in the 50Hz/100Hz range, I'm thinking first, the main filter caps and maybe the rectifiers. If pulling the ECC83 tubes doesn't kill the hum, this is a fair guess.

Then, I'm thinking that plugging the guitar in might be shifting the amp into ultrasonic oscillation (maybe). It may instead be breaking the one good ground connection you have, breaking the circuit.

The real resource here is R.G.Keen's Tube Amp Debug Pages

Best of luck!

[Enzo]

I didn't think of it your way. WHen I read that no sound came out, I got the mental impression he meant no GUITAR sound, but the hum remained. Looking at it afresh, maybe you are right, in which case the problem is odd. Input problems can usually be turned down with the volume control.

[slidincharlie (Carlo P)]

Thanks Don,
Enzo above is right: no GUITAR sound, the hum remains constant with a guitar plugged in.

Does the amp have a 2-wire power cord, or a proper grounded one?

It originally had a 2-prong cord, at some stage it has been fitted a 3-prong cord.

The hum, it comes from the speaker or maybe something else (one of the transformers)?

It comes from the speakers.

Plugging a guitar in kills the hum? Does just a cord do that?

Nope, the amp keeps humming when I plug a guitar or a bare cord.

With guitar plugged in, you getg NO sound? no hiss, no buzz, nothing at all?

See above: a guitar plugged in makes no difference.

No sound as you play with the controls (not even scratchy pots)?

No pot affects the hum. When I turn the tremolo pots, the hum trembles like if it was a guitar note.

Have you double-checked the ground connections?
Tightened the jacks and pots?

Not yet, but it does not sound like a grounding problem to me. I gotta check anyway.

If the hum is in the 50Hz/100Hz range, I'm thinking first, the main filter caps and maybe the rectifiers. If pulling the ECC83 tubes doesn't kill the hum, this is a fair guess.

Gotta check this as well. The filter caps are old (yet no bubbles or other external symptoms of failure).

[Cuff]

Back to the old drawing board

So, now we put some sort of sound into the input, look at it with your scope at the input and take note of what the wave looks like. Then just look at the input and output at each stage until you lose the signal or it becomes a signal that is at a different frequency. Then you will know what stage to look at. This of course after you know that the tubes are biased correctly by checking the DC voltages with your meter.

Cheers

[Don Symes]

I didn't think of it your way. Looking at it afresh, maybe you are right, in which case the problem is odd.

Sometimes the key is the right question.

Not this time. In fact, hardly ever, but I don't like to assume that I understood what was described if something is emphasized but ambiguous.

... or maybe I'm just odd.

[Enzo]

Old? Why my first amp ran on steam. And you had to play uphill and barefoot...in the snow.

We still gotta solve CArlo's problem. I'd also be checking the filters. Any heater to something shorts? If B+ is lumpy, fix it. If it is reasonable smooth, what are the voltages around the power tube? AC and DC. Use your meter set to AC as a substitute for a scope.