Univox 1221 amp hum

[Axtman]

I am totally stumped on this one. The amp in question is a Univox 1221 tube amp with two 6L6GC tubes and a sold state rectifier. I bought it cheap and was worked on by a local amp tech guy. He probably gave up on locating the hum like I am about to do.

I cannot seem to locate the source of the 60 cycle hum. The hum happens even when all the preamp and phase splitter tubes are pulled leaving only two output tubes. I bet the tech was chasing the same problem since he recapped with higher capacitance value. He replaced 3 of the 50uf/350v caps with 100uf/450v caps.

The only only other major change is the output transformer has been changed to a Fender Vibrolux replacement.

Got any ideas?

[The Dude]

Did the other tech install all the new caps with the correct polarity?
Have you tried a new set of output tubes?
Do supply voltages line up roughly with the schematic (including bias supply)?
Have the caps in the bias supply been changed?

[Axtman]

New caps are installed with the correct polarity. I have checked numerous times and will recheck again.

I have tried a new set of output tubes with same hum result.

All around the voltages are higher than listed in the schematic.

Caps in bias supply were changed, I even changed them again and changed out the diode.

[Enzo]

If you are sure it is not 120Hz, then look at the bias supply. It has rather small filters as it is, and it is half wave, so the hum will be 60Hz. Is your bias TOTALLY free of ripple?

And as a long shot, pull the power tubes too. Does it hum with no tubes at all? That could indicate power transformer field coupling to the output transformer.

[Jon Snell]

Ensure the correct value capacitors are used for B1 & B2; 50uF and 100uF.

[mozz]

Be sure pin 2 filament goes to pin 2 on each output tube.

[J M Fahey]

For testing, with power tubes installed, add a, say, 50n or 100uF cap rated 60/100V from the junction of 2 x 100k grid resistors and -bias wire to the point where the 1 ohm 1W resistor grounds.
Does it change hum?
I´m trying to test whether hum gets in through bias line.

Weird amplifier, it has a cheesy ripply voltage doubler rectifier and applies crazy high voltage to poor 6L6 plates.

Not sure modern ones are up to par.

[Axtman]

I thought that I uploaded the schematic. Well here it is again. univox1221.pdf

I am sure it is 60hz. My phone has a audible guitar tuner that also displays frequency.

It does not hum with power tubes pulled.

All three of 50uf filter capacitors were previously changed to 100uf.

Pin 2 filaments go to pin 2 filaments.

[Helmholtz]

What are the 6L6 idle currents?


As others said, 60Hz hum sounds like bias ripple. Make sure that bias filter caps are connected with positive terminal to ground.

[Enzo]

All three of 50uf filter capacitors were previously changed to 100uf.

Those are the B+ caps, I asked about the bias caps in post #4 and Helms did as well in post #9. Please check that.

[Axtman]

The bias caps are connected with positive terminals to ground. I replaced them with higher capacitance caps and it made no difference.

I replaced the bias resistor (7k on the diagram) with a decade box. When I increase the value of the bias resistor thus making the grid more negative the hum on the speaker increases. The opposite is also true the less resistance (lower voltage) the hum reduces.

[Helmholtz]

Don't carelessly change circuit values, especially don't switch bias dropper resistors with powered tubes, as you might damage them.
Measure the voltage across the common 1R cathode resistor. Should not be above 60mV.

[g1]

Measure the voltage across the common 1R cathode resistor. Should not be above 60mV.

And then I would suggest doing it again with one power tube at a time. A big imbalance could cause hum also.