Power Tube mismatch and hum

[dunner84]

Why can some fixed bias amps tolerate a power tube mismatch without any hum, and others can't? Is it an issue with the bias supply? Filtering? or is it just voodoo?

[sluckey]

It's the common mode rejection nature of the circuit. The B+ used for the plates contains a high amplitude 120Hz 'ripple' signal due to poor filtering by the first filter cap. This signal will be applied equally to each tube. Now if the tubes (and the rest of the circuit) are perfectly balanced, these two equal 120Hz signals will perfectly cancel due to the way the perfect OT recombines the plate signals. Equal signals of the same phase (called common mode signals) will subtract resulting in zero signal out of the OT. Plate signals that are equal but 180° out of phase (this is the desired signal coming from the grids) will add and the results is a signal that is twice the amplitude of either plate signal. This becomes the push/pull effect of the circuit. That's the way a perfectly balanced circuit works.

Now if you unbalance (mismatch) the tubes, the B+ ripple signal will now be greater in one tube than the other so they will not perfectly cancel when recombined in the OT. This results in some 120Hz signal being passed on to the speaker causing audible hum. Greater mismatches will cause greater hum. The ultimate mismatch would be to remove one of the output tubes.

Any mismatch should result in a measurable hum even if you don't hear it. There are lots of other factors about the circuit that can also contribute to mismatches and imperfect common mode rejection. And many things contribute to how well you hear the hum. The frequency response of the OT and the speaker are big factors. Even the speaker enclosure has a big effect.

You can completely eliminate the 120Hz hum by improving the power supply filtering (or use batteries) to deliver a pure dc voltage. But this is expensive and impractical, especially with higher power amps.

[dunner84]

Thanks Sluckey, that is exactly what I was looking for!

[PRR]

In fix bias, bias supply smoothing also matters. All the same reasons Sluckey gave for plate and screen smoothing.

[HotBluePlates]

In fix bias, bias supply smoothing also matters. ...

I once fixed a Princeton Reverb that hummed.  Attempting to isolate where the hum was occurring I pulled tubes, starting with the phase inverter.  I still had hum.

Hmmm...  That means this hum is at the output tubes, but it almost sounds like 60Hz hum.  Where could that happen?

The cap in the bias supply had failed.  Not catastrophically, it just lost nearly all capacitance.  That left a large half-wave a.c. as the output in the bias supply, so hum was being injected right at the output tube grids. "Should have cancelled" but perhaps it didn't for the same reason we hear tremolo.  Replacing the bias filter cap resolved the issue.

[PRR]

> we hear tremolo.

We should not hear the 5Hz trem thump, it cancels. Well, never 100% for all the reasons above.

We hear gain-wobble because the gain-change does not cancel.

That sick amp should have put a 60Hz "tremolo" on the signal passing through. Of course that is too fast to be "trem". More an ugly ring-modulator. Locked to the power line so the ear lumps it with the hum. And if it hums real bad you may not have got as far as putting signal through?

[HotBluePlates]

... And if it hums real bad you may not have got as far as putting signal through?

Nope, I never got that far.  Owner complained of "hum" and I turned on the amp with no input and heard hum.  Tested as mentioned above, thought swapping the bias filter was the shortest path to a happy amp, and everything turned out well.

It's been a few years... I can't even be sure I measured volts at pin 5 of the output tubes to notice a.c.