Bizarre hum problem from tone stack

[harry]

Hi there,

Long time reader, first time poster.

I'm a bit of a novice at working with tube amps, but I'm modifying an old PA head and so far I've managed to transform it from a muddy mess into a sparkling, chiming guitar amplifier (beginner's luck I think).

I'm getting some hum that I had assumed was 60-cycle hum coming from a mismatch in the output tubes (as it was present when I accidentally cut the power to all my pre-amp tubes). However, when I turn down the treble control on the pre-amp, the hum progressively disappears until you turn the control completely down, when there is (almost) no hum. The output is fine when you do this, although the treble control still functions correctly, so all the treble is rolled off.

What could be causing this? I have enclosed a (very rough) schematic of the signal path. If it's not much help I will draw a more detailed one.

Any suggestions would be most appreciated!

[Alex R]

A question to help everybody - I assume the hum doesn't go away when you turn down those volume controls?

[Ptron]

Does turnning either or both of the chanel volumes all the way down kill the hum?

[Alex R]

That's what I meant to say but my fancy eddication got in the way.

[harry]

Sorry, I should have clarified

Turning down the volume controls doesn't remove the hum. Neither does turning down the bass control. I just found out that turning the mid control all the way down does take out a (tiny) bit of the hum.

It's just when you turn the treble down that the hum is removed completely.

[harry]

Additionally - I'd imagine the can caps need changing, because they look like the originals and the date on the output transformer reads 1962.

[Alex R]

Is C1 picking up atmospheric hum. Put your finger near it carefully and see. Try the difference if you ground yourself. If it's a ceramic, try a silver mica.

How many gain stages do you have after the tone controls?

[harry]

Yes, that's exactly what it's doing! If i ground myself it reduces the hum to some extent, but not all of it (the treble control still needs to be turned down).

After the tone controls, there's one buffer stage, then it goes to the PI (long-tailed pair going to two EL34s)

I'll try replacing C1 with a silver mica cap once I find one with the correct voltage rating. Thanks for the tip.

[R.G.]

This is a VERY high impedance tone stack. The impedance is so high that it is probably picking up hum. It's likely that constructing it using vanishingly short component leads and shielded cable might reduce or eliminate the hum, but because of the high impedance levels, everything becomes critical. For instance, there is some hum coming in if the control pot bushings are not fully grounded to chassis. Building it inside a shielded cage might help.

A more robust approach would be to put followers on the 12AX7 plates and drop the impedance of every single part by 10X: resistors become 1/10 resistance, caps become 10X capacitance. That should help a lot. Good construction will still matter, but it will be much less critical.

[log1982]

Is the shell of the treble pot grounded?

[harry]

Is the shell of the treble pot grounded?

Yes, I've tried it grounded and ungrounded

I'll try lowering the impedance values too, I hadn't considered that. Thanks for the tip!

[Don Moose]

I think RG was referring (mainly) to those 470k series resistors after the level controls. Unless you use the clean and dirty simultaneously, you should add a channel switching point in place of those resistors.

I think.

Hope this helps!

[Enzo]

Here is a test:
Does the hum get loudest part-way up on the treble control? In other words if the treble control is maxed, the hum is slightly less than it is with the control maybe 3/4 up?

In any case, connect a clip wire to ground, set the treble control at mid point, and now clip the groudn wire to the top of the pot - the end that joins C1. If the hum stops, it is coming from C1 or something before it. If the hum remains, it is coming from the pot itself or something else connected to the circuit.

Now, ground the wiper of teh treble pot. If that kills the hum, then indeed it is coming in from the pot or the following grid.

By the way, is your heater supply grounded? Is one side of the 6VAC to ground, OR is there a center tap to the 6VAC grounded, OR are there a couple resistors from either side of the 6VAC to ground ala Fender? I just dehummed an old Fender COncert last night that lacked any of those measures.

If you change out the filter caps, I suggest fixing the amp first. If the caps were at fault it would sit there humming all the time, no matter what. DOn't add to the troubleshooting by doing mods/updates DURING the problem solving.

[harry]

I replaced the capacitor and the hum is much lower, but still present.

Here is a test:
Does the hum get loudest part-way up on the treble control? In other words if the treble control is maxed, the hum is slightly less than it is with the control maybe 3/4 up?

The hum now disappears when the volume control is about 1/10 of the way round, any lower and it re-appears, any higher and it starts to get much louder. Why should this happen?

[R.G.]

It happens because you're picking up hum two places with different phases. One place is before the volume control, one is after. The one before the volume control is amplified and inverted with respect to the hum after. That means there's a place on the volume control where the mixed hum cancels; and it happens at about 1/10 of the volume control because of the amplifcation of the hum from earlier in the amp.