Having Trouble with the GEO Tube Wah

[Roobin]

Like it!

[Paul Marossy]

I'll like it, too - if I can get rid of this darn buzzing noise! 

[R.G.]

OK, let's look at that buzzing.

Next, listen to it while you turn off the AC power to it. Does the hum/buzz stop when you turn off the AC power but before the tubes shut down? There should be a second or so you can still hear audio through it. If the hum/buzz quits instantly when you turn off the AC, then the noise is generated inside the unit. If it does not, the noise is generated outside the unit.

If it's generated inside, there is one issue with all tube power supplies - how EXACTLY did you wire the rectifiers to the first filter? The winding and rectifiers should connect directly to the + and - of the first filter cap. No part of any of the wires connecting from the winding and rectifiers to the first filter can should be shared with any wiring to the rest of the circuit.

[Paul Marossy]

Next, listen to it while you turn off the AC power to it. Does the hum/buzz stop when you turn off the AC power but before the tubes shut down? There should be a second or so you can still hear audio through it. If the hum/buzz quits instantly when you turn off the AC, then the noise is generated inside the unit. If it does not, the noise is generated outside the unit.

What do you mean by "noise generated inside the unit"? If it's ripple on the B+ supply, then of course it would stop immediately when you turn off the power - wouldn't it? Do you mean the inductor picking up EMI? Or are we talking about the power supply section?

If it's generated inside, there is one issue with all tube power supplies - how EXACTLY did you wire the rectifiers to the first filter? The winding and rectifiers should connect directly to the + and - of the first filter cap. No part of any of the wires connecting from the winding and rectifiers to the first filter can should be shared with any wiring to the rest of the circuit.

I've got it wired exactly like the schematic. I've wired full wave diode rectifiers many a time now. I guess I  should double check it, maybe I screwed up something... 

[Thomas P.]

I thinks he means that you could have created a loop for example which would still be humming if the ac is switched off. Maybe therefore he asks about how you wired it. This (internal) hum could be decreased by grounding the several stages separately to the filtercap's ground point (aka star-ground).

at least that's what i read out of it

regards,
tomboy

p.s.: good work as always

[R.G.]

Sorry - I didn't mean to be cryptic.

There is always a problem with high impedance tube amp circuitry in finding out whether it's picking up external noise or making its own noise. The power-off test is a good one for separating that out, and it works the same way that your heaters-on-battery test did. Since the tubes work for a second or so before they drain the power supply or get too cold, if you flip off the power supply, all power supply generated noise and all noise caused by AC wiring inside the enclosure vanish when the switch opens. That tells you that there is a power supply noise problem.

If the noise does not vanish, it is not being generated by the unit's power supply, and so you have to look elsewhere. It could be grounding, as we've already mentioned. It could be house wiring being funny. It could be noisy flourescent lighting buzzing.

After a few years of debugging tube amps, you get to be a connoissieur of hums and buzzes. Bass hum is 60 Hz, and it's caused by heater leakage, AC line leakage, AC miswiring, faulty transformers, poor transformer placement, or ground loops. 120Hz one octave up that is only slightly buzzy is power supply ripple. 120Hz that's distinctly buzzy is spikes at 120Hz, and that's from rectifier transients caused by poor rectifier wiring, light dimmers and fluorescent lighting in the building, or other things that cause spikes on each half wave.

A wah circuit is a special case of "poor transformer placement" because the inductor can pick up 60 cycle or 120 cycle M-field, as you already know.

[John Lyons]

Yes, I thought that as well.
Something to the extent that when the AC is switched off (turns off instantly) the DC voltage and circuits are still being fed by the filter caps which will drain slower and still produce some voltage and sound. Possibly enough to tell if AC is your problem.
At least I think that's what RG is talking about.

[Paul Marossy]

Sorry - I didn't mean to be cryptic.

That's OK. I figured out what you meant by the tube/power off test. It's definitely from the power supply unit - and I think I know what the problem is: I apparently miswired the rectifier bridge. 

It took a little while to figure out, but that's got to be what it is looking at my picture of the power supply PCB. When I get home tonight, I'll cut one track on the PCB and solder in two jumper wires. That should hopfully fix it! Thanks for all the patience and responses! 

[Paul Marossy]

OK, some progress has been made. I fixed my boo boo on the rectifier section. The buzz is gone and the hum is much less than it was before. You can hear the hum a little bit when you are playing clean, but then it really gets large with a high gain distortion pedal. So, I think I'm back to looking at the tube heaters and the inductor. Maybe this is a situation where you could do the humbucking inductor trick with a pair of inductors?

About the grounding: Inside the PSU, all grounds connect in one place and then get connected to the PSU chassis. The grounds inside the pedal all connect to the input and output jack and then to the ground in the PSU. The input and output wires to the circuit are shielded wires.