Help with hum in AC15H1TV

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Help with hum in AC15H1TV

Postby straleno » Mon Nov 30, 2009 11:41 pm

Picked one of these up last week, and it sounds amazing... except for a fairly loud 60 cycle hum that's audible even with nothing plugged in and both volumes all the way down. True, you can't really hear it when I'm playing, and a quick search reveals I'm not the only Vox owner with this issue, but still.

Clues:
-The hum is affected by the bass knob on the top boost channel. When I turn it up, the hum gets louder. When I turn it all the way down, the hum diminishes, though you can still hear it.
-The hum is worse in pentode mode (vs. triode mode). Again, this is with no instrument plugged in, and the volume knobs all the way down. The volume knobs have no effect on the hum.
-When I turn off the power, the hum stops right away.

I did some tube-pulling, and found that the hum went away when I pulled V3 (Sovtek 12AX7WA). I swapped in some known good substitutes--JAN Philips; NOS GE; Groove Tubes "Mullard" style"; JJ ECC83. The JJ--and ONLY the JJ--was quieter, quiet enough that I think I can live with it for the time being. (I tried several JJ's in this spot--they were ALL quieter than any other choice.) V3 is part of the top boost channel, though I'll admit I can't read schematics well enough to know just what it does in the circuit. (Schematic is here: http://www.blueguitar.org/new/schem/vox ... dwired.pdf)

I also poked around the wiring, but it didn't make a difference. I can see that the preamp socket wiring isn't twisted together, and that the wires are close to the sockets themselves, if that means anything.

I'd love to hear any thoughts on what might be going on.

Also, what could it be about the JJ's that makes them uniquely quieter in this spot?

Thanks!
John
straleno
 

Re: Help with hum in AC15H1TV

Postby Epidrake » Tue Dec 01, 2009 10:03 am

First of all, welcome John! Hope you stick around.

All tube amps hum somewhat. Sounds like you tried everything. Did you try switching power outlets? You could have a noise making appliance or fluorescent light on the same circuit.
Overhead fluorescents are a big culprit too. Check that the outlet is truly grounded with a circuit ground tester. They are dirt cheap and available at Radio Shack. Many homes have bad wiring where the electricians got lazy and didn't properly ground the outlets. That would do it too. Yes, there are tubes that are naturally quieter than others.

Hope this helps.
“Do not answer a fool according to his folly, or you will also be like him. Answer a fool as his folly deserves, that he not be wise in his own eyes. (Prov. 26:4–5).
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Re: Help with hum in AC15H1TV

Postby straleno » Tue Dec 01, 2009 3:39 pm

Hey, Epidrake, thanks for the suggestions. I have indeed covered off on most of those things, and none made a difference. Also, none of my several other tube amps hum like this one.

I guess the main question is, what specifically does V3 do in the top boost channel, and what might be causing the hum to be centered there?

I'd love to hear any other thoughts.
Thanks!
straleno
 

Re: Help with hum in AC15H1TV

Postby Epidrake » Tue Dec 01, 2009 8:56 pm

Sounds like a question for Don Butler. Hope he chimes in.
“Do not answer a fool according to his folly, or you will also be like him. Answer a fool as his folly deserves, that he not be wise in his own eyes. (Prov. 26:4–5).
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Re: Help with hum in AC15H1TV

Postby Don Butler » Tue Dec 08, 2009 3:52 pm

V3 is the Top Boost circuit. Could be a couple of culprits... Check the filter cap that goes to pin 6 on V3 and also check that all the screws are tight that attach to the chassis. I don't really like the low quality filter caps they used in this model. Could be a simple as a shitty cap. Hard to tell w/o seeing it. But, pretty normal to have some hum when turning up the bass control. Also, make sure the volume pot is off for the channel you're not using.
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