hum from aa764 champ

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phsyconoodler
Junior tube assistant
Posts: 11
(3/19/04 8:21 pm)
  hum from aa764 champ
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 Hi there!
I have a hum that comes from my champ build that is present when there is no input.I removed the 12ax7 and it is still there.i replaced the filter cap and it's still there.I rechecked every component and no luck.It comes through the speaker and doesn't change at any volume level.It's not real loud but drives the sound guy nuts.I am at a loss.I know it's in the power amp side because it's still there when the 12ax7 is removed. Any ideas?
Thanks phsyconoodler
 
Andy Poupart
Senior tube assistant
Posts: 134
(3/19/04 11:42 pm)
  Re: hum from aa764 champ
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 Champs hum. All single ended amps hum to one degree or another. If your hum is unafffected by the volume control, it could be hum introduced by the filament supply to the power tube.

Also, the way the heaters are wired in a Champ is noisy. You can fix that by ungrounding pins 7 and 9 on the power and preamp tube sockets and ungrounding the green wire from the PT that is soldered to the chassis. Run that wire to the side of the pilot light that is grounded (ungrounding it) and to pins 7 and 9 on the tube sockets. Add a pair of 100 ohm resistors from pin 2 and pin 7 from the power tube socket to pin 8.

That should reduce the hum, some.
 
Tyrannocaster
Junior tube assistant
Posts: 55
(3/20/04 7:57 am)
  Champ hum
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 "Champs hum."

Technically, this may be correct - they may hum more than a push-pull amp. But I have two Fender Champs - a '68 and a '73 and neither one puts out any more hum than any other amp I own. I also built a single-ended High Octane (AX84) which is very quiet, so I don't think his problem is the design itself. It's likely a wiring issue.
 
phsyconoodler
Junior tube assistant
Posts: 12
(3/20/04 2:13 pm)
  Re: hum from aa764 champ
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 Hi There!
I should mention that this is an AA764 champ,and it had no hum when the tubes were facing perpendicular to the controls.I changed the layout by mounting the tubes down for that 'glowing tube' look.I think it may be the layout as it did not hum before.But it doesn't look out of wack,so I think it may be the filament wiring.Any other thoughts?
Thanks...phsyconoodler
 
Andy Poupart
Senior tube assistant
Posts: 135
(3/20/04 4:13 pm)
  Re: hum from aa764 champ
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 But I have two Fender Champs - a '68 and a '73 and neither one puts out any more hum than any other amp I own.

The plural of anecdote is not "data".

I should mention that this is an AA764 champ,and it had no hum when the tubes were facing perpendicular to the controls.I changed the layout by mounting the tubes down for that 'glowing tube' look.I think it may be the layout as it did not hum before.

What? In my Champs, the tubes already point down. I don't understand what you've done. But, whatever it was, if the amp didn't hum before, and it does now, then you've done something to make it hum. Check your grounds. Consider raising the heater's ground reference above the cathode ground, all the usual things.

 
phsyconoodler
Junior tube assistant
Posts: 13
(3/21/04 6:59 pm)
  Re: hum from aa764 champ
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 Thanks for your input,and if you've never seen a champ layout with the tubes pointing upward,you have now!My tubes in the original build were straight up and the wiring is all point to point as opposed to using an eyelet board.I believe it pre-dates the builds you have.I actually re-routed the filament heater wiring and the hum changed to an acceptable level.My build also uses a can style filter cap instead of three separate caps.It seems the eyelet board versions have less problems with noise.This is my first champ build and it is still 'in progress' as far as mods and wiring are concerned.I do believe a board is much neater and easier by far.The wiring is not as neat as I'd like and that is probably where the problems are. Thanks guys!
 
Tyrannocaster
Junior tube assistant
Posts: 56
(3/22/04 8:25 am)
  thanks
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 "The plural of anecdote is not "data"."

The adjectival (or gerund, for that matter) form of "politeness" is not "patronizing". If you have a constructive comment I'm sure we'd all welcome it, though. "Data" IS the plural of "datum" and my two Champs seem legitimate examples of quiet single-ended amps. It can be done.
 
 
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