New AA964 build vibrato hum

[dwhutchens]

Hey guys,
I just completed a Blackface Princeton clone. The only mod to the circuit is the 2 .01 caps in the trem circuit were changed to .02 to slow down the tremolo speed. The chassis layout is somewhat different. I used a generic board and the filter caps are mounted on one end of the board. It all works and sounds good except there is a hum riding on the lfo signal. The hum is there with or without the guitar plugged in. It goes away when maxing out the speed pot, intensity doesn't seem to affect it. The only other thing that kills it is grounding either the second stage or the phase inverter grid. Another odd thing is grounding the circuit where the footswitch would connect sometimes caused a loud rapid popping noise, when that doesn't happen grounding that point makes it dead quiet other than the guitar pickup noise.

Any ideas appreciated.

Thanks for reading,
David

[Chuck H]

Try grounding the trem circuit elsewhere. If it's grounded at the preamp node change it to the power amp node. Of course I don't even know that you have the two separated, but you should.

[dwhutchens]

chassis pictures

Looks like my schematic didn't post, also here's some chassis pics

[dwhutchens]

Thanks Chuck, I'll give that a try tonight.

[dwhutchens]

moved the ground

Try grounding the trem circuit elsewhere. If it's grounded at the preamp node change it to the power amp node. Of course I don't even know that you have the two separated, but you should.

I moved the ground to the transformer bolt and no change, I found a mistake, I had connected the tremolo circuit to the first filter cap before the 1k resistor. I moved it to the next cap where it should be but that doesn't help. I found when I put the meter on either the plate or the cathode it's dead quiet except for the tremolo heart beat as long as my hand is on the probe???

[Chuck H]

The trem should be grounded from two places in the circuit. one at the 0V reverence for the oscillator and another at the footswitch jack. Be certain they're both grounded in the same place and NOT with any preamp grounds.

[dwhutchens]

Fixed it!!!

Thanks Chuck, that was it. I had only moved the ground for the 1meg on the grid. I forgot all about the cathode ground. I move it to the transformer bolt as well and now it's working as it should. Now the only noise i have is from the pickups on the cheap Yamaha strat clone I keep in the basement to test amps.

[Enzo]

You say the hum is riding the LFO, so you mean to say the hum is pulsing to the tempo? Or do you just mean to say the hum is there when the LFO is on?

Grounding the second stage grid stops it? Turning the volume to zero essentially grounds that grid, so does that kill the hum?


I don't see mention of trying a different tube in the #1 and #2 socket. Did you?

The trem inserts itself into the bias voltage, so the grids of the power tubes, yet you can kill the hum by grounding off the grids to the second stage or the PI, well before that point. So I have to look for something about that tube circuit itself. If the trem triode were amplifying hum, it would insert it into those power tube grids, so I am thinking we have a shared ground situation, or a tube with crosstalk.

Does pulling the first tube kill hum?

Do the volume bass and treble controls affect the sound of the hum in ANY way?

Is the hum 60Hz or 120Hz? That is important.


Touching your meter to WHICH plate and cathode quiets it? That COULD be a case of introducing a hum out of phase with the noise and they cancel.


I see a ground bus soldered to the rear of the first three pots. I cannot quite see where it is itself grounded and just what all connects. The original layout has three chassis grounds along the edge of the board. This looks different. For example it kinda looks like there is a wire from the bottom end of the cathode cap and resistor of the trem tube up to that ground bus. There LOOKS like a short wire from that cap lead over to the PI circuit bottom. But on the "real" circuit, those two points are chassis grounded at different places, while tours joins them together and they then share a wire to the buss and from there on back to chassis. Just where does that bus get grounded to chassis?

[Enzo]

Oh darn, I am busy writing, and you guys get it fixed.

Well way to go then.

[dwhutchens]

amp pics

Hey guys,
Thanks again for the help, here's some pics of the finished amp.

[Chuck H]

Dats reel purty.

Is the wood crossed over the speaker (visible because of a flash?) there for beam blocking or just speaker protection?

[dwhutchens]

Thanks

Dats reel purty.

Is the wood crossed over the speaker (visible because of a flash?) there for beam blocking or just speaker protection?

I though I'd try it as a beam blocker, the first amp I built will pierce your eardrums if your directly in front of it. I did it on this 5F2a clone for appearance.

[dwhutchens]

That built in beam blocker might not have been such a good idea. I've been playing it a couple of weeks and noticed some rattling in the cabinet. I put a frequency generator through it and at certain frequencies it seems the speaker cone may be hitting the baffle board. I put plywood spacer ring between the speaker and the baffle and so far it seems to be cured.

[Chuck H]

Beam blocker type baffles were common. So were ring spacers You're just catching up on old technology.

[überfuzz]

Looks nice! Your little white beast will be visible from way back.