What does this scope signal look like?

beasleybodyshop · Post by **beasleybodyshop** » Thu Sep 11, 2014 4:04 am

Ok. so i attach a jumper to pin 8 to ground. hum doesnt go away.

I jumper point H, hum is still there.

I jumper point N (pin 2 of V3) still humming.

YouTube · Post by **xtian** » Thu Sep 11, 2014 4:20 am

Pin 8 of the 6AN8A?!? Stop there. Hold the presses. You're telling me if you ground the grid on this tube, the hum stays, but if you lift one leg of C21 the hum vanishes?

I don't see how that could work.

beasleybodyshop · Post by **beasleybodyshop** » Thu Sep 11, 2014 4:20 am

Ok so I did some more work here. Attached are two scope shots I took from the amp output into my dummy load. First one is the signal output with the preamp disconnected from the PI (pin

. Second shot is when I ground Pin 8 (pre still disconnected). This is coincidentally the same waveform I get when I connect everything back to the preamp like normal.

martin manning · Post by **martin manning** » Thu Sep 11, 2014 12:31 pm

Are you sure the FWB rectifier is good? You can't test it in-circuit.

beasleybodyshop · Post by **beasleybodyshop** » Thu Sep 11, 2014 12:34 pm

martin manning wrote:Are you sure the FWB rectifier is good? You can't test it in-circuit.

I replaced all 4 rectifier diodes already with no change.

beasleybodyshop · Post by **beasleybodyshop** » Mon Sep 15, 2014 2:07 am

UPDATE:

No more hum!

I had originally moved all the filter cap grounds to one central spot on the amp - apparently, the main OT filter cap isnt actually grounded, its referenced across the separate screen winding as labeled in the schematic. Im betting it was specifically placed there because in a normal grounding scheme it was creating that nasty 120Hz hum.

I moved it back to where it was supposed to go, and now it doesnt hum anymore!

I have been beating myself over the head for over a week on this thing. Thank you everyone for the advice you gave me on this sumbitch. Now where is my Jim Beam...

Reeltarded · Post by **Reeltarded** » Mon Sep 15, 2014 2:20 am

Good deal!! That feels great to be done with, huh?

Phew!

beasleybodyshop · Post by **beasleybodyshop** » Mon Sep 15, 2014 2:25 am

Reeltarded wrote:Good deal!! That feels great to be done with, huh?

Phew!

Oh man, it does. I almost passed out i was so damn happy. Now im gonna burn it in on my bench for the next 2 hours to ensure nothing blows up.

Oh man it feels so good. But that might be the bourbon yee yee!

eniam rognab · Post by **eniam rognab** » Mon Sep 15, 2014 3:45 am

you definitely need an adult beverage after all that headache!! congrats buddy!!!

martin manning · Post by **martin manning** » Mon Sep 15, 2014 11:03 am

You actually have two levels of ground there. The main reservoir caps, the screen supply, and the bias supply have a common ground, but D206 is between that and the chassis so the power section is about 0.7V below chassis ground. Mixing them together is what got you in trouble. Those Ampeg engineers could be creative at times.

dorrisant · Post by **dorrisant** » Mon Sep 15, 2014 11:18 am

Glad to hear you got it sorted out.

Tony

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What does this scope signal look like?

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