Ampeg SB12 Hum

[jlonigro]

My SB12 has had all caps replaced. New output tubes installed and all others check strong. Power transformer was replaced by previous owner. When I pull the phase splitter the hum goes away. When I wiggle the yellow wires going to pins 2 & 8 of the 5AR4 the hums gets worse. A tech replaced the power plug with a polarized one. He had the narrow blade going to the white wire. I thought this might be wrong so I swapped them so the black was the narrow blade and going to the fuse. This didn't seem to make any difference. Any thoughts?

[Mars Amp Repair]

Hello,
I'd first tighten the pin holders in the rect socket. I'm thinking that effect is not really part of the hum issue.

Is this one of the variety with the circuit board or turret strip?

One common issue with some of the portaflex's with the turret strip is that when the cap-can is replaced, a wire gets added from the can tab to ground. The can is mounted on a phenolic insulator to keep the can from contacting the chassis directly at that point. This creates a ground loop. The can is supposed to be grounded through all the other ground paths up the turret strip & finally chassis grounded at the input jack.

I don't recall if there is a hum when the phase inverter tube is removed tho. Just thought I'd throw this in.

see if the can has only one connection to the can tab that goes off to the turret strip, or if someone has added another wire that goes directly to the chassis.

Also if the power tubes are grossly mismatched or the bias is set way hot, this also can create a hum. glen

[jlonigro]

This is a turret board configuration.

I was thinking the pins needed to be re-tensioned on the Rectifier as well. There seems to be much play in the socket and when I gently wiggle this the hum goes from bad to worse.

The can caps are clamped into the turret board and there is only one lead from each going to ground of the turret board.

I tried to adjust the bias pot a little bit to see if that made any difference but it didn't seem to. I'm not exactly sure the best way to test the bias on this amp.

Here are some pictures inside.

[jlonigro]

I also noticed in my amp that the white AC wire is going to the on/off switch. Is that correct? I thought it should be the opposite with the (hot) black wire going to the on/off and white(neutral) going to the fuse. The 2 prong polarized receptacle has the black wire going to the small blade and white to the larger blade.

[Mars Amp Repair]

I see now, no cap-can in this amp.
Try putting the PI tube back in & pull out the next one back from the PI to try & determine if the hum is getting being injected at some particular stage.

If the PI is the only tube that reduces the hum, that really only proves that the rawest of power supplies is clean enough & not causing the hum.

Also with all the tubes in place, doe the hum vary with the volume control or other tone controls. All of this will help isolate where the hum is coming from.

Also, is the noise more of a 'hum' or a 'buzz'. Buzz like a bad guitar cord, can be more of a signal issue that power supply issue. Really hard to tell not hearing it.

glen

[jlonigro]

Thanks so much for this Glen. I really appreciate your help.

First I did re-tension the 5AR4 and that helped with the varying noise while wiggling the wires going to it.

I pulled the first 12AX7 and that did not eliminate the hum. The PI seems to be the only tube that completely eliminates the hum.

If I turn the volume all the way down as well as the bass and treble pots the hum is present. If I turn any of them up, the hum doesn't necessarily get louder it subtly changes in tone.

I can record the hum and post it if that would be helpful.

I tried varying the bias pot and that did not seem to affect the amp in any way. Right the now the cathodes of the 7868's are going to two different legs of the balance pot with the center leg of the pot going to ground. It states 100ohms on the schematic. Can I read the bias through that or would I have to solder a 10 or 100ohm resistor to the cathode and ground in order to get an accurate reading of the bias?

[jlonigro]

Well I guess you can say it does get louder when I turn up the volume, treble, or Bass pots.
Any further advice on what to look for?

[Enzo]

Any control that affects the hum sound is after the source of that sound. That helps isolate the problem to a certain part of hte ccircuit.

[jlonigro]

That has become clear t me now. It seems it should be around the PI circuit or tone controls. I'm just not sure what to look for (voltages, out of tolerance resistors, ect.)

[jlonigro]

There's not much before the volume pot. Are you saying the source has to be before the volume pot? What should I look into?

[Enzo]

Not much before the volume pot? There is a whole tube stage, plus the wiring to the input jacks and the jacks themselves. And yes, conceivably the pot itself could be involved. Make sure the pot body is grounded to chassis.

If the source of the hum was AFTER the control, how would the control have any effect on it? If the volume control can turn the hum up and down, then the hum entered the system before that control.

[jlonigro]

I was thinking that because I tries pulling as well as swapping V5 the first 12AX7 stage and nothing changed that it would be before that. It does indeed sound like a ground issue. I will check out the jacks and volume pot.
Thanks for your help.

[jlonigro]

Here is a sound sample of the hum I am hearing. It does appear to me like a 60Hz hum. I checked all resistor values of everything before the volume pot and they all meet specs. I am really at a loss as to what else I should check.

The recording is an AKG D12E through a Neve 1272pre into PT. Peak level at
-8.53 and average at -13.7RMS. Just to give an indication that the recording wasn't set exceptionally high in order to hear the noise from the amp. My vintage Fender Twin and Champ display any noise at these settings.

Also when volume bass and treble are completely turned down the hum is present. When they are turned up the hum gets louder.

[oc disorder]

Geez with the AKG,Neve and PT couldn't you buy another one...?
Just jealous....um far from me to say the Emperor is naked but with the
computer monitors I've got couldn't detect the hum only crosstalk...
One observation though , the pictures show only one 20uF cap the diagram shows 3. Also just because they are new doesn't mean they are without fault. I would try piggy backing (hope thats not an offensive term) another 20 to 100uF 450v after the 22k droping resistor and see if there is any improvement. Of course be careful.. some big voltages there .. discharge with a resistor to ground or leave it off for a long while .

[jlonigro]

The black can cap next to the blue 20uF is a dual 33uF450V. It also is new. What exactly do you mean by "piggy back?" What would this determine?
I am perplexed as to why when the PI is pulled it is dead quite. It must be before that stage correct?