Load 120Hz hum in Ampeg B12N: running out of ideas

[Ptron]

Sorry for another humming problem thread but I could use some suggestions. The amp in question is a 1961 Ampeg B-12N portaflex with some pretty excessive 120Hz hum. It's worse on channel 1 but both do it. looking with a scope, the power supply rails are clean but the plates of the preamp tubes have significant 120Hz sawtooth on them. Like a volt or two on stage 2. The filter caps are pretty new. I replaced the corroded input jacks and isolated them from the chassis and installed ground wires. I did the same for the ch1 tone control ground. The volume control was never grounded to the chassis. I also isolated the ext amp jack. I cleaned the connection between circuit and chassis ground. I looked for obvious ground loops or bad connections. I switched the preamp tubes around. I poked around for bad components or connections. It seems like it's gotta be a ground thing somewhere but I can't seem to find it. Any ideas?

The schematic for this thing is a bit different than the oldest one I found online (1962), epecially in the simplicity of the Bandaxall tone control, but it's pretty close.

[stokes]

Are you sure it is 120HZ hum?When the 60 cycle AC gets rectified it becomes 120 cycle (HZ) pulsating DC,this is what the filter caps smooth out.If it is not the filter cap,sometimes coupling caps can become conductive draining current from the supply causing the filter cap to become less effewctive.Check the voltage on the plates of the preamp tubes and see if any of them are noticably low,if so the coupling cap from that plate is bad.I found that on an old Ampeg Gemini II once.

[Ptron]

Thanks for the reply.

For both channels stage 1 reads ~115V and stage 2 ~150V. Since supply voltage is something over 300 I suppose that might be a little low for stage 1. It is the same on both channels, but then again both channels are buzzing. I'll try replacing the coupling caps and see what happens.

Are you sure it is 120HZ hum?When the 60 cycle AC gets rectified it becomes 120 cycle (HZ) pulsating DC,this is what the filter caps smooth out.

Yeah it's 120Hz, or that's what it looks like. Not raw rectified AC but the sawtooth shape of rectified and filtered AC.

[stokes]

According to the scematic I have it should be 180v on both halves of the input tube.I know you wont always see what the schem shows but they both are low.I would also check the next tube in each channel-the schem shows 220v on that tube.If you say you are sure the filter caps are good,then I would say the coupling caps need to be changed.

[Enzo]

You report isolating jacks. Does that mean you have UNgrounded things that used to be grounded? Your new grounding scheme might be less cool than the old.

[Ptron]

I've "isolated" (but not solved) the problem

Could this really be a capacitive thing with the input jacks? I changed most of the coupling caps. It did bring up plate voltages anywhere from ten to thirty volts but I can't say it made a big difference in hum. I disconnected every single circuit ground that used the chassis and wired them to the most appropriate spot on the eyelet board. I now have the quietest '61 B-15 you ever heard except...

... when I tighten down the nut on one of the ch1 input jacks, there's that nasty hum again. I keep taking the jacks out to see if they're contacting the chassis, but my isolation looks intact. Just loosening the nut a bit makes it go away. First, it was doing it on one jack while the other seemed ok. Now it's switched. I checked wires, reflowed solder joints, it still hapens.

Some possible important factors:
Jack isolation technique - So that I don't have to drill out the chassis, I wrap a strip of electrical tape around the threads. I then use two non-shoulder isolating washers with the holes enlarged so that they will slide easily over the electrical tape. This technique has always worked for me before.

Input circuit - I replaced both ch1 jacks with shorting jacks. Instead of using the original scheme with the 5.6M input resistor, I put 1M resistors from tip to ground ala Fender/Marshall.

[Ptron]

Didn't see your post, Enzo. I grounded the jacks to the same place as the 1st stage 2.2k cathode resistor.

[stokes]

When you tighten the nut on the jack it is most likely causing some contact at your jack.The jacks on that amp are supposed to be grounded at the jack itself.If you have 120cycle hum it has to be originating in the power rail-it is the only place 120hz can come from,it is rectified 60 cycle.I suspect you cured the 120hz hum with the cap change but now you introduced a ground loop by changing the ground scheme.You said you now have the quietestB-15 till you tighten the nut on the jack.Like Enzo said your new grounding scheme is less cool.I would go back to the original,including the change you made with the 1meg and 5.6m resistors.

[Ptron]

When you tighten the nut on the jack it is most likely causing some contact at your jack

Yeah, I guess that pretty much has to be it. It's just weird that when I take it apart and examine it, I can't see where or how it would be contacting the chassis.

I have to disagree about switching back to the original grounding scheme. As long as I do it right (i.e. don't have the jacks serruptitiously touching the chassis ) , grounding them to same spot as the 1st stage cathode resistor should be an improvement, especialy since the 45 year old chassis has seen it's better days and damp basements. I've quieted down several old amps this way. I will put back the original resistors.

There were probably several sources of hum/buzz in this amp. Replacing the caps probably helped, but what really shut this amp up is when I removed all circuit grounds from the chassis and grounded them to the most apprpriate spot in the circuit.

[stokes]

New grounds will quiet ground induced hum,but the 120 cycle hum can only come from one place.If changing the coupling caps brought the voltage up 10-30 volts that was the source of your 120HZ hum.Those caps are meant to block dc voltage,when they go bad they either leak or become conductive.If they were leaky they would allow dc volts to get to the grid of the next stage,when they become conductive they draw current from the preceding plate,that is why the voltage is low at that plate.An amp stored in a damp basement would do more damage to the components than the chassis,the chassis would only get rusty-easily remedied with a piece of sandpaper,caps and resistors,especially carbon comp type absorb moisture and can make all sorts of noise.You say the noise now appears when you tighten the nut,that is why I suggested going back to the original ground scheme.By changing the 5.6meg grid resistor to 1meg puts your input signal much closer to ground,leaking signal to ground but could also be allowing any current in the chassis to leak back into the signal.By the way are we talking about a B-12 or 15,you made reference to both in 2 different posts here.Any way I would try putting that 5.6m back that could be the cause and perhaps your jacks are isolated okay.