120 Hz Hum in Bassman Clone

[SBax]

I have a 5F6a Bassman power section with a hot rodded Marshall preamp homebrew. While it was on, but not being played through, a slight hum was slowly building up until it got loud enough for me to shut it down. New, good tubes didn't help. Pulled preamp tubes one by one until it stopped when the PI was pulled out. Dead quiet with the two preamp and PI tube out.

Checked the recorded hum with a frequency analyzer and it shows 120 Hz and harmonics, no 60 Hz. Hum gets much louder when bias pot is raised. Guitar signal still seems ok on top of the loud hum now that the bias is set to minimum. Quick check of power tube voltages are ok.

When I turn off the standby there is a long funny sucking squeaky sound with a rising tone.

Before I dive in with a big project of rebuilding the entire power section, I would like to find the offending power cap as a short cut Any ideas how to narrow this down to one cap? Thanks much.

[Enzo]

Who says it is a bad cap?

120Hz means power supply ripple, but I suspect the real culprit is ripple return currents in your grounds. If any of the preamp tube grounds share a ground path with the filter cap grounds, then the ripple currents will impress on the preamp supply.

[robrob]

Since it builds in volume it's feeding back into itself in the preamp and/or phase inverter.

Does the hum occur when the volume is at min? Does turning up the volume higher make the hum build faster?

[mhuss]

Bias ok? The sound you're describing I've heard when a push-pull amp starts red-plating.

[SBax]

Some history: The build is 10 years old. 2 years ago I was running full out and the speaker wire fell off from a bad solder connection. This over heated and melted and shorted power section wires and they wielded to the chassis. I re-built the entire power section and bias supply. Everything was good for 2 years.

Last week the amp was on, standby on, no guitar pugged in. I heard a slight hum and it slowly got louder. In 10 minutes it got so loud I shut it down.

The hum is not affected by the tone or volume controls except for the presence pot.
When I turn up the vol and master vol all the way, the chassis is microphonic. A guitar signal rides of on top of the hum and sounds normal.
I changed all the tubes and checked the bias and it was very high at 60 mA and turned down the bias pot all the way down to 25 mA (each tube) and the hum went a lot lower. Both power tubes show the same bias at about 25 mA. Both plate voltages were normal at about 450. Pin 5 gave a negative voltage that looked normal, I forget, maybe -50?

I will re-evaluate the bias supply again. Maybe look for AC on the bias voltage? Forget that, it's not 60 Hz hum.

[g1]

I will re-evaluate the bias supply again. Maybe look for AC on the bias voltage? Forget that, it's not 60 Hz hum.

Check it for AC anyway.

As for the main filter caps, you can try adding a known good one in parallel with the existing caps one at a time. If adding the good one in parallel makes a big difference for any one cap in particular, then you have found the bad one.

[robrob]

I agree with mhuss above, have you watched the power tubes power up in a dark room? Redplating power tubes will bring on an increasing hum even with no input signal.

[Enzo]

Don't get in your own way. You report the hum is not affected by volume controls, so leave them at zero. turning them up all the way and finding microphonics does nothing to help us cure the hum, it just adds more symptoms to the mess.

Unbalance on the power tubes, possibly one red plating (or two) is worth checking, turn the lights out.

Monitor tube current while the hum grows, is tube current ramping up on one or more of the tubes?

[SBax]

Thanks guys.

No red plating in a dark room.

The growing hum only happened the very first time. The hum has been steady since except when the bias pot is raised.

The bias pot is all the way down now with 23 and 18 mA on my bias probe. Pin 5 is reading -60V both tubes. I am also measuring 0.35 V AC at both Pin 5.

When I raise the bias pot half way. I get 3.5V AC on Pin 5 therefore I have AC in my bias circuit, correct?

Now I need to find the source. What component would have failed slowly over 10 min when this started? Must be one of the two 10 mF caps? Diode?

[Enzo]

3v of AC? If you suspect the bias is hummy, then go back to the bias supply. Does that same 3v (or more)of AC appear on the bias supply itself under these conditins.? When you get that 3vAC by turning up the bias, what has the tube current risen to? And has the ripple on the B+ increased? Remember, we are dealing with circuits, not parts.

You mentioned that pulling the PI killed the hum. is that still the case? Does raising the bias current with no PI still increase the grid AC voltage?

Since you have a steady 3vAC on those grids, it should not be hard to track down where it comes from.

[SBax]

The blue circles have AC, the green circles show no AC.



That that means the B+ and bias circuits are clean. Not sure how to work backwards on this.

[g1]

Then it would seem the hum is coming through from the PI.
Do you have a scope?

[SBax]

No, I don't have a scope. My instinct tells me a component failed instead of a short because the hum built up from a very slight hum to a massive hum over a period of 10 minutes and stayed that way.

BTW, on the PI tube, Pin 1 has 3.2V AC and Pin 6 has 0.57V AC. So, it's not even.

[g1]

A solder connection could also go bad over a time period like that.
Hypotheses are fine, but don't let them limit your troubleshooting.
See if you can track that AC backwards from the PI plates.

[SBax]

Still cannot find the source of the AC.

I put a different PI tube in and measured over 5V AC on Pin 1. How could a different PI tube give higher AC voltage? The other PI tube gave 3.2V.

I reflowed the heater connections to the PI socket and checked for any shorts. I traced the all the heater lines with a magnifying glass and saw no shorts or funny stuff.