Ampeg B25 restoration - Bad Hum

[qeopmizwal]

What things have you done so far to try and cure your hum issue?

First let me describe the hum: 60Hz with Volume of both channels all the way down, turning up the volume does not increase the hum, its just always there.

Well so far I have changed all the tubes including the rectifier, this had no effect. So then I changed the power supply filter caps, and a few of the power resistors as well (The amp has a 3 wire AC plug with the earth ground connected to the chassis). Still had hum, so I rewired the grounds for most of the amp, kind of star grounded but without isolating the jacks and pots, this seemed to help a very small amount if any at all. To hlep debug I have taken out the two preamp tubes, turned on the amp and still get the hum. I have twisted the heater wires going to both of the power tubes, still hum.

I was thinking of re-running the ground wires in a different configuration, kind of isolating each ground stage in the system with a separate connection to the chassis. Some of the hookup wire in the amp I think is from the original build, maybe I should use new wire?

Also another thought was this: This amp has a circuit board. So the heater voltage going to the preamp and phase inverter tubes travels on the board, maybe it is causing noise on a signal trace somewhere?
Maybe I should cut the heater traces and run twisted pairs to the preamp and PI tubes...

[Phil_S]

... So the heater voltage going to the preamp and phase inverter tubes travels on the board, maybe it is causing noise on a signal trace somewhere?
Maybe I should cut the heater traces and run twisted pairs to the preamp and PI tubes...

I doubt there is another source of 60Hz. Just unhook the filament supply from the PCB and run twisted pair if you can.

[Stevem]

Bias up the center lug of the hum balance pot with about 50 volts of B+.
Pull if off the last preamp tube power supply node and thru a dropping resistor and another filter cap (16 uf at 400 is fine) to un couple it from the preamp.
One other thing to check since you have this hum with just the outputs in would be to confirm that pin 2 goes to pin 2 and that 7 goes to 7, if not the output stage will not cancel out hum!

[qeopmizwal]

Bias up the center lug of the hum balance pot with about 50 volts of B+.
Pull if off the last preamp tube power supply node and thru a dropping resistor and another filter cap (16 uf at 400 is fine) to un couple it from the preamp.
One other thing to check since you have this hum with just the outputs in would be to confirm that pin 2 goes to pin 2 and that 7 goes to 7, if not the output stage will not cancel out hum!

So you are saying rather than grounding the center lug, give it 50VDC? This pot is attached to the chassis and I assume the can of the pot is connected to the center lug, so I would be putting 50VDC on the chassis which has other ground connected to it. So I would have to isolate the pot from the chassis if I do this. Is there an easier way to ad 50VDC to the heater lines or maybe you didn't mean literally the center lug of my hum balance pot? I want to do this because from what I have read elsewhere it seems like this is my problem, but I want to make sure I do it right.

[Reeltarded]

No lugs are ground.

[ampgeek]

Kind of like what is shown in the attached.

The "Heater Reference" connection goes to the center lug of your hum balance pot.

The 220K:22K voltage divider should be moved down the PS string to the "D" location shown in the schematic early on in this thread.

Yes! Do make sure that none of the hum balance pot lugs are connected to ground. That is often done by bending up and soldering one of the pot lugs to the pot body. Undo that condition if present.

Good luck!
Dave O.

[qeopmizwal]

Kind of like what is shown in the attached.

The "Heater Reference" connection goes to the center lug of your hum balance pot.

The 220K:22K voltage divider should be moved down the PS string to the "D" location shown in the schematic early on in this thread.

Yes! Do make sure that none of the hum balance pot lugs are connected to ground. That is often done by bending up and soldering one of the pot lugs to the pot body. Undo that condition if present.

Good luck!
Dave O.

What if I did a 1M:100K divider, that way I could use 1/2W?

[martin manning]

You could do that but it will take a long time to charge a 47uF cap through a 1M.

10uF is big enough for the cap, and if you don't have any higher wattage resistors handy, I would parallel two 470k 1/2W for the upper resistor and use a 47k 1/2W for the lower. Adjust the 47k to get ~50V out.