[jimbop]

I am trying to reduce/eliminate an annoying hum in my 2A3 amps. Looking for some help.

Jim D

[Niteshade]

Some more information is required:

1. Do they hum without a source connected to them? Do they hum if the inputs are shorted to ground?
2. Do they hum when only the 2A3 & rectifier tube (if it has one) are plugged in?

Somebody else on here is going through something similar.

[mgalusha]

Jim,

If we can't sort this out using the questions that Blair is asking and others I'd be willing to take poke at them with the scope. Do you have the schematic? Not that SET's have many parts but knowing what the designer intended for voltages at various places in the circuit can be very helpful.

Along with Blair's questions, do they have a hum balance pot? If so, does it make any appreciable difference? I would certainly try a shorting plug on the input first to eliminate a ground loop as the cause.

Both amps or one? If it's both then a ground loop of some type is pretty likely.

Mike

[jimbop]

The amps are George Wright's WPA-3.5 (2A3) monoblocks.

Both amps hum:
- With or without a source connected
- With or without cheater plugs on the power cords of all components in various combinations
 - With or without the 6SN7 driver tubes installed

Hum is 16mV and 23mV, measured at the output jacks

There are no hum pots.

There is no schematic available (other than the rough draft I've drawn up)

You can see further questions and replies at AudioAsylum, in the SET Amp forum, under the heading of "Hum in 2A3 Amps"

I've had these amps for several years, and they've always hummed. I've tried various things to minimize the hum, with little success. I've had some free time lately, and a renewed interest in fixing these things.

Any help would be appreciated.

Thanks,

Jim Dombrowski
Colorado Springs

[Niteshade]

There is one more test to try and it will sound strange: Take all the tubes out and then turn it on. If it hums with no B+ (it would be much lower) then there is EMI being communicated from power to output transformer. It's rare, but can happen and should be checked.

(NOTE!) Discharge capacitors after trying the above experiment, or re-install tubes, let them get to operating temps & then turn off again. That helps reduce lethal voltages.

If it's not that, then it's your filament supply.  Low voltage, high current DC isn't nice to work with. If it's AC, you may want to install a hum balance pot.  At this point it's hard to move forward without a schematic or seeing them.

Another note: Are they DC biased? If so- the bias voltage might have ripple. Check the caps and (again) for stupid mods or anything else that may not make sense.

Have any mods been made to them?  If there were mods done to the filament supply, remove them- unless they help.

[jimbop]

Yes, there were mods done to these amps by the original owner, but they were primarily in the way of component upgrades - not circuit changes. I verified this last year by comparing them to a pair of stock amps that a generous soul lent me for this purpose. As I recall, the stock amps hummed also, but not as much.

I'd like to figure out how to make these amps as quiet as possible.

[mgalusha]

Jim,

I read the thread on AA and there are plenty of good suggestions. It sounds like you are somewhat comfortable looking around inside the amps. Have you tried drawing up a schematic for them? While it would be great if Georges family will release one who knows if or when that may occur.

I live east of Parker and North of Elizabeth, so the drive from CS isn't terrible since I'm east of Denver but still quite a trek. I don't own any SET amps but have a fair knowledge of how circuits work and if you like we could poke around and take some measurements and draw up a schematic if you can't scare one up. I'm guessing that with good measurements and perhaps some scope photos the folks on AA could set you straight if we can't figure them out.

mike

[jimbop]

Mike,

I would love to come up sometime to go over these amps with you. I have a rough draft of a schematic, that just needs to be verified by someone with more knowledge than me.

When are the best times for you?

You can contact me directly at: jimmyjohn54@comcast.net

Thanks!

Jim

[mgalusha]

Email sent.