Perplexing 60 Hz hum - bad new can cap?

[sonicmojo]

Hello all,

I am restoring a beat up Sunn Solarus and while I have everything working again, I am chasing down a minor pesky hum. It is a 60Hz background hum, not very loud, that is not affected by the overall volume of the amp. I have a brand new can cap in it. If I place a parallel cap to the filter supply node of the can caps (right before where the choke is inserted), about 80% of the hum goes away. The amp has all new transformers and choke, same as original specs. I'm suspecting the can cap itself but I'd be surprised if that is the problem since it is brand new. Does anyone have any other suggestions I can check before I go and replace it?

Thanks!

[David Root]

60Hz, I would suspect a ground issue somewhere, dry joint maybe?
Might be a ground loop but if that I wouldn't think the second cap in parallel would make any difference.

[sonicmojo]

60Hz, I would suspect a ground issue somewhere, dry joint maybe?
Might be a ground loop but if that I wouldn't think the second cap in parallel would make any difference.

I've been searching high and low and I've touched up every solder joint in the amp. If I stand back 5 feet from my "bench test" speaker which is just a 10" EV, I can't hear it. Maybe I'm overthinking this but I think there is something to find. I think I have to rule out the can cap before I keep searching otherwise. Instead of just going parallel, I tried bypassing that first node completely with a separate cap and it is definitely the same less hum. The can cap is not doing as good a job as a separate cap. But there could be something else going on....

Bryan

[Firestorm]

What kind of can cap? Some of them suck.

[sonicmojo]

CE Manufacturing 40/40/40 (525) from Antique Electronic Supply.

[David Root]

Those have a history of problems. The old Mallory twist lock caps.

I have used a couple without problems but they are made on the original Mallory machines I believe, which have to be at least 30 years old now. Something that old can be hard to maintain. They're also very expensive for what they are.

[V2]

60Hz hum wouldn't be coming from the power supply filter caps; those would give you 120Hz 'buzz'.

If it is truly hum, try hooking up your heaters to a 6V lantern battery to see if the hum disappears. If it does, its a problem with your heaters.

Do you have a scope?

[Stevem]

My stock Sonaro with all new, yet beefed up filtering has the same minor issue and it's the stock grounding layout!
Install a star grounding layout in at least the preamp section and make sure all the grid wires are lifted up off of the heater wires and all should be well.

You also need to confirm that the input Jack shorting switch is working and another really big thing is to have the amp / chassis bolted down to its steel plate in the its cabinet slot!

[bal704]

Does the hum change if you swap/replace tubes?

[billyz]

Check the 7199. Test the film caps connected to cathode of the pentode section. If they test good, then try elevating the filament supply with a voltage divider off the the last preamp supply node. 1/10 ratio should get you about 25 v , connect to the filament center tap . Also bypass the voltage divider resistor that is grounded with a film cap to further eliminate the last residual buzz.

[Leo_Gnardo]

Some good suggestions from stevem and Billyz. I'll add, wire dress. Sunn routed their main B+ wire very close to the pre tubes in some I've seen, and it radiates buzz into nearby signal wiring. I detach the buzz transmitter wire, replace it with a fresh piece of hi voltage rated wire & direct B+ where it needs to go, not some circuitous route. You can remove the offending, now detached wire, or solder one end to a chassis ground & leave it in place. Good amps, them ol' Sunns!

[sonicmojo]

I only have one 7199 at the moment but an identical Solarus running with it does not have the same nominal hum. With everything except power tubes pulled, I can still detect the same small residual hum with my ear to the speaker. Everything else being added increases this a little bit, but primarily the V1 main preamp tube and that just boosts the signal that is already there so leads me to believe it is pre-signal chain. V2 is just for reverb and does not make any difference at all as far as I can tell. I'll report back after I get a chance to swap the can cap. I have another CE with slightly different values that would be a good test to rule it out. Thanks for all the suggestions. Hopefully I don't have to go down any crazy paths to eliminate the issue.

[Stevem]

One thing you can do in regards to improving the stock hum level since these amps gave a choke right after the Recto is to kick up the first filter node to 60 uf.

[sonicmojo]

My stock Sonaro with all new, yet beefed up filtering has the same minor issue and it's the stock grounding layout!
Install a star grounding layout in at least the preamp section and make sure all the grid wires are lifted up off of the heater wires and all should be well.

You also need to confirm that the input Jack shorting switch is working and another really big thing is to have the amp / chassis bolted down to its steel plate in the its cabinet slot!

Thanks for the Sonaro comparison. I guess what is driving me on this is that I have an identical comparison with no hum. Mine has all new transformers and caps. Once again, it is minor and acceptable for the most part but I need something to fix. On this amp the input jacks are simple mono and no shorting so all unused inputs are wide open but doesn't seem to be a problem. I have to build a new cab for it still

[Structo]

I'm sure you have probably tried it but have you moved things around with a chopstick?

Lead dress?

60Hz is usually heater related.

How are the heaters wired?