Perplexing 60 Hz hum - bad new can cap?

[sonicmojo]

Tom,

Thanks for the suggestions. Except for the leads from transformers, the heater wires are along the bottom of the chassis and bundled together from the factory. In fact, most of the wires are bundled together in this thing, very tidy, and it is mostly point to point, wired on standoffs and directly to sockets with the exception of the solid state PCB for reverb/trem circuitry. I haven't had a chance to debug more but I had chopsticked around and at least moved the transformer wires that are free to no effect. I tagged the extra 20uF cap back in before I put it in a head cab and have played it quite a bit since through different speaker cabs. Given how loud this thing is and how quickly it deafens you, the hum really doesn't register unless I really listen for it close to the speaker when not playing so this may be a case of leaving well enough alone and moving on to the next amp. I'll probably revisit again someday and if I ever swap and can prove that the CE can cap was the ultimate cause, I'll update this thread.

Thanks everyone!

[sluckey]

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.

I'm convinced that you already proved the cap can is the culprit. Bridging another e-cap across an in-circuit e-cap is a common, reliable method to troubleshoot a hummy amp. Calling it 60HZ hum rather than 120HZ hum caused a lot of comments to look elsewhere.

[Leo_Gnardo]

Given how loud this thing is and how quickly it deafens you, the hum really doesn't register unless I really listen for it close to the speaker when not playing so this may be a case of leaving well enough alone

Working in a nice quiet place, one can get too sensitive about little noises & lose focus of the signal/noise ratio which may be just fine. Happens to me . . . way too often.

Next amp!

[renshen1957]

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!

Hi,

I had a problem with a job I was called in on for a guitar which kept humming when plugged into an amp. After doing everything three times. I told him the owner the problem was within the buildings wiring. I unplugged went to another location, everything worked fine. I came back, said the guitar was okay, it was a problem with either the wiring of the building or one of the overhead fluorescent light ballasts. The owner blew his top as he had wired the building himself. I went over unplugged the nearest fluorescent light, Voilà!, the hum immediately ceased.

He didn't pay me for the extra hours time (not that I would have asked),

On the way out he complained he had a hum and this buildings PA system. I told him it was another amp on the circuit forming a ground loop, same reaction on his part, and when I unplugged the offending amp, the hum went away.

I was vindicated, but he was still the last part of the human digestive tract, and still grumpy over the matter.

Moral of story. Always try any misbehaving electronics on another circuit breaker or location. If the problem goes away, you save yourself time and needless effort, if not, you know the problem is in what you are working on.

Best regards,

Steve

[sonicmojo]

I couldn't leave well enough alone. The can cap wasn't the solution. I put an original spec Dynakit 30-20-20-20 cap in it. It is probably just a rebranded CE but still not perfect. I swapped in a different choke to rule that out as well.

If I pull the V1 preamp tube, the hum is not there, if that offers a new clue. I've tried a few other tubes and they all create the same hum. I touched up and checked everything around that tube with no help, as far as pulling each cap, testing resistor values, etc. A real hair puller this one.

It may be something buried in the reverb/trem PCB circuit. That is unfortunately part of the chain at all times. Here is the schematic if anyone has any suggestions on where to dig.

One thing to note - I had to double up the 5W 6.8K resistor after the choke to get my voltages more in line with spec. I'm getting about 503V at the standby switch and not much less after the choke.

Bryan

[renshen1957]

I couldn't leave well enough alone. The can cap wasn't the solution. I put an original spec Dynakit 30-20-20-20 cap in it. It is probably just a rebranded CE but still not perfect. I swapped in a different choke to rule that out as well.

If I pull the V1 preamp tube, the hum is not there, if that offers a new clue. I've tried a few other tubes and they all create the same hum. I touched up and checked everything around that tube with no help, as far as pulling each cap, testing resistor values, etc. A real hair puller this one.

It may be something buried in the reverb/trem PCB circuit. That is unfortunately part of the chain at all times. Here is the schematic if anyone has any suggestions on where to dig.

One thing to note - I had to double up the 5W 6.8K resistor after the choke to get my voltages more in line with spec. I'm getting about 503V at the standby switch and not much less after the choke.

Bryan

Hi Bryan,

Have you check input Jack's ground connection? Is the Jack tight? Have you checked the every component to make sure it is within specification? Have you tried a shielded cable between the input jack and the V1?

What type of grounding are you using? Random? Star? Or galactic (bunch of smaller stars going to a the main ground?

I doubt that it is the tube, probability that other the tubes would all hum?

As an aside CE Cap CANs are manufactured in the USA. The multi section can capacitors are reproduced to exact Mallory specs on the original Mallory machinery.

I never use Cap Cans unless each cap has its own ground rather than the common ground (which describes almost all cap cans). In fact I use proper grounding, decouple the nodes of each valve section with Capacitors.

Have you considered isolating printed circuit board?

Have you tried decoupling the first Valve node from the other circuit nodes?'

Well these are my suggestions, no doubt their be others.

Best regards,

Steve

[sonicmojo]

This amp is a single channel, with one single gain valve. The four input jacks are simple mono and not shunted but are wired directly to that socket which sits right behind the jacks. In fact, all four jacks are wired to the same tube pin! The signal chain up to that point is direct.

I like your suggesting of decoupling the first valve from the rest of the circuit which in this amp is just the reverb/trem effect. There is a 12AU7 for the reverb but most of the reverb/trem circuit is powered by the same AC tap off the bias and has it's own rectification. If something is going amiss down in there, it could be the source of the hum. Pulling V1 basically opens the circuit that feeds into the reverb/trem array of things so I should rule that out. I should test the 1N2070 diodes while I am at it.

Bryan

[sonicmojo]

What type of grounding are you using? Random? Star? Or galactic (bunch of smaller stars going to a the main ground?

Steve

The stock grounding scheme of the amp is best described as galactic. There are several standoffs that provide grounding on the middle lug via rivets to the chassis. Given the original dirty/oxidzed state of the chassis, I dremeled around the rivets and soldered with a large iron to be sure where I could. I get good readings to ground and from lug to lug when metering around but that's not to say that the grounding scheme couldn't be better. I don't think that is it though. I'll attempt with the dissection of the reverb/trem circuit next. I'm starting to convince myself that it is down in that part.

[sonicmojo]

SOLVED! SOLVED! SOLVED!

Like a moth to the flame, I was back at this amp again tonight trying a hum balance pot for the heater wires. No difference so I removed it.

BUT, then I found the source of the hum quite by accident just looking closer at the Volume pot which I thought might be the next possible try. The grounded lug of the Volume pot is wired to the grounded lug of the Contour pot and both lugs have separate wires to different ground points. I disconnected the extra wire and the hum is totally gone! It was a ground loop problem after all although you can say there are many opportunities for ground loops in this amp's layout. This is stock wiring since it is the same on the same other Solarus amp I had that does not hum so go figure.....

In the process, I realized the schematic that I posted does not show the ground point for the volume pot nor does it show the connection between the pots......

Thanks to all for helping me look at this. This was not a futile exercise after all.

[sonicmojo]

Here is the final restore pics for this '68 SUNN Solarus. I had a pretty crappy starting point but it was a challenge. I feel like this was worth the effort.

[martin manning]

Interesting to see it, and glad you got it solved!

[Structo]

Good job finding the ground loop.

Finding the causes of hum in an amp can be very frustrating!

[sonicmojo]

Yes it was quite the journey to find the problem. I'm sure I will tinker some more. I may have to replace sockets to get it perfect. This thing was pretty dirty and might have been a flood victim.

Here's the final pair of SUNN Solarus'es that I have now. The top one is the '68 40W one with EL34s from this thread and the bottom one is a '71 80W SS rectified with 6550s. Different power but mostly the same except for the Mid Boost switch. I have to figure out which one I'll keep. I made the missing amp badges today out of a cardstock printout and eyelet board material glued together and coated with a clear matte coat... they are impossible to obtain. Not exact, but turned out okay I think for $1.00 of material.

[Structo]

Sweet!

[sonicmojo]

Yay, the hum is back! I just swapped my homemade tremolo bug for a Vactrol bug and zip tied a few wires together to tidy up around the power transformer. I'll have to figure out what I did (if anything). I'm getting really good taking the chassis back out....
On the bright side, I just found some correct knobs for it from a busted PA control amp. Hopefully will pick them up tomorrow.