Sunn 1200s hum

[tdlunsfo]

I have a read quite a few posts about issues with Sunn amps and hum. Granted the grounding on these amps are all over the place and its throwing me for a loop (pun intended). Curious of what you guys have found in the past on these, or similar, amps.

Here are my symptoms:

• Hum stops when PI tube (6an8) is pulled
• All electrolytic's replaced
• 12AX7 and 12AU7 when pulled do not make a difference
• Inputs are not the grounding type but grounding them does not make a difference
• Grounding the riveted grounds with gator clips doesn't make a difference.
• Scope shows noise out of the reverb section but it rides the noise/buzz already present
• Running two 5AR4s as the amp was purchased with a bad diode recitifier. I had two 5AR4's laying around and wired up the second otherwise empty tube socket.
• Bias has been reset to the -55v per the schematic
• Death cap removed.
• 3 prong cord in place.

I was thinking of lifting the wires to pin 8 of the 6AN8 to see if I can narrow down where this is coming from. The gain stage of the 6AN8 with the mid boost prior to the Cathodyne is confusing. I really don't understand the purpose of the dual voltage feeds from B and C.

[tdlunsfo]

Here are a few scope shots of the speaker output. First is with volume at 0 and the second with volume about 1.5. Scope set on AC, internal trigger, 10mv and 5ms.

[tdlunsfo]

I've been looking at this and it seems I have a pretty nasty signal at pin2/6 of the phase inverter. My power supply ripple looks good though. Tracing parts and such around this PI (rats nest), I discovered that my pin 8 lacks the 1M resistor from grid to ground. Every Sunn schematic from 1970 shows this resistor. Could this be causing some kind of blocking distortion on the plate pin6? Any sunn 1200s owners ever find this resistor missing? Listed as R31 on 1200s schematic found on Google images.

[Enzo]

Do you have a 1 meg resistor? TAck it in and find out. That would be a lot faster than us pondering it.

[tdlunsfo]

Beat you to it. Well.. tried it with an aligator clip anyways. Didnt make a difference. Looks like I need to get another PI next to rule that out.

[tdlunsfo]

I ended up figuring this out. One of my bias caps was not properly grounded and was injecting hum into the PI and 6550s. Wanted to post what I found in case anyone comes across this kind of hum and waveform.

[Jazz P Bass]

Thanks for finalizing the repair.

[nevetslab]

After having restored the original grounding (thru the terminal strip rivets), the biggest source of hum I had was from the half-wave rectifier power supply feeding the Tremolo circuit. On the 1200S Restoration project I completed in May 2016, it took additional supply filter caps to drop that hum level down to what I was getting when I powered that circuit externally.

I've attached the revised 1200S schematic, re-drawn so it's far more readable, and has additional refinements to the original 1200S.

Sunn 1200S Guitar Amp-2.pdf

[tdlunsfo]

Awesome job. My hum ended up being an error on my part but even after fixing that it's still has a little more hum than I am used to. I think my issue is poorly matched output tubes. I am still trying to utilize the original tung sol 6550s and they are not matched very well. I just can't bring myself to replace them.

[Jazz P Bass]

My 2 cents.

Measure the hum at the speaker jack.
Meter set to read Volts ac.
No input signal.

4 mvs is ideal.
10 & above, you have an issue.