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Sunn Studio PA - Flawed Design (MAJOR Hum issue)

1/15/2006 5:38 PM
Bob Clear
Sunn Studio PA - Flawed Design (MAJOR Hum issue)
I recently acquired a '67-ish Sunn Studio PA head. This is the two-tube (60W) version of the Coliseum PA as used by Leslie West on "Mississippi Queen". It's essentially the standard Sunn 60W chassis with one 12AX7, one 6AN8, two 6550s, and a 5AR4 - Dynaco Mark III style - with the addition of two extra 12AX7s for mic preamps. It sounds HUGE with P90s or humbuckers, way more gainy than something like a 200S.  
The problem is it hums like a mutha. I've replaced all the electrolytics and checked all the coupling caps for leakage. I've tried about every grounding scheme I can devise (I'm pretty sure it's a ground loop issue). No go.  
The schematic for the Coliseum is here:  
This amp is 99% identical except for the number of output tubes. There are some component differences - the mic volume pots in mine are 1M rather than 500K, the mixer resistors are 100K rather than 1M, and the input load resistor to the second gain stage is 2M2 rather than 1M.  
If I disconnect the two mic pre tubes and wire a jack to the input of the second gain stage, it works fine, no hum or noise. With the mic pre sections wired up, it has a pretty severe hum that is upstream of the overall volume control. With the volume dimed I get about 1.5VAC on the plates of the power tubes, about 35mVAC on the input of the 6AN8.  
Position of the mic volumes has no effect.  
Pulling either of the mic tubes has no effect.  
Pulling one tube and disconnecting the plate, grid, and cathode of one section of the remaining tube has no effect.  
Grounding the input of the second gain stage (at the junction of the .05uF caps) eliminates the hum.  
I'm tempted to tear out the entire mic pre circuits and rebuild it all with discrete components for each triode section. Does anyone see anything obviously funky with this circuit?
1/15/2006 7:13 PM
Ray Ivers
If it hums with either V1 or V2 pulled (as you know, don't pull them both!), IMO it seem likely that the problem's a failure of the 20uF, 250V cap at PS point 'D'. Having said this I'm sure they'll be 0.00 volts of B+ ripple at that point, but it seems to be a likely culprit, especially as that cap would be unlikely to survive both V1 and V2 being pulled even for a short time - something that IMO is likely to have occurred in this amp before you got it. FWIW, I consider this choice of cap voltage a design flaw, and would get that 250V cap out of there ASAP just on general principles. If you do replace it with a 450V or higher unit (I would also go with 50uF minimum, YMMV), then try unplugging both V1 and V2 and see what happens.  
If that's not it, then I would do a thorough inspection of all preamp grounds and the non-schematic-value components, especially the pots (do these components all look original?), then bring the circuit back to stock - or as close as feasible to the schematic - just to 'cross every T'.  
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1/15/2006 7:46 PM
Bob Clear

Ray, when I replaced the filters I used a 500V cap there.  
When I said that a few components differ from the schem, I didn't mean to imply that it was modded - it's totally stock, the values just differ. I don't know if it was intentional, and that the Studio PA values differ from the Coliseum, or it was a case of 'use what you have'. All grounds are stock, and tight, albeit rather different from any scheme I'd have used.  
The four inputs all ground to the chassis (they're non-isolated switchcrafts). The mic volume pots are all ganged on a single ground wire to one of the inputs. The cathodes of the tubes are grounded to the chassis through a terminal strip, one per tube (Sunns are terminal strip amps).  
When the amp is operating there's no appreciable ripple on the rail side of the plate load resistors of these tubes. Only on the tube side.
1/16/2006 8:09 AM
Ray Ivers
"Ray, when I replaced the filters I used a 500V cap there."
OK, cool... so if you still have hum when both V1 and V2 are pulled, and there's no B+ ripple, the only place for a hum problem to develop that I can see is the 1M V3 grid resistor grounding point (along with Tom's excellent suggestion about the heater winding CT, which would surely cause a major hum problem). Unless there's some bizarre conductive terminal strip or socket problem, that is... :) Can you lift the wire from the junction of the two mic-preamp coupling caps to V3's grid, leaving the 1M in-circuit?  
1/16/2006 8:25 AM
Bob Clear

Filament winding is fine. The 1M grid resistor - which happens to be 2M2 in this amp - is grounded through the mounting tab of a terminal strip. The stock location is also the grounding point of the cathode resistor for the 12AX7 used by channels 3 and 4.  
I don't recall that I've tried your last suggestion, so I'll give that a go tonight. I actually DID consider a conductive terminal strip, though!  
Thanks all!
1/15/2006 9:45 PM
Tom Phillips

After you follow Ray's ideas I'd suggest that you measure each side of the heater voltage with respect to chassis ground and verify that each is resonably close to half of the nomial 6.3V. If not then then the PT heater center tap may be open.  
Also, did it look like someone has worked on the amp and moved atound the wiring before you got it?  
Good luck,  
1/18/2006 5:16 AM
I had a fender ages ago that the local techs could not exorcise the hum from. They just about resoldered and rewired the amp. Turned out that adding 2-100ohm resistors from the 6v heater winding to ground and lifting the heater center tap from ground cured it all.  
This is the same as Toms suggestion....but couldnt resist repeating it after noticing that they strangely show the heater center tap off-center on the schematic.
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