Sunn Studio PA

[Ryan Phelps]

Hi Sunn Fans!
I'm in the process of restoring a tube Sunn Studio PA. I have the factory schematic, but the mic preamps had been modified by the previous owner (V1 & V2). Sunn published the schematic diagrams, however I've never found a physical layout diagram (like the ones that Fender did). Do any of you have a "gut-shot" of a tube Sunn Studio PA (or a Sunn Coliseum PA) amp that shows the physical layout of the parts on at V1 and V2 on the chassis? This would be a huge help. As some of you know, the Sunn Studio PA and the Coliseum PA had an extra amplification stage that the Sunn guitar / bass amps did not have. This makes these amps very "gainy" so that the lead dress is critical. This is the aspect that I am looking for.
Thanks so much for your help!

Ryan

[loudthud]

Ryan, It's late and the pic didn't turn out very well but I thought I would post it. I can probably do better on Monday.

[Ryan Phelps]

Thanks a ton Loudthud! if it isn't too much trouble, could you take a picture of V3 as well? These photos are really helpful....I can't thank you enough!

Ryan

[loudthud]

These V3 pics are from a different amp. No mistery cap on V3.

[Ryan Phelps]

Dear Loudthud,
You are a prince! Thanks so much! This should help me put things right with my amp.

Ryan

[Ryan Phelps]

After a long hiatus, I got back to the restoration of this Sunn Studio PA. The final problem I am having is a very loud hum that I've localized to V3 (thanks to Soundmasterg for confirming this). I've already done the following: re-capped (filter & bias), replaced all caps and resistors in V1 - V3 (and many in V4), tried new grounding schemes, and matched the lead dress shown in photos of Studio and Coliseum PA's (thanks Loudthud!). I even replaced the V3 socket and terminal strip to eliminate any conductive issues.

I found a 10 year-old article in the Sunn Archives about this exact same issue. The OP deduced that there was some incompatibility between the mic preamps (V1 & V2) and the next stage with the master volume (V3). The OP finally rewired V1 & V2 in a more conventional topology (input jacks direct to grids, then volume pots, mixing resistors and coupling caps) and this solved his hum problem.

Since these Studio PA amp must have worked when they left the factory, I'm wondering:

1. Is the factory schematic incorrect? I did find a 470K resistor at pin 2 of V3 in photos that is not shown on the schematic. I have also found errors in other Sunn schematics.
2. Was there a change made during production to correct the hum problem? 

My hunch is there is a design problem with the factory schematic. Thoughts? Any insight would be greatly appreciated!

The old article I referred to can be found here:
http://archive.ampage.org/threads/6/gadc/346233/Sunn_Studio_PA_-_Flawed_Design_MAJOR_Hum_issue-1.html

And I've attached the factory schematic.

Thanks!
Ryan