1970 1200s...and Hello!

[Wally]

Newbie here. I am new to Sunn amps...just found the first for me and need a bit of help. Does anyone have a source for a schemtatic for a 1200S...tube gutiar head from '70. This one is running 6X12's...Eminence with aluminum dust covers. The amp has a slight hum with controls at zero and a big thump,thump,thump with the volumes pushed over halfway. I am gonig to replace filter caps and would like to view the schematic. It looks like the 2000 circuit has some simularities??
Any help wold be apprecaited.
Wally Sanders

[Enzo]

My 1200s copy won't scan worth beans. The circuit is just about identical to the Sentura amps with exception of OT and power tubes and the PT and HV supply. The output tubes and OT are pretty much like the Colisseum PA other than 6550 instead of KT88. The Col PA has a tube recto instead of the 1200s diodes. Sentura is close enough to troubleshoot with.

FInd those at Schematic Heaven.

[Wally]

Enzo, I appreciate the info. I did look at Schematic HEaven and noticed the similarities.

[Wally]

Well, I replaced the multi-section cap in this beast and got rid of the hum and the deep motorboating. This is a BIG sounding amp. the reverb doesn't overwhelm me, but there is no denying the draw if one is looking for big and clean. The midboost pumps up the OD right where a guitar fits in. All in all, it is an interesting amp. Quality construction.
I have to wonder what it would sound like without those aluminum dust cover speakers. Surely it would have smoother high end. I am sure these speakers cut, though.

[AdmiralB]

What did you use for filters? That amp's gotta be close to 600V on the plates and screens; my 2000S did 570V with a pair of GZ34s. I wouldn't trust the Aero cans for that.

[Wally]

Admiral B, good point. I did use an Aero which I surprisingly had on hand which was rated similarly to the original at 525 volts in all sections. The owner of the amp just wanted to hear the amp produce some sound....he is a miserly fellow and doesn't want to even replace the very weak power tubes....against my advice. I think I will pull the chassis again and measure the voltages just to be familiar with it.
Now, what I will do if I find voltages similar to yours, I don't know. Did you wire in separate caps to replace the multisection can? Tomeone had already subbed in one 600 votl axial for one section.

[AdmiralB]

IIRC 'stock' values are 30uF at the rectifier, 20uF at the choke output, 20uF at the PI, and 40uF (two 20uF sections in parallel) at the preamp.

I used two 60uF/350V in series (with resistors) for the rectifier, a 20uF/600V for the choke, and then I replaced the can with an LCR-style 32x32/500V can for the PI/preamp. Close enough.

You could go with bigger values since yours is SS-rectified, but I think the low values are part of the sound and feel.

[Wally]

Thanks for the tips. By the way, this one is rectified by 2 Mullard GZ34's, too. Do you think that subbing in two 80mfd/450's for the first stage and leaving the rest of the Aero can in place do the job?

[AdmiralB]

No, the B+ is still well over 550V at the second node, which is the one that feeds the plates and screens.

[Wally]

Well, thanks for the heads-up, Admiral. Now one more question...was the original can undersized and running on luck? Or is the Aero just not up to snuff at the same 'ratings' as the original?
Or..are our 'modern' wall voltages enough to pump this thing over the limit of the original can's specs?

[AdmiralB]

I think the key is that the original was speced by builders that were counting on 110VAC mains. If your customer variacs it down to 110, he'll be fine.

I've owned plenty of Sunns that were still on the original caps, with 560V+. But I think they're time bombs.

[Wally]

These high wall voltages are a concern. I am going to measure these votlages out of curiousity. If it measures around the 560 you mention, then 9% high on the wall voltages would point to voltages at 110 off thewall would be just over 500V.
Thanks for the heads up.

[soundmasterg]

Wally, the relationship between the wall voltage rising 9% and the voltages in the amp rising isn't linear, so you may find that the voltages in the amp are considerably more than 500v. My 200S that I rebuilt is around 560v, on a 125v supply, and my 2000S that I haven't rebuilt yet is more like 580v. I too went with series connected caps in the 200S and still needed a 500v cap at the third node. I went with larger caps in the first two nodes, with 110uf (two 220uf caps in series with resistors) and 50uf (two 100uf caps in series with resistors) and kept the other nodes stock. I used a Weber Copper Cap WZ34 that emulates the voltage drop and sag characteristics of the GZ34 tube rectifier but is solid state. With the bigger than 60uf caps in the first section you can't use a tube rectifier anymore. For bass I think the larger cap values work great. For guitar I'd have to experiment, but I would hazard a guess that I would settle on smaller values. I've never liked Sunn amps for guitar tones though except for clean stuff when I'm looking for a different voice than a Fender or a Vox. Personally I would never use the 525v can cap in one of these Sunns. They aren't rated enough for today's voltages, and cost more than series caps do.

Greg

[Wally]

Thanks for the insight. I may go back in and restructure those first two nodes.

[Gtr_tech]

I'd recommend going with something like this:
http://store.triodestore.com/sdslabmk3cap1.html
Or if you want to use your own parts they offer an unstuffed pcb:
http://store.triodestore.com/sdslabmk3cap.html