Sunn Spectrum II question

[gui_tarzan]

I just finished resurrecting a 1966 Spectrum II that I've had for a very long time. When I initially fired it up with just the rectifier tube and turned off the standby, the rectifier started flashing inside and blew the fuse. I replaced the fuse, tried again, same thing. Tried two other 5AR4s, both new, same thing. So obviously I didn't leave it on very long while it was doing that. I remember starting this rebuild years ago and stopping for some reason, this was probably why. When I started, I bought a 40/20/20/20 can to replace the original 30/20/20/20. Long story short, that was the problem. I replaced it with separate caps and no more problems.

So my question is about a couple of capacitors on the feedback circuit. The 390pf on the power tube screen that goes to the FB, and the 750pf cap on the FB line coming off the 16 ohm tap. Are these there to stop potential oscillation or just to protect against DC in the signal?

[g1]

The 750pF/1K parallel combination is frequency compensated NFB (cap bypasses 1K for some frequencies). There should be no DC from the OT secondary.

The cap from the screen is also for NFB. In this case there is DC. A little baffled by the use of the double micro uuF value, as they have already used the pF designation elsewhere. Not sure about the oscillation thing.

[loudthud]

Although oscillation is a possibility, it is more likely IMHO that those caps are there to iron out square wave response into a dummy load. They were copied from the original Dynaco design which had a very advanced ( for the time ) output transformer and GEC KT88s.