Hi Guys,
I picked up a 1970 Sonaro a couple of weeks ago at the Seattle - Tacoma Guitar Show. It was in as-is condition, blowing fuses. So I figured it likely needed some major work and bought it for $150. I found it needed a new power transformer (not surprised), so I ordered one from Triode Electronics. I also installed a 3-prong AC cord, disabled the polarity switch and installed discrete power supply caps on terminal strips under the chassis. I have not fired it up yet.
The amp has volume, treble, bass and hi & lo switches. This version has a pair of 6550's, 6AN8 phase inverter and a 12AX7 preamp tube. I could not find a schematic for this model online, but it is similar to a 200S, Sorado and the early 100S (although the 100S has the contour control instead of the hi - lo switches).
This amp lacks the cathode cap on pin 8 of the 12AX7 (like the early 100S). The odd thing is that the 12PF cap on pin 6 of the 6AN8 (pentode side) is connected to the B node of the power supply (425V) rather than the C node (320V) as I've seen in most all Sunn schematics. Everything looked bone-stock under the chassis, so I think this was the original factory wiring. I vaguely recall seeing this on one of my other Sunn amps too. And this also appears to match the Dynaco Mark III that all of these early amps were based upon.
So, my question is......was this just a factory variant or is there an error in the typical schematics? Any of you ever seen this? This cap seems like a feedback loop from the triode plate back to its grid. If it is connected to the C node (like most Sunn schematics indicate) it would parallel the 270K plate load resistor to the pentode ....what would be its purpose?
Thanks!
Edit: OK, I found that the 2000S also has the 12PF cap connecting pin 6 ( & pin 2 as they are directly coupled) and power supply node B.
