Eric,
After visiting the site for the schematics, I think the only error on the Sonic 1-40 schematic would be the output taps. I believe we used an 8 Ohm speaker on this unit which means the 8 ohm tap for the speaker jack and the 4 Ohm tap for the extension speaker jack. The proper drive tube is the 7199.
Conrad
: Finding the Sonic-I schematic on Rich Briere’s Sunn web page (and like this one, an excellent site it is) has only added confusion where I hoped it would be cleared up. The diagram shows several circuit and component value differences from what’s in my amp – in fact the wiring in mine looks identical to the comparable PP 6550/KT88 circuits (Sonic II, 200S, Sorado etc.). This includes the 6AN8 driver/inverter tube and 8/4-ohm speaker wiring, instead of the 7199 tube and 16/8-ohm output shown on the Sonic-I schematic.
: What bothers me most is I’m still not certain what output tubes the thing should be running. When I bought it back in ’86, it had Japanese EL34’s installed. This seems consistent with the transformers in it, a "3-4" power transformer and a Dynaco A-470 output; where the 6550 schematics show "3-6" output/power transformers. Date codes on its pots are very early ‘67.
: The bias circuit adds to the confusion. I can only get an adjustment range of about –40 to –55 volts (makes sense, considering the 20K resistor at the bottom of the bias divider stack) and the Sonic-I schematic specifies –35 V. So the situation is: 1) The amp seems under-biased for EL34’s, at least as far as the schematic indicates, and 2) Not knowing the power transformer rating, I don’t want to risk cooking it by trying 6550’s that should run on the *other* margin of this amp’s bias adjustment range.
: So: Any new clues?
: Many thanks in advance!
