Stromberg Carlson stereo amp conversion

[tee_roo]

I have just finished (mostly, waiting on pots for trem channel) a frankenstein conversion of a 1961 Stromberg Carlson ASR-433 stereo amp to an 18 watt variation using the Pedal Monkey R5 schematic. I have a couple of questions. Since I have two output transformers and 2 pair of 6BQ5's, I want to use the second transformer for more power. 18w +18w. I may have jumped the gun in my impatience and now have a working model. Nothing seems to get hot and the sound is great. My knowledge of tube circuits is limited so I would like some guidance. I have tapped off from the anodes of V2 and ran a duplicate circuit for the second channel. The only change I made was replacing R25 (100/3w) with 1k/1w resistors going into each the 6BQ5's. Also I have kept the SS power supply.
Is this amp going to hold up over time? Should I instead tap off the Volume and add another PI to the second set of outputs? How can I add a switch to turn off one of the amp sections?

[zaphod_phil]

One PI can drive two sets of EL84s without any problem. You can see that in the 36W schematics. You have a choice with what you do with the OTs. You can parallel them up, and run the amp like a regular 36W. Or you can keep them separate, allowing you to switch off one set of power tubes, without any mismatch occurring. There are various ways of switching off power tubes. One is to place a very high value resistor, like 100k say, between the cathodes and ground, so that the tubes get biased into cutoff. In an 18W power amp you could have the 100k resistor and bypass cap permanently connected, and just switch your regular cathode resistor (say 150 ohm) in parallel with those, when you want the tubes to start running.

[tee_roo]

Thanks for the reply. Is this circuit for cutting off a channel correct?


I didn't even think of parallel OTs and after playing with the amp I think I would like to switch in the second amp to gain more headroom. How hard would it be to switch in/out the parallel OT without mismatch?