Sound City 120 Schematic question

[hylaphone]

Can anyone experienced with these amps shed light on a little something?


Does that V4b cathode voltage look like 4.5V or 45V? If the latter, how is 45V generated there?
Thanks a bunch!

[Tom Phillips]

If you mean V4 pin 8, then it is 4.5V.

[g1]

4.5V. If you look at the voltages at pin1 & 6 you can figure the voltage across pin6 plate resistor. That is the same current that will flow through the cathode resistors. From that you can solve for the voltage.
It may be a convoluted method, but it comes in handy for checking schematic accuracy.

[J M Fahey]

Can't tell A from B, but if you refer to V4 pin 8 which is a cathode, the reasonable value is 4.5V .
Grid is DC grounded and cathode voltage is bias produced across cathode resistors.

The apparently too high value is because to the proper 2k2 resistor, they added extra 4k7 to make the active presence control work both ways, boost and cut.

As a side note, I always found Sound City designs klunky, only tube fanatics care about them.

FWIW, here's a slightly cleaned up schematic:

[hylaphone]

As I figured, thanks for confirming gentlemen!