HELP ! Reading the Dynaco SCA-35 Schematic

redpackman

Active Member
OK, there are a number of sources for the Dynaco SCA-35 schematic. Here's one:
http://www.hifiengine.com/manual_library/dynaco/sca-35.shtml

I'm a newbie, obviously, but have a running SCA-35 after replacing the power transformer and the two filter caps (the quad and the dual 50 uf). Before that it just blew fuses immediately.

1. I know something about bias voltage and understand that if it's not right the expensive tubes in this unit can work but overheat and have their life shortened significantly. The bias voltage kind of acts like a voltage regulator of sorts preventing too much voltage and the resulting burnout. Or is that only true in the Dynaco Stereo 70?

2. And this is the primary reason I'm posting this: I'm having a hard time interpreting the voltage chart given in that schematic (see the example at Hifiengine above). I understand that the pins on the tubes are supposed to have different voltages associated with them, for instance the 7199 tubes are supposed to have 255 v. DC at their #1 pin taking the reading between the #1 pin and ground. OK, got that. What I don't get is the reading at pins 4 & 5 on all three types of tubes. I am guessing that's the bias voltage and that it's -6.3 volts. I'm not sure if I have even made that assumption correctly. AND what does it mean when pins 4 & 5 have the notation of 6.3- v with the line above them in the 4 & 5 pin columns of all three types of tubes?? Then for the 12ax tube there's a line over the readings for pins 6, 7 and 8. I don't know what that means, and at this point guessing is probably not a smart thing to do.

Thanks for the help.
 
4 and 5 are the heater - 6.3VAC between thee two pins. The heater windings are biased positive through the hum balance pots which are connected to the output tube cathodes. The output tubes are biased using cathode bis - the cathode current from all 4 tubes goes through a power resistor which drops about 15V (positive). The grid resistors go to ground, making them 15V negative with respect to the cathode - there's no adjustment. Some will increase the cathode bias resistor from 95 to 100 Ohms or more which does improve tube life, but costs a little power. Best mod is "Enhanced Fixed Bias" or EFB.
 
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