Hello Stephe, and thanks for your diligent work on this amp. I'm getting ready to perform the mods on my A10, and as you suggest, I want to check the B+ before I start. I will go by the two pictures of the schematic (before and after on the A12), so my question is, what tolerance for the voltages? Also, if you don't mind helping a newbie, at what point (in the schematic) is the voltage to the EL34 increased to bring it closer to operation?
Thanks again,
Luis
Why? Originally the B1 is 342V and the B2 is 323V. After you mod the B1 becomes 340V and the B2 is 328V.
BUT, once you mod, the current capacity increases, you are increasing the current capacity (with the improved rectifier, doing away with the 10 ohm limiting resistor, and changed the value of the resistor on the B2 feed in the power supply), rather than the voltage.
If you really must voltage check, look at the transformer output of 290V.
Looking at the schematic the "business" happens at the EL34 because you swapped out a 500 ohm wire wound resistor for one half it's value (250 ohm), increasing the current to the tube.
Some "magic" happens when you install the feedback resistors. The "820K" resistors between pin 1 on the driver tube (12ax7) and pin 3 on the driven tube (EL34). Some claim 820K is too high, preferring in the neighbourhood of 620K, You will be removing (and discarding) a pair of 470K resistors from the front end (replacing them with resistors with a value of 1M), maybe give the 470K's a try?