Working on a "AM modulator" from the late 50s. Some very smart EEs designed this piece and they have their 6SN7 tubes running at low voltages when compared to the tube's max voltages. My 32S-3 Xmtr has 1000V on the plates at idle and the tubes don't seem to mind. 800V is suppose to be the ballpark figure for the 6146s I believe. It was conventional wisdom to run the tubes at high voltages, so the tube will be the most efficient in operation. Going thru this piece, I have replaced all PS caps and want to get the voltages to where there are "designed" via the schematic. Max plate voltage for the 6SN7 is 400V. The factory info has the plate voltage of 1 side of the first 6SN7 pin 5 @ 67V and the cathode pin 6 running @ 2.6V. The other side of the 6SN7 has a plate voltage of 180v on pin 2 and a cathode supply of 73V on pin 3. This then gets pumped into a 2nd 6SN7 that is running a plate voltage of 105V on both sides of the triode and a cathode voltage of 3.4v to each side. This then feeds the final PA tubes. The EEs that designed this piece are far smarter then I am in these matters, so I am asking the Forum here, why is the first 6SN7 running two different voltages on the plate and cathodes? This design was from back in the late 50s and 60s and these EEs weren't running HV on their tubes. I am guessing that these EEs must have done their homework before going against the conventional thinking at the time? How close is close enough to the factory voltage numbers? 67V on the plate is a low voltage so at 10% tolerance level, would 74v be the max? Detail A is that a 250K pot will be used in place of the 220K fixed resistor coming off of pin 4 when called for.