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#1
07-13-2010, 06:37 PM
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Bingo!
Will look at your PM later, just found out something important (from what I just read)! When in phono mode, the power supply does not feed the two radio tubes, so current through R18 is lower, and so the screen voltage on the output tubes does get higher, increasing cathode current. Also touching the control grid does induce hum as your whole body, meter, etc. act as a coupling capacitor to feed hum to the amplifier stage. Doing this to the grid of the previous stage (6AU6) should induce even louder hum as you now have two tubes amplifying. The funny voltages on the 6AU6 tubes: do they remain funny when you set the switch to radio? If the two channels have different readings, something is wrong with components... Bye for now. 
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#2
07-13-2010, 06:54 PM
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6AQ5 screen grid voltage
You mentioned that the voltage on pin 6 of the 6AQ5 tubes was 265 instead
of 230. Something might be open in the network formed by R21, R24 and R13, which would explain the wrong voltages measured on the 6AU6 tubes. Those parts are nearly 50 years old (just like me), don't forget, but fortunately there are not too many of them. Good luck.
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