Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Apexelectric on December 26, 2023, 01:02:38 pm
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First, Merry Christmas/Happy Holidays to you all.
Second, I have a silverface non-reverb Princeton on the bench that’s got me scratching my head. Came in with a hum that was fixed with a new cap can, even though the one in there was fairly new. First stage couldn’t take the near limit voltage during startup, I guess. Anyway, after getting that changed out with a 525V rated cap can, the volume level is weak, clear and clean, but weak. Tubes checked out, speaker is ok, components seem to test to spec, no coupling cap leaks, OT was subbed for another too. The only odd thing I’m getting is the voltage at the PI grid is only about 20V when it seems like it should be more like 55-60 and close to the cathode. I checked the 10K, 1M and 56K that make up that part of the circuit and all seems to test ok. Voltages through the 10K are good at idle and plate voltage to the PI holds steady at around 200. Can’t figure out why the volume and the PI grid voltage is low. Seems like it should be simple, not a lot going on there but it’s got me a bit stumped at the moment.
What am I missing here?
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I checked the 10K, 1M and 56K that make up that part of the circuit and all seems to test ok.
There should not be a 10K resistor. It should be 1K. You can't accurately check the voltage on the grid of a boot-strapped cathodyne PI. Your meter loads the voltage considerably. Instead, check the voltage at the junction of those three resistors. That will be the correct voltage for the grid. Schematic says it should be 63.8V and the cathode will be 65V.
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Sorry, meant 1K
Thanks for the tip, that would explain the reading on the grid. The voltage at the junction and cathode are good. Varies a bit when I switch out tubes but stays right around 60V