04-27-2017, 02:12 PM
The reason you are only seeing 200 mV is because the tubes are very weak. They cannot deliver the full cathode current, regardless of the bias.
That's also why you are only seeing 14 V on the cathodes. That voltage is dependent on the total cathode current of all four output tubes. You should get 20 to 24 V there, but because the tubes are so weak they cannot supply the amount of current necessary to get the full voltage across the 12AX7 filaments which serve as the cathode resistor.
I think if you install the new tubes you will be fine. Preset all four bias pots to the setting which gives the lowest voltage from A,B,C and D to ground. Then power up slowly with your variac while monitoring the voltage across each 10 ohm cathode resistor.
That's also why you are only seeing 14 V on the cathodes. That voltage is dependent on the total cathode current of all four output tubes. You should get 20 to 24 V there, but because the tubes are so weak they cannot supply the amount of current necessary to get the full voltage across the 12AX7 filaments which serve as the cathode resistor.
I think if you install the new tubes you will be fine. Preset all four bias pots to the setting which gives the lowest voltage from A,B,C and D to ground. Then power up slowly with your variac while monitoring the voltage across each 10 ohm cathode resistor.
