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Old 21st March 2010, 03:06 AM   #1
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Default Voltage Measurement

Hey All,

I'm modifying a pair of Dynaco Mark VI power amps. I recieved a schematic from Vintage Amp Restoration in Georgia. It uses two 12AU7's as a preamp tube and as phase splitter. His spec calls for 420 volts B+ into the driver circuit. I had 543 volts.

I installed a voltage divider and brought it down to 408v's. The problem is I still have 383 volts at the plates for the 12AU7's. According to JJ's website the 12AU7 has a maximum plate voltage of 300v. (With the 543v's I fried the preamp tube.) I replaced the two tubes. Unfortunately I'm still not seeing any voltage across the cathode resistors. The circuit is drawing 21ma so the power is going somewhere.

When you measure plate voltage do you measure between the tube socket pin and ground? Or across to another pin with the tube in?
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Old 21st March 2010, 04:52 AM   #2
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Default Never Mind

I already found the answer. You measure across the socket from the plate connection to the cathode connection.
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