04-10-2013, 02:23 PM
Do you measure any positive voltage on the grids of the 45's? Since the center tap of the driver transformer secondary is grounded, there should be very little DC voltage at the grids unless the 45's are gassy or the sockets have leakage.
If you remove the 45's, can you measure any DC on the grid contacts of the socket? If there is no DC, this would tend to eliminate leakage from the sockets.
My guess is that one or both of the 45's are bad and drawing excessive current. Normal bias should be 45 V, so if you are measuring 90 V the tubes are drawing twice their normal current. As you said, with 90 V bias, the tubes should be cutoff, and drawing very little current, unless the center tap of the driver transformer secondary is not grounded. If the center tap is floating, the grids could assume a positive voltage and cause excessive current regardless of the bias developed across the filament return resistor.
If you remove the 45's, can you measure any DC on the grid contacts of the socket? If there is no DC, this would tend to eliminate leakage from the sockets.
My guess is that one or both of the 45's are bad and drawing excessive current. Normal bias should be 45 V, so if you are measuring 90 V the tubes are drawing twice their normal current. As you said, with 90 V bias, the tubes should be cutoff, and drawing very little current, unless the center tap of the driver transformer secondary is not grounded. If the center tap is floating, the grids could assume a positive voltage and cause excessive current regardless of the bias developed across the filament return resistor.
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