I'm working on a Blackface 1967 Fender Bandmaster AB763, and the thing that's keeping me from being done is there is some out of spec voltages in the vibrato circuit, hoping someone can enlighten me.
Link to schematic
When I turn off the vibrato switch, the 12AX7 is getting over 400V at the first plate (pin 1) and both cathodes show around 0V. The grids are both receiving around -40V.
Then when I turn on the vibrato, the cathode of the second section (pin 8) is showing around 18V, while the pin 3 cathode bounces back and forth between -1 and 2V.
Bottom line, the amp is sound good and the vibrato is working, I just don't know a lot about these circuits and I'm not sure if this is just fine. 400V seems a bit high to me. The 220K resistor got replaced and it doesn't seem to do much to drop that voltage to 280 where it belongs.
Any help would be appreciated.
Max
Link to schematic
When I turn off the vibrato switch, the 12AX7 is getting over 400V at the first plate (pin 1) and both cathodes show around 0V. The grids are both receiving around -40V.
Then when I turn on the vibrato, the cathode of the second section (pin 8) is showing around 18V, while the pin 3 cathode bounces back and forth between -1 and 2V.
Bottom line, the amp is sound good and the vibrato is working, I just don't know a lot about these circuits and I'm not sure if this is just fine. 400V seems a bit high to me. The 220K resistor got replaced and it doesn't seem to do much to drop that voltage to 280 where it belongs.
Any help would be appreciated.
Max
It's fine. Look what is supposed to happen.
OFF is got by applying a big negative voltage to both grids. Looks to me like -65V, but off is off.
When OFF, no plate current, no plate resistor drop, the plates want to go to B+, which is marked +440V. The neon-driver will read much less through the 10Meg resistor which is comparable to your meter. The LFO a little less, sure, 400V.
But who cares what voltages when off?? The readings are obviously for ON. With the -65V shorted to ground through the pedal.
ON, voltages should bounce with the throb. In this very low frequency, neither AC nor DC, the exact readings may depend on your meter. Those numbers are from a VTVM, which is ideal for many things but sadly out of fashion. The LFO plate should drop a few hundred Volts, 250-300V.
OFF is got by applying a big negative voltage to both grids. Looks to me like -65V, but off is off.
When OFF, no plate current, no plate resistor drop, the plates want to go to B+, which is marked +440V. The neon-driver will read much less through the 10Meg resistor which is comparable to your meter. The LFO a little less, sure, 400V.
But who cares what voltages when off?? The readings are obviously for ON. With the -65V shorted to ground through the pedal.
ON, voltages should bounce with the throb. In this very low frequency, neither AC nor DC, the exact readings may depend on your meter. Those numbers are from a VTVM, which is ideal for many things but sadly out of fashion. The LFO plate should drop a few hundred Volts, 250-300V.