Fender Bandmaster 50 Vibrato Issue - AB763

[misterc57]

Dazed and confused.

No pulsing. The roach lights up but does not flash.

V3 voltages. Plates are off from schematic.

pin 1: 423 V
pin 2: 0.6 V
pin 3: 7m V

pin 6: 183 V
pin 7: 2.4 V
pin 8: 3.4 V

I then replaced the cathode caps and now see the following readings

pin 1: 423 V
pin 2: -32 V
pin 3: 0 V

pin 6: 372 V
pin 7: -31.7V
pin 8: 80mV

Grid V are confusing me. Schematic does not show NEG. V but the Tom Mitchell amp repair manual does.

Fender_bandmaster_ab763_schem.pdf

[Helmholtz]

Look at the schematic: The 2.2M resistor connects to the negative bias supply and puts a negative voltage on both grids.
The negative grid voltage puts the triodes in cutoff and stops the tremolo.
To start the tremolo you need the footswitch which shorts the negative voltage to ground.
Or use a wire.

[Jon Snell]

V3 vibrato?
If it is a 12AX7 there isn't 420volts available.
Pin1 anode sgould have less than the supply voltage of 370volts.
Pin2 is the grid and should be around zero.
Pin3 is the cathode an should have a few volts on it; roughly 3.
If the cathode and the grid are at zero, the valve is not conducting or you have an error in your measurements/no heaters.

[Helmholtz]

V3 vibrato?
If it is a 12AX7 there isn't 420volts available.
Pin1 anode sgould have less than the supply voltage of 370volts.
Pin2 is the grid and should be around zero.
Pin3 is the cathode an should have a few volts on it; roughly 3.
If the cathode and the grid are at zero, the valve is not conducting or you have an error in your measurements/no heaters.

When the triodes are in cutoff (by negative grid voltage), the anode voltage equals supply voltage and cathode voltage is zero.
See post#2.

[misterc57]

Makes sense now. Thank you

Turned out to be a bad V3 tube.