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I was trying a few things to try to alleviate a hum issue on this Kalamazoo Bass 30 amp and I touched a wire to a B+ node and caused a spark and a sudden loss in output power. Not long after that I discovered the power tubes were red plating.
When I measured voltages yesterday (before the incident) I had -22V on the power tube grids, 1V at the cathodes and about 470V on the plates. Right now the plates are sitting at around 435V, cathodes are 4.0V and grids are about -10V. I just spent most of the afternoon trying to get the bias voltage back to -22V but no matter what I tried I couldn't get it better than -10V. This included swapping out components in that bias circuit and trying different values in some cases. I even hooked up a pot in place of the resistor shown as 47K (actual is a 62K). Decreasing the resistance from roughly 62K pushes the bias toward 0V but going the other way did not yield better than -10V. I also tried lower values for the 390K resistor, I was surprised that the voltage drop remains the same with different resistance values. I don't know where to go from here, any takers?
When I measured voltages yesterday (before the incident) I had -22V on the power tube grids, 1V at the cathodes and about 470V on the plates. Right now the plates are sitting at around 435V, cathodes are 4.0V and grids are about -10V. I just spent most of the afternoon trying to get the bias voltage back to -22V but no matter what I tried I couldn't get it better than -10V. This included swapping out components in that bias circuit and trying different values in some cases. I even hooked up a pot in place of the resistor shown as 47K (actual is a 62K). Decreasing the resistance from roughly 62K pushes the bias toward 0V but going the other way did not yield better than -10V. I also tried lower values for the 390K resistor, I was surprised that the voltage drop remains the same with different resistance values. I don't know where to go from here, any takers?
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