Bias shifting

[SmashBruh]

Hey guys, I’ve encountered something strange with my mainline- I can’t seem to get the tubes to stay exactly at 145V anymore. I try to bias my tubes once a month and usually the bias is stable as I turn the knobs but this time around the A side seems to swing +-.5V and the B side swings +-.2V. Is this just the tubes aging/a sign that they need replacing? Or do you think there’s something else at play? Just figured I’d ask what you guys thought since searching didn’t reveal any light on the matter and I want to make sure my amp is in tip top shape!

[Paul Birkeland]

The OA and OB pads on the center PC board provide the bias voltage that's adjustable.  You should be able to read the voltage at either terminal and see if move as you twist the pot. 

What voltages do you get at those pads to see 145V on the 6C45PI plates?

[SmashBruh]

The OA and OB pads on the center PC board provide the bias voltage that's adjustable.  You should be able to read the voltage at either terminal and see if move as you twist the pot. 

What voltages do you get at those pads to see 145V on the 6C45PI plates?

Hey Paul,

The tubes seem a bit more stable today as they only seem to be shifting +/-.1V today. I did measure the Voltages on the two pads; and unless I'm reading the wrong areas it seems like the OA pad is at 1.9V and the OB pad is at 1.7V when both tubes are set to 145 (with the +/-.1V). Is it normal for the values to be different?

[Paul Birkeland]

Yes, that's normal for those value to be different.  The plate voltage will wander a little bit as the amp runs, the solid state components in the circuit and the 6C45PI will have some performance deviations as the thermal conditions cycle.