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Hey guys, I have a VS65R here which was brought to me for an intermittency problem. That is sorted out now, but the clean channel has a low frequency hum (60 cycle), not overpowering but enough to be annoying. A bloke on a Marshall forum said that this amp is sensitive to different tubes and indeed when I tried a newish JJ 12AX7 the hum was reduced, but not eliminated. It is not audible whatsoever in the Lead channel (at least not at moderate volume levels). I did some probing around and it seems the hum originates at the cathode output of the tube on the side where it splits off to tone stacks etc. for the respective channels. However, the Lead channel has additional circuitry that for all intents and purposes kills the hum. The hum is present in the Lead channel to the same extent as the Clean channel until it reaches C56 which reduces it quite a bit and then whatever is left over would only get through if VR9 is turned way up. So in the end there is significant hum on pin 2 of IC5 and negligible hum on pin 7. IC5 switches the preamp output (pin 5) to and from pins 2 (clean) and 7 (lead) via the channel select switch and the switching function of pin 1.
So before I start rolling tubes I wanted to check if the voltages on the tube pins seem right to y'all. Note the right half of the 12AX7 in the diagram corresponds with the "A" triode (pins 1, 2, and 3). Pin 1 ->371VDC, Pin 2 -> 200VDC, Pin 3 -> 203VDC. Pins 4 and 5 are 6.7VAC measured to ground.
Pin 3 seems suspect to me but I'm not experienced with this design so any feedback would be much appreciated.
- BL
So before I start rolling tubes I wanted to check if the voltages on the tube pins seem right to y'all. Note the right half of the 12AX7 in the diagram corresponds with the "A" triode (pins 1, 2, and 3). Pin 1 ->371VDC, Pin 2 -> 200VDC, Pin 3 -> 203VDC. Pins 4 and 5 are 6.7VAC measured to ground.
Pin 3 seems suspect to me but I'm not experienced with this design so any feedback would be much appreciated.
- BL
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