One of the power outputs on my Pilot AA-903 causes the 6V6 tube in it to red plate - but this appears only when I'm using the phono stage,
The tube does not show any signs of red plating when switched to the radio or aux input.
I've attached a pdf of the circuit, and would be very grateful if anyone could offer suggestions on what the likeliest issue might be.
The tube does not show any signs of red plating when switched to the radio or aux input.
I've attached a pdf of the circuit, and would be very grateful if anyone could offer suggestions on what the likeliest issue might be.
My best guess is that some high frequency signal cones from / is generated in the phonostage.
( DC is out since there is a cap right before the tonecontrol circuit)
Any leakage on this input capacitor would dramatically increase the tube's bias. However as you mention the tube only red plates in use. But the simple would be to just disconnect the capacitor and see if the red glow stops. Same for the interstage and output terminals.
What you say is very unlikely. Does this happen with the volume control all the way down?
If so, check the output tube grid coupling capacitors, and other grid related solder joints and connections.
If so, check the output tube grid coupling capacitors, and other grid related solder joints and connections.
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1953?
Has it ever worked right for you?
It is oscillating, only the long-loop through phono preamp. Have you changed or modified ANY routing or shielding around the phono stage input? Is the bottom plate on and in good contact? Any new large parts (magic caps)? OTOH have you done routine maintenance like replace the filter caps? I would not expect supersonic squeal through power filters (and motorboat would thump-thump) but multiple old-part degradation can do odd things (excess ESR and ESL for one thing).
By red plating, do you mean the output tube flashes and dims, or does it remain constant?
Does it change with the position of the volume control?
Low frequency motorboating can cause the output tube to visibly flash, but it will vary.
A problem with bias or grid connections will appear constant.
Does it change with the position of the volume control?
Low frequency motorboating can cause the output tube to visibly flash, but it will vary.
A problem with bias or grid connections will appear constant.