fisher sa-16 very red tube and?

[robertleo]

I have fisher sa-16 one of the tubes was getting very red.I checked the voltage on pin 2 on all three 6bq5 output tubes.Three of the tubes red 6 to 7 dc volts,on the red tube the reading was 11 spiking up to 16 dc volts.So I think I need to change the ceramic capactors on each tube the values are .02 uf.Can someone verify.And also I noticed that Shelton has a mod for this amp.I looked at his shematic but could not read it very well on my screen,Can someone explain what this mod consist of........thanks Robert

[Tom Bavis]

Could be grid leakage on that one tube - does it do the same in a different socket? Ceramics seldom go leaky - they usually fail shorted. But I'd change 'em anyway.

Fisher used VERY high grid resistors in this amp, so it will be sensitive to grid leakage or emission... and the common cathode resistor means you need a matched quad of tubes.

[Seraph]

The culprit is probably a leaky coupling cap to the tube going cherry red. DC voltage appearing at the output of the cap is also partially appearing on the grid of the tube in question adding to grid voltage making it more positive hence the added/excessive plate current. I would replace all coupling caps. If they are stock, it is time to replace them all. Please be extra careful observing all safety measures when working with high voltages. High voltages can be lethal.