6bq5 red plating

[maciek]

i have 9302 magnavox,working great ,but one of 6bq5's is red plating,it is not bad tube i keep changing them and each in that socket starts glowing after some time,could it be bad socket,or something else,thanks for any advice

[HollowState]

Most likely suspect is the coupling capacitor feeding the grid is leaking B+ and overriding the negative bias voltage that keeps the tube under control. Compare the grid voltage with the other one and see if it's drifting less negative. If so, replace the capacitor. Best to replace both if it's push-pull and both channels if it's stereo.

Could also be a screen resistor off value and pushing the voltage higher.

[maciek]

thanks hollowstate,gonna replace both and see if it helps,greetings

[Tarzan]

First measure as said by HollowState before changing parts.
By taking measurements you learn something.
By changing parts you don't.
And if it isn't the case you lost some money and still keep the problem.
Let us know what you measure...

[Frank Berry]

The Magnavox amplifier probably has cathode bias. If so, the control grids should be at zero d.c. potential. If the coupling cap is leaking, the grid will be at some positive voltage.
It may be helpful to know whether the two output tubes share a common cathode resistor and capacitor. If each tube has its own cathode resistor and capacitor, one of the cathode capacitors may have shorted or become leaky.

Do let us know what you find.

[artosalo]

The Magnavox amplifier probably has cathode bias.

Here is the schematic:

[Frank Berry]

Since both tubes share the same cathode resistor and capacitor, it's likely that your problem is the coupling capacitor on the control grid. I would replace all four of them (C104, C106, C204, C206). For good measure, I would also replace the electrolytic capacitors on the cathodes.

[maciek]

thanks guys for advice im going to replace coupling caps with some russian pio i have,let you know if it helped

[Tom Bavis]

This amp uses one cathode resistor for all four tubes. Weak tubes will cause other tubes to draw more current. Ideally, this would have a matched quad of 6BQ5s. But check grid voltage - if positive, it's a leaky cap or grid leakage in the tube. If it's still there with tube pulled, it's a leaky cap for sure. I usually wouldn't recommend running an amp without a tube, but the Maggie only draws about 110 mA total - still safe split 3 ways.

[radiostar]

Originally Posted by artosalo

Here is the schematic:

Thanks for the schematic!