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Marshall '69 plexi Super Lead: Red Plating V4 and V5

Fri, Jun 14, 2019 7:47am

Hi guys,

Really hoping someone can help me out.

I just got what appears to be a mostly stock '69 plexi-panel Super Lead. Consistent across all four power tubes: I measured 482v plate voltage (pin 3, with the tubes in). I measured -44v on pin 5 (with the tubes out). Tubes bias fine to 35mA (no input, volumes turned down). I've dropped the bias as low as about 20mA just to check.

At idle (guitar plugged in but not playing): everything seems fine.

But if I play for about 60 seconds or so, one side of both V4 and V5 starts to red plate (the side toward the back of the amp; the other side of the tubes is fine). As soon as I stop playing, the tubes cool and return to normal.

I've swapped the tubes around: the problem says with the sockets for V4 and V5.

Any ideas what's going wrong here? The amp sounds glorious. But it would be catastrophic to lose the output transformer from red plating tubes.
Last edited by cardinal on Fri, Jun 14, 2019 8:04am, edited 2 times in total.

Re: Marshall Super Lead: Red Plating V4 and V5

Fri, Jun 14, 2019 7:51am

To the extent pictures help:

Red plating tubes (V4 and V5):
Image

Turret Board (someone clipped out the v2a cathode bypass cap, WTF):
Image

Phase inverter and bias circuit close up (I think that's what this is):
Image

The two problem tube sockets:
Image

The two normal tube sockets (one screen resistor was replaced, I think):
Image

Re: Marshall '69 plexi Super Lead: Red Plating V4 and V5

Fri, Jun 14, 2019 4:21pm

Check the 1k screen resistors.

Re: Marshall '69 plexi Super Lead: Red Plating V4 and V5

Sun, Jul 07, 2019 7:07am

I'd check the grid resistors as well as check for a PI coupling cap leaking DC current

Re: Marshall '69 plexi Super Lead: Red Plating V4 and V5

Sun, Jul 07, 2019 8:58am

Maybe its those straw hats :D
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