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

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cardinal

cardinal

Well-known member
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.
 
To the extent pictures help:

Red plating tubes (V4 and V5):
3-F9-E19-DE-327-D-42-F1-A179-91-B8-C4-E6-ACB7.jpg


Turret Board (someone clipped out the v2a cathode bypass cap, WTF):
659-A0449-FC1-A-4-C20-B4-ED-5-B882-E6059-D5.jpg


Phase inverter and bias circuit close up (I think that's what this is):
A06-FE00-A-5474-4-F5-C-9582-205-F69090-B51.jpg


The two problem tube sockets:
5348-C26-D-6426-4-BC4-9929-A76-ABCE84-EB6.jpg


The two normal tube sockets (one screen resistor was replaced, I think):
B84-E28-C9-1-CD3-4-E99-B895-79-B54-CE5-CC28.jpg
 
I'd check the grid resistors as well as check for a PI coupling cap leaking DC current
 
cardinal said:
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.
Click to expand...

How did you solve this issue please?
 
I know this is old but those grid resistors look like there is only one per pair. That is, the green wires from the .1uF/220k junctions on board go straight to pin 5 (no grid resistor). Then there is a grid resistor connecting the second tube of the pair.
 
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