Rig-Talk
https://www.rig-talk.com/forum/

Marshall '69 plexi Super Lead: Red Plating V4 and V5
https://www.rig-talk.com/forum/viewtopic.php?f=93&t=203841
Page 1 of 1

Author:  cardinal [ Fri, Jun 14, 2019 7:47am ]
Post subject:  Marshall '69 plexi Super Lead: Red Plating V4 and V5

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.

Author:  cardinal [ Fri, Jun 14, 2019 7:51am ]
Post subject:  Re: Marshall Super Lead: Red Plating V4 and V5

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

Author:  fusedbrain [ Fri, Jun 14, 2019 4:21pm ]
Post subject:  Re: Marshall '69 plexi Super Lead: Red Plating V4 and V5

Check the 1k screen resistors.

Author:  scottosan [ Sun, Jul 07, 2019 7:07am ]
Post subject:  Re: Marshall '69 plexi Super Lead: Red Plating V4 and V5

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

Author:  MetalHeadMike [ Sun, Jul 07, 2019 8:58am ]
Post subject:  Re: Marshall '69 plexi Super Lead: Red Plating V4 and V5

Maybe its those straw hats :D

Page 1 of 1 All times are UTC - 5 hours
Powered by phpBB® Forum Software © phpBB Group
https://www.phpbb.com/