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| | |-+  36w TMB EF86 red plating and blowing valves
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Author Topic: 36w TMB EF86 red plating and blowing valves  (Read 196 times)
Ade71
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« on: November 06, 2020, 08:12:45 PM »

Hi.

Iíve owned the above amp for about a decade. I ordered it as a built amp in chassis that I retro fitted into an old JMP 2104 2x12 (sans amp but with the original Ď78 blackbacks) Iíve owned since the late Ď80s.

I havenít used it regularly for a while simply because itís so loud. A few months ago I had it out and it and it blew an EL84. I got a new set of reissue Mullards and re-valved the whole amp. I had to also replace the internal HT fuse that must have been taken out when the valve went.

Almost immediately I noticed two of the new valves were red plating. I kept the chassis out of the combo for a week or so and played it on and off. Using it in both Hi (36w all four valves) and Lo (18w the two inner EL84s), I rotated the matched quad through the various positions and the red plating ceased. Rightly or wrongly, I put the red plating down to new valves burning in.

I put the chassis back in the combo and have been playing it for the past few weeks without incident. Today after playing in 18w mode for about 30mins I left it on and was giving my ears a rest when I heard a crackle then pop. I picked up my guitar but the sound seemed unaltered. Smoke was rising from the vent. I lent it forward and peered in the back and noticed of the two inner EL84s, one was glowing red, the other was dark. I switched it off, let it cool, then pulled the chassis out. One inner valve was dead, cracked in the middle. I should add I had put some tube rings on them and it seems to have cracked where the ring was touching.

Any ideas why my amp is cooking EL84s?
Should I ditch the tube rings?
In 18w mode can I run it with the two outer valves removed (cause now my matched quad is essentially a matched pair and a spare...).

Any help greatly appreciated.
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