JVM410 Redplated

[texhex]

Was jammin' some Iron Maiden riffs (there's my first problem).

Volume starts dropping out of nowhere. Look behind the amp and V1 and V2 are glowing bright as can be. Powered it down and popped out the two tubes... melted the Marshall logos off a bit.

Put the tubes back in, hooked up a multi-meter and the bias was measuring ok after 10 minutes. Plugged in my guitar and stummed a chord and "pop" V1 starts squealing and that multi-meter blows an internal fuse.

Popped in some spare EL34s and run it a bit, play through for 30 minutes... all OK.

First time I've ever had tubes redplate like that. Amp seems just fine now.

Guess I'm getting some new tubes this week.

 

[gearJunkie]

Glad it was just the tubes. How many hours did they have on them?

 

[texhex]

Glad it was just the tubes. How many hours did they have on them?

Not sure. Bought it from the original owner. It’s a 2021 though. Hope it’s just the tubes at this point. The one’s I swapped in are working fine so I’m feeling pretty positive it was just a bad tube. Pretty wild to see both of them lit up like that!

 

[Byrdman]

Glad it was only that! Do you run it wide open through an attenuator? If you don't, the last guy may have.

 

[texhex]

New tubes went in, biased ‘em up and played through for about an hour, zero issues. Actually sounds better with the new tubes than before. Something was definitely up.

 

[James Freeman]

I think all tubes increase idle current while hot but they should stabilize... some don't.
I like my bias at 50% of power dissipation., around 13W.