Weird behavior (blue light) on V8 and V9 after replacing Rectifier Tube.

Hello,

The tubes on V8 and V9 on my Mark V started to show a blue/white light and blow the fuse after I replaced the 5U4G rectifier tube that was gone a few days ago.

You can see what happens in the picture below. When I switch from standby to ON, both V8 and V9 behave like that at 45/90w and only V8 at 10w (as the circuit only use V8 and V10. At 45w the fuse is blown after about a minute. I saw a much more intense white light just before the fuse blown. I switched the matched pairs in V10-V11 and V8-V9 and the same happens, always on V8 and V9.

At first I thought that it was a faulty tube set of the new EL34 I have ordered from Mesa along with the rectifier tubes, but then the same problem occurred with the other EL34 pair and also with the old 6L6 set, so the problem is not with the new tubes.

Does anyone have and idea of what could be the problem? All this happened in Variac mode (but I guess full power wont make a difference) and when testing the EL34's the bias was correctly set.

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Hello,

I won't try to fix it myself. I'm only trying to find out if the problem is with the tubes or the amp. This is the first time I try EL34's and I bought 2 brand new STR 450 sets. Too bad that this is happening after all that money was spent in this tubes.

Today I replaced the rectifier tube again with a brand new (the other was also brand new), put back the old original 6L6's and everything went fine with the amp, in all channels and all power modes 10/45/90w. Then I put the EL34's, switched from standby to on, with mute on, and after about a minute (I was arranging my cables, etc) the fuse blows. Replaced it and put the amp on channel 3, 10w mode, and was able to play for a little while. When I tried to switch to channel 2, the tubes started to flash and pop very loud. I had put my phone recording a video in the back of the amp. You can see what happened in the link below.

https://youtu.be/FColx-3up6Y

So, if the power tubes are bad, why I can play on channel 3, but not on channel 1 and 2? The fuse blown on channel 1. Could it be a bad preamp tube that does not cause a problem with the 6L6's?

Problem solved. It was the sets of EL34 STR 450 NOS I recently bought from Mesa. I have just tried two other pairs os EL34's from a different brand and the amp played fine. Will send them for replacement.