[SteveFord]

You turn on an amp and after a few minutes a little cloud of electricity appears around a tube acoompanied by a small puff of smoke.
I say, something appears to be amiss...

[twitch54]

A new type of incense perhaps ? 

[SteveFord]

Eau de OH NO!

I got lucky, it was just the tube which took out the main fuse.
Of course I can't find my spares so new fuses on order.

[SET Man]

Hey!

    You are very lucky indeed! The fuse did it job.
 
    I say a little pray every time I turn on my tube amps, especially my 845

[FullRangeMan]

845 are expensive, you would change to GM70 around $20 each in Russia, military tube not commercial Chinese product.
Change the socket and add a 20V heating transformer.

[SteveFord]

I'm not sure if you posted on the wrong thread or not?
This amp takes eight 6550s, I replaced it with a spare Sovtek I had in the drawer.

[FullRangeMan]

I'm not sure if you posted on the wrong thread or not?
This amp takes eight 6550s, I replaced it with a spare Sovtek I had in the drawer.

SetMan use a expensive 845 tube but could use a cheap GM70.
They are dirty cheap in Russia, I dont have a GM70 amp but have many GM70 tubes just for collection, great value, glass seems to be 2 or 3mm on sides and 5mm at the bottom with aluminium base.

Hey!

    You are very lucky indeed! The fuse did it job.
 
    I say a little pray every time I turn on my tube amps, especially my 845

[rockadanny]

Opa!  ( sorry)

[SteveFord]

These MB250s came to me with KT88s and the manual says 6550s.
I called VTL many years ago and they said use 6550s.
More 6550s were getting smoked recently and AVNerdguy sent me a post where someone said forget modern 6550s, use KT88s in these so I did.
So far, so good.  No explosions, no fires and the bias is not fluctuating too much after 3 hours use.