One output valve blown, VBA, do I replace all 8, or just 1, 2, 4?

[Bassman Rich]

I guess the title says it all, during the last rehearsal halfway through the last song, my Marshall VBA amp goes quiet.
Replaced the HT fuse which had blown today, and loud bang when the standby switch turned on. So I took the back off and looked inside and 1 of the output valves has no glow from the heater like the other 7 valves,
Do I need to replace just 1, them all (they are tung sol ones), or just replace 2 or even 4 with 'balanced pairs', and if so, which brand should I choose?

Thanks in anticipation of the usually great advice I get on here

Rich

[Mr. Foxen]

Dead valves can still glow, so might be other duds in there. Or it might be just that one has lost its heater supply. Should get it looked at properly rather than chuck more valves at what could be a separate problem that eats valves.

[icastle]

If it were me in this position then I'd replace the suspect valve first and see if the amp starts working again as it should.

If it does, then I'd leave it running for a few hours and, if all is still well, replace all the valves (as they all work at the same time, if one has gone then the others probably won't be far behind).

If it doesn't come back to life then it's going to need a more serious inspection.

[Mr. Foxen]

Valves aren't so consistent that one going indicates the rest are close.

[paul_5]

erm, I think output valves need to be biased, so if you're replacing one I'd recommend having a technician look at it - preferably someone who knows enough about valves not to kill anyone (or your amp).

[3below]

As I remember (if memory serves me) from my VBA, there are instructions for biasing stuck on the insides and it has test points. If you are electrically confident it is quite straightforward. If not seek help.

[bertbass]

I had a valve go in my VBA400, replaced the valve and all good again.

[Bassman Rich]

Thanks for all your replies, I have ordered 2 replacements, unfortunately, 2 x el 34s have turned up, not 6550s, so this may take longer than I thought!!