Odd biasing question

[Gromit]

I've recently bought a Marshall JCM2000 DSL 201 like the one in the attached picture. The amp uses two EL84s in the output stage.

 

Now, I have absolutely no problem with biasing an amp, but my question is this: Imagine I'm out at a gig (which admittedly I don't do very often) and a tube (or valve as we call them over here) goes lame. That'd be a bit of a pain if I had to get out a DMM and bias the replacement EL84s that I'd carry. This got me thinking.

 

If I bought a matched quad of EL84s, put 2 of them in the amp, biased them up and then 6 months later had one fail, would the other 2 that I'd not used, but that were matched originally be just a drop-in replacement, or would I need to rebias, even though they were 'matched'?

 

I hope that made some sense.

[onelife]

If you have a matched quartet then I would think that six months down the road your replacement tube would be close enough to get you through the rest of the gig.

 

If you are really concerned about a possible mis-match between the new tube and the used one then swap them both.