valve grades

[gordiji]

thomann sells engl output tubes with grades such as 22, 24 , 13 , 17. what do these grades indicate?
i've no intention of buying them as i'm going to try a eurotube retube for my screamer, and bias it myself with
a bias probe.i've also noticed there's very little info on t'net on this amp. has anyone a photo of the circuit board
showing the wherabouts of the trim pot.
thanks

[Keven]

i think if you previously bought tubes there and say bought a '22'' and had the amp biased for such, you can safely buy another 22 without rebiasing.