Advice on changing the Preamp Valves on my Hughes & Kettner wanted.

bignorm

I'm after some advice on substituting the Chinese Ecc83 preamp valves in my H&K Grandemeister 36 amp.
I've read on a H&K usergroup that placing a JJ 5751 in V1, a JJ 12AT7 in V2, and V3 on the amp, will not only tame the ridiculous amount of gain on tap, but will also warm the amp up a bit on the clean and crunch channels.
Not being one for just doing it coz someone else has done it, I'd rather ask the advice of someone with much more experience.
will replacing the valves work as described or are there any pitfalls I should be aware of?

ICBM

Without knowing the amp or seeing a schematic I can't be sure, but *in general* these sorts of swaps are usually OK. The pitfalls are mainly if the amp uses a 'cathode follower' (eg V2 in old Marshalls and some modern Voxes) in which case a 12AT7 is not suitable and you may blow the valve, and if the plate resistors are on the small side (eg some modern Fenders) in which case a 12AT7 is also not suitable and may damage the amp.

These problems might not show up immediately either, so be careful about accepting that it's OK just because someone else has done it and it seems to work.

A 5751 is always safe in any position that calls for a 12AX7, since it's really just a lower-gain one.

I wouldn't change all three at once either - I would get one new valve of each type and see what effect either of them has in each position in the amp before changing everything.

You could also try a 12AU7 which has a larger reduction in gain and is fine in a cathode-follower, but has the same potential issue as the 12AT7 with plate resistors.

Hope that helps!

bignorm

Thanks for the advice. I intended to swap out one at a time anyway just to see what worked well in what position. I have an old brimar ecc83 in the drawer somewhere which I was going to put in to see what it sounded like... I suspect it won't have that much effect as this amp is packed with electronic doohickeys