Hi all, I have this problem with my little Kustom V5.
It's absolutely fine with the stock Chinese el84 valve, but when I change to a JJ EL84 it distorts really badly. I sent the JJ back thinking it was faulty, got 2 more (this time low plate voltage) still no luck.
Funny thing is that the tone control really effects how much it distorts with the JJ, but acts like a normal tone circuit should when the stock Chinese valve is used.
Can anyone please advise? Here's a little clip showing what's happening...