I am using an Ampeg SVP-CL bass preamp, which has begun to crackle and pop intermittently while warming up. Thinking it was a tube going, I tested them all (V1-V4 / EH 12AX7, Tung Sol 12AU7, EH 12AX7, Tung Sol 12AU7) - and all tubes tested OK.
I changed them out for JJ ECC82 and 83's, but lost...