I’ve got a Marshamps 6G6 replica, bought second hand from its original owner. Has been making a sound like thunder and buzzing occasionally. It usually settles after a while or a good tap on top. I’ve tried swapping out pre amp tubes but my spares are pretty useless by the resulting sound. Anyhow, we were in the studio and it started playing up. Got so bad we had to call off recording until it is cured. The amp seemed hotter than it should be. Tubes looked fine though. A little microphonic. What I need to know is, can I use small bottle 6l6 without anything but a rebias and will they lower the overall output volume and what’s the best pre amp tube to compliment them. Failing that. Same question regarding regular 6l6. Also, is there a way to tell from the transformer code what impedance it needs from the speakers? I was told it was fine at 4ohm so that’s what i’ve been using. They are Heyboers. (sp) Thanks.