Just for starters - make sure you are connecting your cabinet to the SPEAKER jack, and *not* the EXT speaker jack. That's common.
If that fails...plug your guitar directly into the EXT AMP jack and see if you get a big loud sound or not...plugging into it bypasses most of the preamp--which doesn't sound like your issue, but will narrow it down. If it still has the same problem, you know it is somewhere in the circuit after that and not that part of the circuit (and/or the power supply etc.)
If same, I'd next check/test the flyback diodes, and/or remove them (given that you know how to work on amps from your post?), as that may be the issue.
If none of that does it (or anything anyone else recommends as a starting point), let us know.
PS - I had an EH KT88 blow without much use, sadly. I'm back to running cheap/rugged/generic 6L6GCs.
If that fails...plug your guitar directly into the EXT AMP jack and see if you get a big loud sound or not...plugging into it bypasses most of the preamp--which doesn't sound like your issue, but will narrow it down. If it still has the same problem, you know it is somewhere in the circuit after that and not that part of the circuit (and/or the power supply etc.)
If same, I'd next check/test the flyback diodes, and/or remove them (given that you know how to work on amps from your post?), as that may be the issue.
If none of that does it (or anything anyone else recommends as a starting point), let us know.
PS - I had an EH KT88 blow without much use, sadly. I'm back to running cheap/rugged/generic 6L6GCs.