A possible solution to the tube supply issue - converting your amp to EL84s, wich seem to be in slightly better supply (for now) and at lower prices that 6L6s, etc.
This pair is used but tested and working great,.
These units are shorter than THD's Yellow Jackets, allowing use of the converters and EL84 tubes in more amps due to the smaller clearance.
From the GT instruction manual:
"THE GT 928PR POWER REDUCER Congratulations! Your new GT 928PR power reducer will enable you to quickly, simply and safely modify your amp’s power and tone by “substitube-ing” your amp’s higher power EL34, 6L6 or 6V6 output tubes with low power EL84 tubes. The GT 928PR also converts your amp into a “self biasing” Class A circuit, thereby decreasing power and increasing tone while requiring no re-biasing of the amp. You simply plug them in and play. Please follow this direction to ensure happiness! INSTALLATION DIRECTIONS: 1 - Turn your amp off during replacement of tubes, always. 2 - Remove your old tubes with a gentle circular motion to avoid breaking the plastic guide pin on your old tubes. 3 - Plug in the 928PRs, and make sure that you line up the 928PR plug’s center plastic guide key to the corresponding key- way on each socket. This sounds obvious, but it is possible to force the plug part way into the socket if not keyed correctly. And, if not keyed correctly, and you turn your amp on in this incorrect position, it can cause serious damage! When it is keyed right, the 928PR should easily insert fully into the socket. 3a - Normally you would choose a matched pair of 928PRs for a two power tube amp, and a matched quartet for amps with four power tubes. However, you can use just two 928PRs for the job instead of four to achieve further power reduction. When changing from four power tubes to two 928PRs, it is recommended to drop you impedance selector switch to half what you have been using for four power tube operation (i.e. from 8 Ohms to 4). You can also just reduce the speaker load impedance by half too. Either way, this better balances your amp’s output transformer to the new lower primary load of two power tubes instead of four. 4 - When you want to get louder again for those bigger gigs, sim- ply plug back in your original power tubes! (NOTE: you do not need to readjust your bias IF you haven’t changed it) NOTE: the GT 928 – PR is designed for amps with plate voltages at or below 450 DCV. This covers the vast majority of tube amps, but there are a few exceptions such as early Marshall 100 amps from the late 60’s thru1974, and Ampeg SVTs."