If his is an older JTM45 and not a reissue, then you can get closer to that sound by retrofitting a reissue JTM45 this way:

1. Use a Mullard GZ34 or a new Chinese GZ34.

2. Use GEC KT66 tubes or new Valve Arts KT66 tubes.

3. Use Telefunken, Mullard, or new Ei ECC83 tubes.

4. Change the filter caps to 16u x 16u for the preamp side and 32u x 32u for the power amp side.

5. Change the OT to a Mercury Magnetics model.

6. Change the circuit board to point-to-point board using better grade caps and carbon comp resistors. You should be able to get one of these from hoffmanamps.com

The more of these upgrades you do, the closer you'll get to the sound of the original. Keep in mind that you'll have invested hundreds (and hundreds!) of dollars by the time you're done. Good luck!

The rectifier tube in your amp is labeled "GZ34", but it is not a GZ34, but more like a 5Y3. Get a Chinese GZ34 (or NOS type) and install it. This will raise your B+ to around 410V. The KT66s will not cause stress to the power supply. You'll want to turn the bias trim pot to get those KT66s to around 40ma at idle. I have many hours on mine, and neither I nor anyone I've ever heard of has had any problems relatd to the increase in filament current. When you play the amp after setting it up this way, you'll want to take those 5881s and flush them down the toilet.

Oh, BTW, while you are in there, get a 32X32uF and a 16X16uF and change the filter caps as I pointed out in a post near the top. This will improve the clarity of the amp (as per the original JTM45 circuit) and will make it easier on your rectifier tube.