JTM45 to JTM50

[Paul G.]

Bought a JTM45 from a builder. It squealed, it howled, it buzzed. I won't name him because he offered to make it good.

When I opened it up just to do a chopstick test before sending it back, I couldn't believe the lousy workmanship. Wires cut too short, too long, cold solder joints, "lead dress" resembling a dropped plate of spaghetti.

I didn't want to waste months going back and forth and just wrote it off.

I ordered a built chassis from Ceriatone and swapped out the trannys, tubes and box. Works a treat.

Now I have a complete chassis, loaded with the best quality components (I paid for 'em) just staring at me. I was thinking of stripping it down and just doing a complete rewire at my convenience, since I'm not in a hurry anymore.

I checked the difference between a JTM45 and early JTM50 layout and all I see is 6 resistors and 2 filter caps are different. Preamp filtering layout is oh, so slightly different, as is choke wiring. Add a death cap, and nothing else changes. Is this right?

I might do that just to get a bit more power. Should I go to SS rectifier like JMP? Just looking to recoup something here, and don't actually need 2 JTM45s.

P.

[Smokebreak]

I think the most significant change from JTM45 to 50 was the move from KT66 to EL34. Apparently some of the JTM50s had SS rectification, too. Also, as you mentioned, higher preamp filtering, and 470k mixers instead of 270k. I'm not sure offhand the others resistors you mention, maybe screen resistors and bias stuff. No need to add a death cap.
Sure, build a 50 w/SS rectification. You'll basically be into the territory many call "plexi". Should sound and feel a bit different than a 45. The OT will be different for 50. Don't use a trad 45 OT for it. Good luck!

[Roe]

the most important changes are:
- OT (from 784-103 to 784-128)
- PT (from 55 to 118)
- increased filtering
- the first el34 amps had 270k mixer resistors

[xtian]

If your good JTM45 has no master volume, add one to your new build for a lot of added flexibility.

[Paul G.]

OK. My plan is JTM 50, higher filtering, SS Rectifier, 470K mixers, and PPIMV.

Used my JTM45 trannies in other amp, so I'll get a nice new set for the 50W.

Making a list of parts needed now, drawing up a plan.

Will take my time and have fun.

P.

[rp]

If you have a hole for the rectifier in your old chassis you could build it with hi-lo taps and use a plug in ss thingy

<http://www.mojotone.com/amp-parts/vacuu ... -Rectifier>

Marstran has a Marshall PT with hi-lo secs and a 5V tap. I'm pretty sure it's the PT option for the first EL34s that started w/ the 5AR4 then quickly went ss. You need to check Marshall voltages histories if exact cloning matters. Otherwise it's a nice option, IIRC the Duane Allman JTM50 was a tube rectified model. If you're selling the MV is a good option if you don't have to buy a new faceplate. Same for the hi-lo switch, but you could mount that on top, forcing peeps to use it judiciously, they'd have to remove the back cover and (maybe) think a bit.