jtm 30

[bigmik]

i have a jtm 30 in the closet that's in need of repair, i'm shure anyone who has had or delt with one of these amps know that they are notorious for overheating cracking tube sockets ect. so I'v come to the point after having it sitting around for a few years of either trashing it or trying to salvage what i can and building something else.do any of you know anything about the OT/PT in these amps and are they worth useing in another project? if they are useable oppinions on what to build would be appreciated

thanks
big mik

[tarzanalog]

Schematic says that the transformers are as follows:

Power - Drake 1961_166
Output - Drake 292_772

Get the specs on those and go from there.

[zaphod_phil]

Also what tubes does the amp have in it? That will also tell us some things about those transformers.

[martinw]

With my 2 JTM60's I removed the PCB ouput valve bases, and fitted 2 normal ceramic bases vertically to the chassis, and linked them up with short lengths of wire. The valves then hang vertically upside down which improves the cooling situation.
The JTM600 head has a cooling vent in the top, unlike the combos, so it makes sense to add this.
Studying the schematic, I noticed you can bypass a lot of the unnecessary complexity, like the DI output and effects loops. Its surprising how many of those pcb-pcb link cables can be removed like this.
Once simplified like this, you're left with a more robust and a good sounding amp.
Have a go.

[bigmik]

thanks for the help so far really appreciated!,and phil it has 3 {12ax7} and 2 {5881/6l6gc}

thanks
big mik

[zaphod_phil]

And I bet those two 5881/6L6GCs are Sovteks. So they won't give a very Marshall sound. So you could follow Martin's advice, and while you're at it, change those two tubes to KT66s and rebias, for some killer tone. Alternatively, swap in a Plexi or JCM800 board and put in a pair of EL34s. Thundertweak's Classic Plexi or Top Fuel 50 boards would be suitable - see http://www.sonicdeli.com/ThunderTweakWeb/boards.htm

[Plexi]

i'm shure anyone who has had or delt with one of these amps know that they are notorious for overheating cracking tube sockets ect.

And the transformers too.. they are very small.
I had one years ago, and sold it. Never missed it one bit.

[bigmik]

so in your oppinion the trannys aren't worth useing ? I was kind of hyped on that plexi board swap as zapphod sugested

[Plexi]

No i would not use those trannys..just my opinion though.

[bigmik]

thanks for the feedback

big mik

[martinw]

IMO the power tranny is fine, but the OT on the combos is a little small. This is backed up by the fact that the head version (which came out a year later) had a bigger OT despite being otherwise identical. However, I suspect that any potential OT failures will have happened by now, and rather than spend more money, I'd keep the existing OT.

I considered various reconfigurations when I did mine, but the chassis is a funny stepped design that doesn't lend itself to this.

In the end I decided to keep the basic amp and just improve the obvious faults. To remodel it would be too expensive to justify. It's actually a pretty good sounding amp IMO.

[tubetwang]

i would not worry...

I have a Hammond 272JX pumping a 5E3 Deluxe mated to a 35 watts OT and she is doing fine.