Metropoulos Forum

Hi, i am new, i came from Italy!
I need your help for the overheating issue with my Marshall JTM 30. I have already the fun, but on the net i have found only pics and not schematics or instruction for how to install the fan, or better, where to connect the fan wire to the pcb!

Please help me! Thanks in advance

first question is what is the rating on your fan? Is it a 12v or 115v fan?

First...What kind of heat related problem(s) are you experiencing? Tubes and tube amplifiers get hot. We're talking about a JTM30 here but some guys butcher up older, vintage amps putting fans in them because they THINK they have a problem, when nothing really "bad" is happening but a little heat. If your JTM30 is truly over-heating, something is wrong than can be fixed without installing a fan....over-biased power tubes, bad transformer....

@ 54 strat: The JTM30s seem to have a serious overheating issue. The tubes and chassis seem to get a whole lot too hot. Installing a fan seems to be a popular mod to take care of it - I read about it in several places on the web because I was thinking about buying one some time ago, when I wanted "my first tube amp" - that was before I got into plexi amps...

@fede.90k: But - sorry, I can´t help with that. I´m still a tube amp beginner myself. Maybe you should try on other forums too - there seems to be little activity here recently.

Before commenting this time I surfed around some other forums for info on JTM30 issues. The consensus seems to be that what some say are overheating problems in some JTM30's is due to the fixed bias being set too high at the factory. Same well known problem I have had with a Fender Pro-Junior...they come from the factory WAY over biased. This, coupled with the fact that tubes hang underneath a combo amp chassis and heat rises, will make an amp run hot. Since there is no bias trim pot on the JTM30 chassis you will have to locate and change the bias resistor to a higher value.
Also...a guy who used to post here a lot, known as "World of Tone" and highly knowledgeable, posted on another forum that if you also remove the metal tube shield these JTM30's have, the temperature will drop at least twenty degrees! So I think if you remove the tube shield and get the bias down to the correct range, your problem is solved without installing a fan.
Now there's nothing "wrong" with installing a fan until you try to sell the amp and have to lower your price because it has been "butchered" in some way. Even JTM30's will be collectable some day. Removing a tube shield would not be a permanent modification that will decrease the value of your amp, and no one would care that you changed a resistor to get it properly biased either. Good luck!