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1966 JTM-45 BLOWING FUSES

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9/9/2000 9:18 PM

1966 JTM-45 BLOWING FUSES

I was wondering what would cause the smaller 2 amp fuse to blow on a 1966 JTM-45 amplifier. The amp turns on but as soon as I take it off standby the fuse blows what could be causing this ??? I might have had the impedence set wrong but I corrected this and installed a new fuse and they just keep blowing ??? If anyone out there can help I would be indebted for life !!

thanks

P.S. Also does anyone know where I could buy a variac that alows me to use the amp at 240 volts.

9/9/2000 9:33 PM
Peter S	Michael, It could be a shorted tube,shorted cap, wiring or power tranny. Try turning the amp on with the rectifier tube out of the amp. If it stays on then your power tranny is not shorted. Then take the power tubes out and try putting the recto back and turn it on. If it stays on then one of your power tubes has a short. Good luck. Peter S

9/10/2000 12:20 AM

quote:
"If anyone out there can help I would be indebted for life !!"

OK, get out your checkbook 8-)

8-)

Seriously, go read "The Guitar Amp Debugging Page" at GEO. It's specifically intended to lead you through finding things like this.

It's highly likely to be a bad tube, but read the debugging page.

quote:
"Also does anyone know where I could buy a variac that alows me to use the amp at 240 volts."

Sure - use a 120V variac and a 120 to 240V step up autotransformer, both available from Mouser Electronics. However...

Why?

Variacs in general are a bad idea because they offer several ways to damage your amp. Unless there is some specific thing you want and you're really certain that you can actually get it that way, you're about to spend $125 on a variac and another $50 on a step up transformer in order to get the chance to mess the amp up.

9/10/2000 2:29 AM

All older British Amps sound A LOT better when run on 240V instead of 120V. The reason is this:
Most modern amps have 2 individual 120V primaries. Run them in parallel for 120V, or in series for 240V. The entire primary is always "on". Old Marshalls, Voxes, etc. have a single 240V primary which is tapped for the lower voltages. Run them at 120V and you're only using half the primary. The amp is less punchy, compresses more and gets muddy at higher volumes. The difference is huge, much bigger than, say, the difference between different brands of tubes, and immediately noticeable.
Hogy

9/14/2000 10:20 PM
MICHAEL	Re: 1966 JTM-45 BLOWING FUSES just to let you know Peter S. Dear Pete, Thanks for your help. The fuses were blowing because 2 caps were blown, there was a bad GZ-34 valve and an ECC81 in the 1st preamp tube slot. Thanks for all your help it is greatly appreciated !!

9/14/2000 11:07 PM
Trace	Usually when there's a tube recto and the amp is blowing fuses like that the recto usually is bad. It's a good place to look first and RG's debugging page is AMAZING! Trace

9/15/2000 1:25 AM
DavidB	Re: 1966 JTM-45 BLOWING FUSES Hogy,never knew this about old British amps,does this apply to American exports Fender ect as well?is there a way to test a pt to see if it is one of these critters to see if it has a single primary? Unrelated but would you know who manufactured transformers for Klemp amps? Regards David

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