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HELP I'm about to blow a fuse!!

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11/15/1999 11:00 PM

HELP I'm about to blow a fuse!!

Hi guys,
My son brought me his friends Marshall JCM800 lead series 50 watt amp.It had a blowen fuse on the 6.3vac heater tap on the tranny.I replaced the fuse with a 4 amp slowblow and when I turn on the amp sometimes it blows and sometimes it doesn't.It mostly tends to blow when the amp is cold.I figure that three 12ax7s and two el34s should draw about 3.9 amps.I tried a ordenary run of the mill 5 amp fuse and it also had a tendency to blow sometimes.Is there a problem here with something else that could be causing the fuse to blow? when I measure the voltage right of of the taps before the fuses I get a reading of 3.2 odd volts on each tap .
would going to a 6 amp slow blow be alittle to much?
Thanks for any help, it will save me a bundle in fuses!

Gary

11/15/1999 11:27 PM

Have you measured the *current* going out on the 6.3V heater lines? If you calculate 3.9A that your tubes should be using, and you're regularly blowing 4A slowblow fuses, something is wrong. It may well be that there is something eating heater current that you don't know about.

Pull ALL of the tubes out and measure the heater current - should be only the pilot lights or zero if the pilots are not 6.3V powered - then put back in one tube at a time and see if you get neat jumps in the amounts that you expect. Shoot, pull out the pilot lights as well and try to get heater current down to what should be zero, then try to find what's eating current that you don't know about.

There is a turn on surge in heater circuits, but slowblow fuses were designed to take that into account. It takes well over 200% of rated current to blow a fuse quickly, and probably 500% of a slowblow type's rating to pop it quickly.

11/16/1999 12:27 AM

RG,
It may be turn on surge. Here's what I did. I left the 12ax7s in and took out the el34s.I put a 3.5 amp fuse in the posistion that blows all the time and then turned on the amp.So far so good it didnt blow. Next I let it warm up a bit and then I put in one el34, still good it didnt blow ,
I let the tube warm up a bit and then put in the second el34, the fuse began to glow but didnt blow and as the tube heated up the fuse cooled down and stopped glowing.(I bet it really felt like blowing)
It's been on for about 15 minutes now but I'm sure if I turn it off and let it cool it will blow the 3.5 amp fuse .
I'll try measuring the current as you suggested
Thanks
Gary

11/16/1999 1:47 AM
Mike Burgundy	Sounds like the "cold-start" surge. What's the official value of a fuse in this position? If 4A is the official value, we definitely have a problem, but I don't think so. These things are actually usually dimensioned liberally, within safety measures.

11/16/1999 2:07 AM
jb	Something about a glowing fuse really bothers me. Just for kicks, mark the power tubes and do the same warm and insert that you did before and see if reversing the power tubes still makes the fuse glow as before. I hope I made sense there. I'm thinkng the last power tube is possibly bad. Interesting problem for sure. jb

11/17/1999 1:36 AM
Don Symes	Gary... are you inserting tubes with the power on? That's an excellent way to kill expensive parts, like YOU.

11/16/1999 2:37 AM
Tim C.	Say, couldn't a shorted output tube cause that? Just a thought. I think jb was thinking the same thing. Good Luck, Tim C.

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