Marshall JCM 800 Lead Series - 100 Watt - Blowing 1 amp fuse when turning on Standby

cogorman · 12-08-2007, 09:18 PM

Can anyone help me with my Marshall JCM 800 Lead Series - 100 Watt - Blowing 1 amp fuse turning on Standb. I turn the main power on - no problem. Wait a few minnutes & turn standby on & I hear pop through the cabinets & my 1 Amp Slow-blo fuse is blown & my 4 amp fuse is not blown. Is there anything that I can replace/test myself without having any electronic tools?

Enzo · 12-08-2007, 10:08 PM

As long as you are indeed using slow blow fuses, then pull the power tubes and see if it still does it. if not, you have a bad power tube. If it still blows fuses without power tubes, then you have a shorted rectifier, a shorted filter cap, a wiring problem, etc.

I'd bet you have a bad power tube. The little tubes can't really blow fuses for the most part.

And may I request you use model numbers like 4100 or whatever instead of JCM800, which doesn't tell us what you have.

cogorman · 12-09-2007, 05:14 PM

Thanks for the advice. I'll try that hopefully today

cogorman · 12-12-2007, 03:01 AM

6550A Tube was blown. Is it safe to run ampw with 3 tubes until I get another tube? Also, If a get another GT 6550A tube do I need to rebias the amp?

Enzo · 12-12-2007, 04:59 AM

You can safely run with three, or for that matter just two - one removed from each side.

Bias addjustment ought to be done whenever you change tubes. It is not hard to do.

I prefer to replace power tubes as sets. How old are yours?

Liam · 12-12-2007, 06:09 PM

A suggestion.

I'm betting the amp is either a model #1959 or 2203. In both cases consider converting to EL34 output tubes (they were fitted in all the UK amps of that era). You'll get a bit less headroom and a much more classic marshall-like tone. It's not a major job, and I think I have the necessary circuit changes written down somewhere.

In the meantime, 2 tubes will generally sound better than 3 (you'll need to pull the tube "opposite" the blown one), and someone else will be able to tell you which way to turn the impedance selector so it still matches, or I'll have a think about it and see if I can remember!

Liam

cogorman · 12-13-2007, 04:30 AM

Thanks Guys! I'm going to try running it with 2 GT 6550A tubes & I also have the 12AX7 for my 2203. Hopefully, I will be able to get a little bit more distortion at lower volume levels. I appreciate your advice & time! Rock On!

12-09-2007, 05:14 PM	#3
cogorman Junior Member Join Date: Dec 2007 Posts: 4	Model is a 2203 Thanks for the advice. I'll try that hopefully today

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12-08-2007, 09:18 PM	#1
cogorman Junior Member Join Date: Dec 2007 Posts: 4	Marshall JCM 800 Lead Series - 100 Watt - Blowing 1 amp fuse when turning on Standby Can anyone help me with my Marshall JCM 800 Lead Series - 100 Watt - Blowing 1 amp fuse turning on Standb. I turn the main power on - no problem. Wait a few minnutes & turn standby on & I hear pop through the cabinets & my 1 Amp Slow-blo fuse is blown & my 4 amp fuse is not blown. Is there anything that I can replace/test myself without having any electronic tools?

12-08-2007, 10:08 PM	#2
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,361	As long as you are indeed using slow blow fuses, then pull the power tubes and see if it still does it. if not, you have a bad power tube. If it still blows fuses without power tubes, then you have a shorted rectifier, a shorted filter cap, a wiring problem, etc. I'd bet you have a bad power tube. The little tubes can't really blow fuses for the most part. And may I request you use model numbers like 4100 or whatever instead of JCM800, which doesn't tell us what you have.

12-12-2007, 03:01 AM	#4
cogorman Junior Member Join Date: Dec 2007 Posts: 4	6550A Tube was blown. Is it safe to run ampw with 3 tubes until I get another tube? Also, If a get another GT 6550A tube do I need to rebias the amp?

12-12-2007, 04:59 AM	#5
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,361	You can safely run with three, or for that matter just two - one removed from each side. Bias addjustment ought to be done whenever you change tubes. It is not hard to do. I prefer to replace power tubes as sets. How old are yours?

12-12-2007, 06:09 PM	#6
Liam Senior Member Join Date: Sep 2007 Posts: 121	A suggestion. I'm betting the amp is either a model #1959 or 2203. In both cases consider converting to EL34 output tubes (they were fitted in all the UK amps of that era). You'll get a bit less headroom and a much more classic marshall-like tone. It's not a major job, and I think I have the necessary circuit changes written down somewhere. In the meantime, 2 tubes will generally sound better than 3 (you'll need to pull the tube "opposite" the blown one), and someone else will be able to tell you which way to turn the impedance selector so it still matches, or I'll have a think about it and see if I can remember! Liam

12-13-2007, 04:30 AM	#7
cogorman Junior Member Join Date: Dec 2007 Posts: 4	Thanks Guys! I'm going to try running it with 2 GT 6550A tubes & I also have the 12AX7 for my 2203. Hopefully, I will be able to get a little bit more distortion at lower volume levels. I appreciate your advice & time! Rock On!

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