Musician Gear & Tech Discussion

I have a "pre-owned" Marshall TSL 1oo amp head (the former owner is a friend), and after playing through it for thirty minutes or so, the freaking thing blows the T1A fuse. I switch out the fuse - it repeats the destruction with the new fuse. I figured, maybe it's old tubes, so I spent 120 some-odd bucks to completely re-tube the thing (pre-amp AND power amp tubes). The new power tubes are matched, and correctly biased; The speakers are set on the correct impedance (8 Ohms for my 2 cabs) - and, after 30 or 40 minutes or so of playing, once again it blows the freaking T1A fuse. I can not figure out why the bloody thing keeps doing this.
My Peavey 6505+ head works fine and operates off the same power source. I would like to be able to shift to the Marshall sometimes, but at this rate I'd be mid-tune during a gig and the f'ing amp is just gonna drop out on me....
Anyone heard of this problem with a Marshall before? It's pissing me off. What the hell would cause the fuse to keep blowing? I'm beginning to think this particular Marshall head was just a major waste of money 'cause it's beginning to look like a real piece of sheeeeit.

Try checking the ground on the both the power chord and whatever you are plugging it into, be it the outlet or whatever.

Hello Captain, Will check out the plug ground from the Marshall as you suggested. Doubt that it would be the ground in the outlet, since I've run other amps (combis) and my Peavey 6505+ head through the same outlet with no problem at all for anything EXCEPT the Mzrshall. Truly bizarre (which is why I wondered if anyone else had heard of this sort of problem), but I'm about ready to blow off trying to figure it out, it appears to be beyond me. Will see what a check of the plug ground turns up.

Thanks.

Just check the grounds, been through this before with another TSL.

In that case though, it turned out to be the outlet that was the culprit.

Just make sure you try both grounds. I.e. The outlet and the power cord ground.

I have one on the bench right now.
The worst solder joints I have ever seen.

Fact#1; All of the boards should probably be resoldered.
I found a 5 watt 1K grid resistor hanging by one lead!
This alone will cause your problem.Also it is on the main board which is a bitch to get out.

Fact#2; The footswitch and related circuitry have a high failure rate. The failure usually results in the RC4558 op amp failing which means your VPR and mute circuits no longer function properly.Again a factory defect.

Fact#3; If the amp failed within 30 days of the repair, the shop should at least warranty the work and have another look.

Marshall amps are designed to run hotter than most. The plate voltages are 500VDC compared to Fenders which run around 330VDC.
If they do fix it you should play it in the shop for about 30 minutes giving it a chance to heat up. It wouldn't hurt to give it a good bump after it heats up.Any loose components will show up then.
Most amps, especially tubes, will idle all day with some problems that won't surface until you drive the amp hard.

The fuse that failed provides power to the output tubes.
This would be caused by anything from, a bad output tube, shorted output tranny, grid resistor, bias resistor, no bias or drifting bias,
to check these you need;an oscilloscope, signal generator,load,
meter and a lot of technical skills.
If you can get the shop to take another look remember to ask the tech if he set the bias at 90ma.Some guys think hotter is better but you don't have a lot of wiggle room .It (the bias) can run away quickly.

Also. You might try replacing the fuse, then turn on the amp, if the fuse blows right away look for a shorted tube.Pull the output tubes.
Replace the fuse.Turn the amp on with no output tubes.
After a minute turn off power. Check fuse.
Replace either the two middle output tubes or the two outside output tubes.Turn on power. Does it play?
What you are trying to do is isolate the problem to one or two output tubes.If the fuse holds with either pair separately but blows when all four are in place then your bias could be drifting badly. If one set kills the amp immediately but the other does not then you probably have a shorted tube.
I do know that with Marshall amps you can't just throw any old EL 34
in there as a sub and expect good results.You have to re-bias the amp!!
Try not to get discouraged.
I hope I have been helpful.
Let me know.
G.E. PERRY

Hi G.E. Perry

A very long overdue followup on the Marshall issue....!
First, I want to thank you for the extremely detailed advice. I greatly appreciate everyone here who gave me recommendations.

In May of 2008, I moved from up North to Fayetteville, NC. I had bought a house down here, and brought all of my gear down and moved in. I set up the Peavey, I set up the Marshall (not expecting it to work), and damn if it didn't just crank up fine--and stayed that way. I have played it down here for 2 years now, and the TA1 slow blow fuse has never once ever blown again. It is the absolute weirdest thing... I have no freaking clue what changed other than the venue--it must simply prefer southern juice to Yankee voltage.

I got a Marshal Valvestate (S/S) I've been dyin' to get into, but I needed lead in information before I tore into it. Enough has been said to motivate me to have a look see. I really don't have any technical skills in this area except to look for the obvious. This unit does not power up at all. It's in good condition aesthetically, and is my first and only Marshal. Sure would be nice to get it online!

Yes! It'd be a fuse! Hopefully that is all. It shows signs of having been in the shop before, but the work looks good and everything else seems to be intact. It says it's Solid State, but there is a tube. It is very clean and shows no sign of being burnt. I don't have a fuse like that on hand. the fuse is going to be in my pocket until I make it to town. Hope I can get the rivets to snap back together.

Before I run into some confusion,(I almost overlooked this), The circuit board at the fuse location warns to replace fuse with like fuse. This one is burnt, so I don't want to do that! \, but really, it shows on the factory print two fuse kinds that could go there-T1a (for 230v), or T2v (for120v) which is what this unit is and the fuse that came out of it. Pretty cut and dry if you think about it, but if it keeps burning fuses as does in TajIV's case- can you put a heavier fuse there and at what risk?

Had a fuse in the drawer. Replaced the old one with the new, nailed it back together and Viola!- Nothing.

No reply, yeah, yeah, thanks, guys. it's a 5th amp and didn't need it anyways.