1971 Marshall SuperLead blowing mains fuse

[Arthur Winter]

The head was working flawlessly for the months I had it, until I took it to a new rehearsal studio that some friends wanted to meet up and jam at. It was it’s first time out of the house and I connected two random 4x12 cabinets they had, that would’ve hopefully have matched the 8ohms the amp’s been hardwired at. It blew a fuse after playing through it for about 4 hours straight and it coincidentally happened right as we decided to stop playing. I barely palm muted the guitar, to then turn around and switch it off, when it just died in silence. At the moment, it was too hot to touch the chassis and had this distinct electrical burnt scent coming from the open back. The smell was the indication that maybe something worse had happened. I then took it home to my setup and it continues to blow the same type of 4A fuse every time I power it on. The tubes attempt to start up normally and faintly glow before the fuse gets blown in less than 3 seconds each time. One of the pictures shows a chassis bolt that had already broken off and was partly rattling inside. I really hope this isn’t that nightmare when the power or output transformer blows.

 

[Matthews Guitars]

Visually I don't see any evidence of what would cause that problem. In the event you've got a bad output tube, the test is to remove all four output tubes, replace the blown fuse with another of the correct value, and turn the amp back on. If it blows THAT fuse, then the problem definitely requires a professional service tech to check it out carefully.


Since it was recapped recently, I would not be inclined to suspect any bad capacitors.

I would certainly be checking the transformers very carefully. I'd be suspicious of a damaged transformer. Hope it isn't that.

 

[Arthur Winter]

Thanks. I just checked again with all four power tubes out and it does the same. I turned the power switch on before standby, but no difference. The suspicion of what caused this is the outlet in the wall it was plugged into at the rehearsal space. I don’t know. Not sure who to take it to besides Amphole nearby.

 

[Matthews Guitars]

Considering the test outcome, brace yourself for a possible transformer problem. Plugging into cabinets that have unknown impedance values and then running the amp for a few hours was probably the wrong thing to do. I'd suspect a damaged output transformer.

You said the amp was too hot to touch after running for the whole session. Was that with the back cover off? That kind of heat shouldn't happen. It alone would be a symptom of a problem. I've never seen an amp get untouchable myself. Very warm, yes, but never untouchable.

 

[Arthur Winter]

Yes, although the 2 cabinets were supposedly 16ohms each, I don’t know what speakers they had in them. A mesa boogie cab and some other off brand one. I thought the amp got that hot because of how long it was running with full volume on 6-7. I never ran an amp that long either, but always keep the back off anyways.

 

[neikeel]

Likely your PT burned although I have had new filter caps die it usually happens when you switch on
By the fact that it blows fuse without the standby on means that it is all upstream of your stand by switch (heaters, bias are active in this situation but the smell is a concern). Those bias caps are ridiculously huge too! 150v rating is more than adequate.
Correct thing to do is lift all the secondary wires and fire up using a lightbulb limiter to save you wasting fuses.

 

[Spanngitter]

Nice hack jobs being done to that beauty...wonder who had the idea to squeeze that 500V atoms into that cramped space on the turret.
Anyway, I think I spotted already the culprit, as for me, the indicator light seems to short out because it's butt looks quite burnt/molten....
Aunt Edit:
Spottet another isse: look at the blue wiring routing to the HV Cap near the fuse holders...

 

[playloud]

There's a lot of bad work in there. Note all the burnt insulation.

What's the story with the bias dropping resistor peeking out from under the top bias cap? Could that be failing short?

Your output tubes look shot, but apparently not the issue here.

Amphole might be a good idea.

 

[Central Scrutinizer]

Time to call in an amp surgeon.

Hope your repair is easy and the bill not too high.

I learned a valuable lesson here, never take a cabinets ohms rating for granted.

Maybe you guys and gals that gig out or go to practice sessions with unknown equipment can buy a cheap voltmeter, even an old analog one to stuff in your guitar case to check ratings before you plug in.

Again, good luck.

 

[Pete Farrington]

For a transformer bolt to have snapped, the amp might have been subject to quite a bash.
So whatever’s caused the fault might have occurred coincidentally to that.

 

[DaDoc]

Sorry to hear, nice amp..And that doesn't sound good.

I would get it to a tech pronto, and be ready for bad news.

Hauling a 4X12 cab around can be a hassle, but that's the best thing to do to keep this kind of thing from happening.

I was going to ask if your Ohm selector switch had come loose, but I can see from your pics that it's had an upgrade installed.

 

[Matthews Guitars]

I will just mention that the bolt that broke is brass and brass bolts embrittle with age. I've seen some brass bolts fail with surprisingly little torque applied. It's not really a good choice for screws and bolts, particularly if there is any significant load applied to them. They will get weaker just from time under stress.

 

[Arthur Winter]

Thanks everyone. Seems this is the last time I trust another’s cabinet. Whatever work that’s been done with the wiring is unknown to me. It came from Emerald City Guitar’s and I think they partially recapped it. The bolt must’ve broke during the car ride or while it was overheating. I’ll take it to a tech. If I remember clearer, the smell was more apparent near the PT. If it’s the PT, then worse case is I have this other hacked up amp that could potentially be a parts donor? It’s a SuperLead from 1972, with what looks like the same power transformer? The OT has been disguised and both painted over. I don’t know if the PT on that one has had any work done to it. Second to last is a comparison photo. There’s also this on eBay, though I don’t see any part number.

https://www.ebay.com/itm/225330751977?mkcid=16&mkevt=1&mkrid

Comparison above

 

[neikeel]

Those transformers are Drakes. Typical for a short run in 71/72.
Same performance as the Dagnalls of the period - I’ve experimented with switching back and forth in same amp a few years ago.

 

[DaDoc]

Best of luck to you and your amp..I hope it turns out to be an easy fix..

This reminds me of something I've been thinking of doing, which is to change out my Ohm selector switch on my '73. Obviously not what caused your problem, but my Ohm selector, while not really loose, is dodgy enough that I keep it taped down, just in case..I may be taking my amp out for gigs soon, and I don't want to have to deal with a major blow-out! Upgrading it to one like yours is just good sense.

 

[Whatwhatringrang]

I would say listen to experts above . Hope your PT is not done and that would be of course horrible. Good luck hope everything works out to be something else hopefully.

Disregard all this after this sentence. It already mentioned Disregard below sorry I should learn to read more better lol.

Powering amp up with no tubes and new fuse still blow fuse?What do the bases of the tubes smell like ? The ones that where in when it fuses went . Do one or more of the tubes smell strange ,burnt or bad? Have you tried a new set of tubes and see it blows the fuse still?
Btw I can not stand those window selectors Marshall switch to after the first types prior those.

I wish Marshall never switched to those window selectors. Those old bolts (I have seen even the small ones break )can break or become brittle sometimes. All the best

 

[paul-e-mann]

Tech time, good luck!

 

[Arthur Winter]

Thanks. Will see what happens after it’s taken in

 

[PelliX]

Preaching to the choir and after the fact, but I concur on the meter. A cheap multimeter is well worth it to have with you - also generally provides a spare 9V battery for one of your pedals just in case. When I use cabs that I don't "know" I tend to take a look at the amp every now and again, give it a feel for temp - and that's after I've done a resistance check. There's a bunch of things on the pics that look dodgy, I hope the tech you take it to has a good look all round and doesn't merely fix the one thing that's causing it to pop its fuse. Best of luck and fingers crossed for a low cost repair...

 

[Matthews Guitars]

That "hacked up" amp that you might look at as a parts donor is a fully repairable/restorable amp that deserves to be put back into service, too I'd cheerfully buy it from you for a fair price, as is, and bring it back up myself. I love doing that.

There are no unsalvageable Superleads, unless they're mangled beyond recognition.