Repaired Plexi died in 10 mins

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The 1992 SB schematic says 4A and 5A depending on output tube version 6550 or EL34
https://www.thetubestore.com/lib/th...rshall-JMP-Super-Bass-100W-1992-Schematic.pdf
6550-->5A, El34-->4A
Snap some photos of the transformers and the front and back panels. The amp could have some unique features. 1967 was a transitional year.
Here are some pics. It has the original Drake transformers. I’d be a bit worried betting the problem is simply a fuse value (3A vs 4A). If that’s not the issue, couldn’t I cause more damage to the amp?
 

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Gene Ballzz

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First off, that fuse label/sticker does not appear to be factory applied! Is there any chance that this amp has spent most of it's life in the UK or EU? If the experts here say it should be a 4A, I would trust them. Those nice, clear transformer pics should be helpful. A gut shot or two would likely help also!
Just My :2c:,
Gene
 

neikeel

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Those stickers are both correct from the factory. Gold Dymo used for years.
U.K. models would have Bulgin. Export models fixed cord so could have been European model. I’d sill use T4A in it.
 

Gunner64

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Could be stupid simple fuse related shit too, like a corroded arching fuse holder, bad solder joint at the holder ect.
 

Gene Ballzz

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Those stickers are both correct from the factory. Gold Dymo used for years.
U.K. models would have Bulgin. Export models fixed cord so could have been European model. I’d sill use T4A in it.

Thanks for that info. Another reason I love this place! I actually like being corrected, instead of letting my ignorance stand. :D
I Enjoy Learnin'
Gene
 

mickeydg5

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First off, that fuse label/sticker does not appear to be factory applied! Is there any chance that this amp has spent most of it's life in the UK or EU? If the experts here say it should be a 4A, I would trust them. Those nice, clear transformer pics should be helpful. A gut shot or two would likely help also!
Just My :2c:,
Gene
And @Gene Ballzz, did you notice the HT fuse labeled as T1.5A?
Marshall also ended up at the common T1A value for those designs.
 

william vogel

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A 100 watt Marshall 120 volt version uses a 4 amp slow blow mains fuse and a 1 amp HT fuse. If a 1 amp fast blow won’t hold go to a 1 amp slow blow.
 

Matthews Guitars

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Although my own Superleads are healthy, one would eventually pop a 1A fuse after a few minutes playing loud. I went up to a 1 and 1/4 amp quick acting fuse and it's never given me another problem. 1A is just too marginal on that amplifier, but 1.25A is just fine.
 

myersbw

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I agree with all the assessments, too. T4A for the mains and 1.25A for the B+...won't hurt a thing. That's a hefty amount of filter capacitance to deal with, too! While you're at it...inspect all your head-to-Fryette-to-cab cabling. And, pop a meter on the load 1/4 plug before plugging it into the amp...make sure it's close (dc resistance) regarding the load the amp expects to see.
 

Pete Farrington

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It’s worth noting that the UL 248-14 spec fuses which are probably most commonly available in North America will blow at lower currents than the IEC 60127-2 spec fuses most commonly available in Europe (?).
So if the fuse rating is a T or F type (ie IEC spec), then a UL spec fuse with similar fusing characteristics will need to have a higher rating (eg value increased by 25-50%).
So a T4A mains fuse may ‘translate’ to SloBlo 5A, a T1A HT fuse to a SloBlo 1.5A.
See https://www.dalmura.com.au/static/Valve amp fusing.pdf with particular reference to the tables on p2.
 
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Update: This saga has a happy ending! I just got the Super Bass back from Jeff Bober in Md. and it sounds fantastic. Lush and sparkly, huge and growling when jumpered. As far as I understand he did little — and charged a very fair price. He switched out a component (replaced with vintage mustard cap), rebiased the amp, removed the previous tech’s cathode 2 mod, and put in a 4A mains fuse. Whatever it was, it worked! What a fantastic amp! Jeff told me the amp is “stupid clean” and he would have thought it was a reissue if he didn’t know better. So I felt totally vindicated in my belief that it’s totally worth all this trouble.
 

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XTRXTR

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Its a great beginning from a near death experience. Happy to hear you have a solid amp.
 

paul-e-mann

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Update: This saga has a happy ending! I just got the Super Bass back from Jeff Bober in Md. and it sounds fantastic. Lush and sparkly, huge and growling when jumpered. As far as I understand he did little — and charged a very fair price. He switched out a component (replaced with vintage mustard cap), rebiased the amp, removed the previous tech’s cathode 2 mod, and put in a 4A mains fuse. Whatever it was, it worked! What a fantastic amp! Jeff told me the amp is “stupid clean” and he would have thought it was a reissue if he didn’t know better. So I felt totally vindicated in my belief that it’s totally worth all this trouble.
Thats awesome! Jeff is awesome hes everything you say he is, he did a great job fixing my SLP too, I was very impressed with his knowledge and service! :yesway:
 

Gene Ballzz

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Update: This saga has a happy ending! I just got the Super Bass back from Jeff Bober in Md. and it sounds fantastic. Lush and sparkly, huge and growling when jumpered. As far as I understand he did little — and charged a very fair price. He switched out a component (replaced with vintage mustard cap), rebiased the amp, removed the previous tech’s cathode 2 mod, and put in a 4A mains fuse. Whatever it was, it worked! What a fantastic amp! Jeff told me the amp is “stupid clean” and he would have thought it was a reissue if he didn’t know better. So I felt totally vindicated in my belief that it’s totally worth all this trouble.

So nice to hear a happy story, in this age of a little less than happy stories!
HFAD,
Gene
 


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