1973 Super Lead 100 Blowing Main Fuse and Power Tubes

[Mkjames]

Hello All,

I'm new to the forum but so thanks in advance for any help. I should also say that I can find my way around the inside of an amp, however, I am a hobbyist. This is my second Marshall, my first was JCM900 in the early 90's which was sold long before I new anything about repair or modification.

What I've got is a 1973 Super Lead 100. I picked it up in poor condition and not working. At the time it had a blown fuse and showed signs of someone modifying the output jacks. The modification appeared to be two additional speaker jacks and the transformer wired to all the jacks in a fashion that completely bypassed the Ohms selector.

I brought the amp to a local tech that I've used for years and is known for his work on older Marshalls and Fenders among others. I supplied him with new caps for everything except for the filter caps and caps and resistors connected to the tube sockets. These I changed out myself. All caps were either Dijons or TAD.

When I picked up the amp he had it on the bench with an old radio playing through it and it sounded great. An hour later at home, I plugged it in to my 8 ohm V30 cabinet. The first thing I noticed was that at 0 volume on the dial the amp had a good amount of volume, though I could hear my voice over the amp. As I turned up the volume to breakup, the amp sounded great and then blew the 4 amp Main Fuse and one of the power tubes.

I brought the amp back in and he looked it over. I suspected that there might be a carbon buildup on the power sockets and mentioned it to him. He changed a socket and the corresponding tube that he said was blown. Again, it sounded great on his bench. He turned it to a medium volume as well without issue. When I got back to my place and hooked it up I had another blown fuse within seconds as I gave it about 50% volume.

This time I pulled the head out and tested every tube on a test meter. I found the only bad tube to be a preamp tube (Mullard ECC83) in the outer most position. ALL other tubes tested at about 83% +/-1%. I cleaned the all tube sockets with a toothbrush, Deoxit and canned air to dry things out and blow any debris from the sockets. When all was dry, I reinstalled the tubes, a new 12AX7 in the outer position, and replaced the 4 Amp Main Fuse. Upon startup, the amp seemed to be fine. I let it warm up for a few minutes and started playing my Les Paul through it. Again I noticed that at 0 there was significant volume. As I bumped the volume up to about 25% it got nice and punchy, but clean. As soon as I got to about 50% the sound quickly degraded, I saw what looked like lightning in one of the center tubes and the amp went silent. I turned the amp off but noticed that the fuse did not blow this time. This may be because I used a Slo-Blow fuse. I'm not sure what was in there before. Also, the fuse that blew was the replacement fuse that the tech put in there when I brought it back to him. The socket, however, was one of the original sockets, not the socket the tech replaced.

I don't want to bring this amp to another tech. I want to fix it myself and learn what might be going on here. Maybe it's some sort of high frequency oscillation or a transformer issue, I'm not sure. If anyone has experience with similar issues or knowlege trouble shooting I'm eager to hear from you.

Thanks
Mark

[Wildone]

I would start by trying to find the schematic for the amp. There is a section here on the forum where you can download them. From there try to find out what was modified because more then likely that is where you problem may be. Can't offer you any more technical help then that.

[ClassicRock88]

Any chance that this is a 6550 amp? I am not a tech and I don't know if EL34's in a 6550 amp would cause what you are seeing but it might be something to check out. This would be a very late '73 because of the PCB, correct? They might have started using 6550's in the US export amps by then.

[Mkjames]

I would start by trying to find the schematic for the amp. There is a section here on the forum where you can download them. From there try to find out what was modified because more then likely that is where you problem may be. Can't offer you any more technical help then that.

I'm having trouble finding schematics and/or a layout that matches. Even photos from that year seem to be a little different. I found a couple 1959 layouts from metroamps but they were different as well. I put a call into Kevin at US Music Corp to see if they have anything tucked away in an old file cabinet.