HELP ME SAVE 74 SUPERLEAD!!

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heckitar66
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HELP ME SAVE 74 SUPERLEAD!!

Post by heckitar66 » Sat Oct 17, 2015 12:47 pm

Guys, my son bought his first Marshall, it's not in great shape and it's had some things done to it.

- 3 prong cord added
- cross phase type master volume
- impedance selector switch replaced
- changed from 6550 to El34
- bright cap removed
- all filter caps replaced with f&t dual 50 uf

The amp sounds pretty good clean but as you turn up the preamp volume it gets too distorted and very compressed. Sounds like fuzz pedal. Plenty of volume just fuzzy. It starts with the preamp volume at about 3 and gets worse from there.

It's been biased properly, I've tried many different preamp tubes and a different set of power tubes. Removing the mv didn't make a difference. I've taken it to two techs who have only told me its working properly and returned it to me. I've played Marshall amps for 30 years and own the best sounding '78 jmp I've ever heard. This sound is not cool.
Any suggestions? A place to start? Any help would be immensely appreciated.
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neikeel
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Re: HELP ME SAVE 74 SUPERLEAD!!

Post by neikeel » Sat Oct 17, 2015 3:35 pm

The pictures you have posted are not the clearest.

From a distance the board looks broadly correct. I would be tempted to remove the PPIMV in the first instance.

Looks like really weird around the filter cap holes, as if someone has welded the chassis or packed the caps off the chassis with card or paxolin??

Do you find any difference between plugging into channel 1 and channel 2.

This era of Superlead in good order is a pretty fearsome beast and every bit as lively as your 2203.

Try some nice clear pics of the chassis and board and some well lit close ups of the filter cap area :thumbsup:
Neil

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Doug H
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Re: HELP ME SAVE 74 SUPERLEAD!!

Post by Doug H » Sat Oct 17, 2015 3:57 pm

Could start by pulling the death cap, or if that isn't the cap of death I'd like to know what it is.

Do the preamp sockets seem clean and tight?'

that's quite a mess on one end of the output caps, but doesn't seem like it could be the problem you are describing.

heckitar66
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Re: HELP ME SAVE 74 SUPERLEAD!!

Post by heckitar66 » Sat Oct 17, 2015 4:54 pm

Thank you so much for taking an interest in this. Neil, I hope these pics help. The original mains filter caps apparently used a different mount. Whoever changed them just used silicone to hold them in place. Those are the ones that look like cardboard. I ordered two new clamps and will install them as soon as they get here.

Doug, which one are calling the death cap? I'm thinking it's one of the ones Neil is questioning. The wiring around the replacement impedance selector is definitely on my radar.

The sockets seem clean but I'll clean them tonight just to be sure.

Also, I have tried removing the master volume and it didn't seem to make a difference but I'll remove it anyway because I don't think it's a particularly good mv.

The two channels sound different like they're supposed to but still share the same problem. It's just more pronounced in channel 1.
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sixpakldp
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Re: HELP ME SAVE 74 SUPERLEAD!!

Post by sixpakldp » Sun Oct 18, 2015 12:22 am

If the bright channel seems to be worse than the normal channel, I'd start looking at the pre amp section of the amp. It looks like some of the solder joints on the pots have been re connected. As a good starting point I'd compare and check the wiring against a layout just to be sure nothing is amiss. I'd check continuity of the resistors and caps, especially in the preamp section next, just to ensure that isn't it. At the same time, I'd go through the resistors and mark down that the are the correct values so you know that is good. They look ok from the photos but it could be easy to miss without the amp in front of me. Next I'd check for loose connections just to make sure that the connections are sound. If everything seems correct there and nothing gets better, I'd think about looking at the electrolytic caps and if hey are original to the amp, replace those.

Try that as a starting point. Many times if I'm stuck with an amp and just can't figure out what's wrong, going through the basics seems to help or sometimes expose the issue. If bias is correct and the amp is fizzy I'd guess the issue is in the pre amp section somewhere. Another thing I would try is pulling two tubes and run on both the inside pair and outside pair to see if there is any difference there. I doubt it's the power section but it can't hurt to try and rule out that issue.

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neikeel
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Re: HELP ME SAVE 74 SUPERLEAD!!

Post by neikeel » Sun Oct 18, 2015 11:29 am

Those pics are a help.

I agree with the above post.

The input jacks look ok (some cleaner/lube on the contacts doesn't go amiss)
I think the solder joints affected on the vol 1 pot is where the 5nF cap was removed.
All the B+ line 10k resistors were replaced, presumably like for like (73/74 used cc) but I would have used MO on a new amp or found some Pihers for that amp.
The rest of the board looks ok but I would check the soldering on the 220k grid lead resistors and the bias feeder as the solder is a bit blobby (you can flip the board up without desoldering anything by removing the input jack chrome nuts and the pots).

The death cap referred to is the black (Sprague - I think) cap on the polarity switch. I personally would be removing all the polarity cap wiring when I tidied up the loom, you don't need it now you have L/N/E input wiring.

I've never seen those white rectifier diodes before but they do look original tho'.

I would also shorten most of the wires to the preamp valves and dress them for least noise.

Basically clean it up and remove the crap, making decent joints with good clean lugs and wires. F&T are usually pretty good reliable filter caps

Keep us posted!
Neil

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Re: HELP ME SAVE 74 SUPERLEAD!!

Post by Haze13 » Sun Oct 18, 2015 12:16 pm

Fuzzy sound and too much gain on lower volumes can be caused by disconnected feedback... I've seen this on a Twin Reverb.

heckitar66
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Re: HELP ME SAVE 74 SUPERLEAD!!

Post by heckitar66 » Mon Oct 19, 2015 5:24 pm

Thank you! I'll be checking the the things neikeel and sixpakldp mentioned for for the next couple of days as well as removing the death cap and mv. I'll post again when that's all done. Also haze13, the problem isn't so much fizziness but a ton of gain like the tubes are over saturated. Maybe that description helps?

Fingers crossed and thanks again!
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Doug H
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Re: HELP ME SAVE 74 SUPERLEAD!!

Post by Doug H » Mon Oct 19, 2015 6:08 pm

might be an idea to quickly rule out those fly back shunt diodes, I think that's what they are, the diodes on the PT socket. Can;t imagine them causing a problem, but you never know I suppose.

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Doug H
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Re: HELP ME SAVE 74 SUPERLEAD!!

Post by Doug H » Mon Oct 19, 2015 6:13 pm

The question I'd like to know the answer to is what's up with those funny looking resistors connected to the PI grids? Are they original? They look like some CC ones I used to have, I've never seen an old marshal with those in it before. If they are replacements and the values are screwy that might be causing something like your issue. I can't really read them from the pic.

heckitar66
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Re: HELP ME SAVE 74 SUPERLEAD!!

Post by heckitar66 » Mon Oct 19, 2015 8:30 pm

Doug H, I'll remove the flyback shunt diodes first since that's easy and quick. I'll let you know about those residue as soon as I pull the board.
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Doug H
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Re: HELP ME SAVE 74 SUPERLEAD!!

Post by Doug H » Mon Oct 19, 2015 9:45 pm

doing the full monty, probably the best idea since you know there's been at least some mods done on it.

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neikeel
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Re: HELP ME SAVE 74 SUPERLEAD!!

Post by neikeel » Tue Oct 20, 2015 3:45 am

Maybe I am not seeing al the pics you can see but I see no flyback diodes on this amp PT. Usually see flyback diodes on OT.
Neil

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Doug H
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Re: HELP ME SAVE 74 SUPERLEAD!!

Post by Doug H » Tue Oct 20, 2015 12:31 pm

I think that's what they are, I know of no other reason for diodes on a PT socket. They sort of look to be in the right place for plate to ground, but it's been a while since I was inside a big amp.

heckitar66
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Re: HELP ME SAVE 74 SUPERLEAD!!

Post by heckitar66 » Tue Oct 20, 2015 1:01 pm

Sorry, guys. I thought Doug H was referring to the diodes on v8. I removed those, removed the death cap and master volume and I'm in the process of shortening the wires going to v1 and v2. I've also ordered a new impedence selector switch so I can tidy up that area as well. Is there a way to test diodes without a diode tester?

I peeked under the board last night and everything looks okay to me. Neikeel, good catch on the 10k resistors. They've definitely been replaced.

Doug H, those resistors you mentioned seem to be original if I'm looking at the right ones. We're you talking about the 1megs off the PI?
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