What's a Marshall Silver Jubilee supposed to look like inside?

[WaltC]

I'm working on a customer's 25/50 Marshall Silver Jubilee trying to quiet some 120hz hum and clean it up for extended use. Got most of the hum taken care of by tightening and/or re-soldering all the ground connections and cleaning up the lead dress. Still some hum but not nearly as bad as it was. But....<G>

Looking at the inside of this (my second in ten years) some of the stuff looks kludged and or like it might have been added on by someone else. There are two block-style polyester caps silicone sealed to the chassis and connected by flying leads back to the pc-board, a couple of old Fender-looking blue turd caps have been added as well as a couple of other poly caps earlier in the preamp section of the board.

Both of the push-pull pots are scratchy and the master vol pot causes a loud hum when turned all the way off (got replacements on order). Plus the channel switching push-pull pot *seems* to work backwards, i.e. pull out for "Rhythm" (led off) and push in for lead (led on). Is this right?

Any of you have any pictures of what the insides is *supposed* to look like you'd care to share?

And finally----

Is it just me but what's the deal with this giant chassis, and that teenie little pc-board with all the pc-board mounted pots and jacks. There enough room left over inside this chassis to put in three Champs and a BiasRite!<G>

The amp's playable now, but I'd like to get it as close to 100% as possible, if possible.

Thanks,

[alltone]

Have you tried looking here http://forum.metroamp.com/ for help with a schematic??

[michael patrick]

Walt,

I sent you a PM with a link to a pic. The pic was too big to post here... but because its big, it shows a good amount of detail.

[WaltC]

thanks guys for the help...

Mike, the pic you sent verified my suspicions , the amp has been seriously hacked, most of the coupling caps swapped out for non-stock, most much larger in size and so had to be either flown over the board or (a couple) silicone-sealed to the chassis with flying leads to the pc-board, 2K worth of screen resistors for each tube, extra ground wires and the rhythm/lead push-pull pot reversed.

I'll leave it up to the owner as to how much surgery he wants done, the amps working pretty well now and sounds ok...

Thanks again,

[mikey]

The push-pull pot is correct. Pulled out it is "rhythm", pushed in is "lead".

[michael patrick]

Cool, glad it helped...

[AintNoEddie]

Note that there were several versions/revisions of the pcb/the wiring. When I tried to reverse my hacked 2550 I found at least 3 different schematics. If you need some more pictures drop me a line, though I have no complete pcb picture I think. http://wishingwell.lim-music.net/stuff/scratchchin.gif

[WaltC]

Thanks ANE, the pics I can live without at this point, but if you've got the schematics handy I could use those...you could attach them to an email if you've got 'em and I'd much appreciate it.

Thanks everyone, lots of help as usual!

[AintNoEddie]

Hey Walt,

I was trying to send you an eMail but I got back an error (...not allowed...). I'll try a different mail address of mine.

Cheers,
Mike

[Adwex]

Here's mine...it's the 100 watt version, model 2555. Not sure if the pic is helpful, but here's the schematics too:


http://i32.tinypic.com/rkuc8p.jpg
http://i28.tinypic.com/35i4u13.gif
http://i25.tinypic.com/xfuhjs.gif

Adam

[WaltC]

Man, I wish my customer's Jube looked as good as that one inside... but we're working on it...<G>

[JerryP]

The push-pull pot is correct. Pulled out it is "rhythm", pushed in is "lead".

The different versions switch differently. I forget which is which, but they flipped the switching at some point.
Jerry

[Lyle Caldwell]

The ones I've worked on looked like Adwex's. The hum with the volume all the way down might be a bad pot but it might also be a ground loop - especially with suspect mods in the amp.

The stock circuit won't let you bias EL34s near hot enough - not even 60%. You'll want to change R23 - I think I used a 33K or a 27K, which gave the bias adjustment a good margin to really make some EL34s come alive.

Much better sounding amp at that point, though there are still huge mistakes (IMO) in that and in all JCM800s. Really bright fizzy amps without much real sustain or low end. But then that was the style around '87.

Go ahead and put some 1 ohm bias resistors on the output tubes while you're in there.

[WaltC]

thanks Lyle for the info. There are two added "ground wires" that are questionable and one was loose and part of the original hum problem. Your comments also reminded me to check for leaking coupling caps that may have been added in the pre-amp section that may be putting DC on the other push-pull pot and causing the noise on it as well.

I'll let you guys know when I've finished what I come up with.

Thanks again to all...<G>!

[salev]

Hey everyone, thanks for all the help on the Jubilee, especially the schematics. I'm the customer who brought Walt the "FUBAR" 2550 to try and rejuvenate. Got a couple that sound really good, but this one arrived with a lot of mods the previous owner doesn't know anything about. Not sure the guy that did them knows anything about them, either. I trust WaltC and his advice completely and know I'll get an amp back that may sound different, but it will sound really good!

Cheers,

[WaltC]

As Selim said, this amp (as I've recently found out, thanks guys) bears pretty much *no* resemblance to a stock 2550. I've taken out some of the over-the-top crunch mods and the amp sounds pretty good at this point but still breaks up more than stock in the clean/rhythm mode. Short of completely re-building it this may be as close as I can get it at this point.

The two push-pull pots are definitely shot and, as soon as the back-ordered parts come in <G> I'll be replacing them and we'll see what we really have here.

Hey Selim, need a '58 custom shop VOS chambered LP? I know where you can get one pretty cheap <G>!

[salev]

Walt, not fair. You know I'm a sucker for '50s Les Pauls, so let's talk. BTW just got a CS '57 Gold Top.

Wanna trade the '58 for a "FUBAR" 2550 jube that was partially de-modded by no other than the master himself, the famous "Walt C"?!? :crazy

Selim