Any good mod for a 2550 Marshall?

[JC@]

I have one of these to check and maybe to mod, but except for the clean, the whole amp sucks to me. New tubes, proper bias, but the thing only sounds acceptable on the clean setting. The rest of the settings is like adding a low gain fuzz. And looking at the schem that is what I think there is.
I plan to use the effects loop more like an extra gain stage.
Does any one have mod to share or comment, please?

Thanks,
JC

[bulldogguitars]

Have you checked out fja mods. I think he has some mods for the 2550.

[JC@]

I meant to say, something I could do myself.

[Satamax]

http://music-electronics-forum.com/showthread.php?t=223

[Matt T.]

That amp should sound the opposite. The clean channel should suck and the lead channel and rhythm clip mode should sound good. There shouldn't be any fuzz tones.

The only way to use the effects loop as a boost would be to either put a boost in the loop or to reduce the massive attenuation just prior to the Send jack. But if you do the latter then the signal will clip most things you put in the loop. A Send Level pot would work. The Return stage looks to be pretty much maxed out although I'm curious why they would use a 220K plate load resistor and an unbypassed 470R cathode resistor instead of bypassing the cathode resistor and (perhaps) using different plate and cathode resistor values. Maybe they wanted some compression (these amps are somewhat compressed)? Reduced bandwidth?

[Chris / CMW amps]

Replacing the OT and choke with some fine plexi replica's does improve the sound. Did help a buddy tech with such amp and the whole amp did sound nicer, tighter and the high-end was way more pleasing to the ears.

Hope this helps,

[Ray Ivers]

Matt,

By leaving the return stage Rk unbypassed they could have been looking to increase return-stage headroom, for occasions when a too-hot return level was encountered at the return jack - and they may have used a 220K Rp to ensure they still had sufficient recovery gain available for pedals.

Ray

[Matt T.]

Thanks Ray. I thought there had to be a reason since they chose a combination of values that I haven't seen before (not that that means much).

Chris,
That surprises me about the trannies. I replaced the stock OT in my 2555 with a MM 100W Plexi OT and it sounded almost exactly the same and if anything...the original OT sounded a tiny bit better (I attributed the 'tiny bit' to other human factors though). I verified this when I swapped the original OT back in. I then used the MM in another amp and it sounded stellar. So I think the stock OT in the 100 watters is fine.

[mocklah]

what serial# range is yours? mine is early 600's and is a little different from later ones. someone on another forum helped me mod my amp to be more like a later serial# jubilee. sounds awsome now.
it's seems sorta messed up to just send you to another forum (it's down right now anyway), so the next chance i get, i'll see if the guy who helped me, minds if i re-post it here.

-mike

[Chris / CMW amps]

Thanks Ray. I thought there had to be a reason since they chose a combination of values that I haven't seen before (not that that means much).

Chris,
That surprises me about the trannies. I replaced the stock OT in my 2555 with a MM 100W Plexi OT and it sounded almost exactly the same and if anything...the original OT sounded a tiny bit better (I attributed the 'tiny bit' to other human factors though). I verified this when I swapped the original OT back in. I then used the MM in another amp and it sounded stellar. So I think the stock OT in the 100 watters is fine.

Strange, I always experience better results with MM OTs in post '68 Marshalls. Preamp/circuit has a huge influence but every customer over here did really like the OT and choke upgrade.