Wolfeman: got a tip for me? (Amp Tech)

[Lasse Lammert]

glad you seem to be getting somewhere. why not try another 33k over the existing one?

before you go to far with modding the stack; there seems to be a lot of components in there.
you sure there is not some kind of "voicing" switch on that amp?
can you find out more about the mod first?

what about posting some clips with the stack set to midnight and then various other setting so we can hear what it's doing?

what resistor is the "load resistor"?

yeah, am gonna try another 33k paralleled tomoz.



There's very little info about the mod, it seems like there are only a handful of these around (I think it's the one AIC used on facelift), it looks a bit confusing in there to say the least, it's got an entire new channel with it's own PCB and 3 12ax7 (so 6 in total in this amp), as well as another little PCB with the tone stack on it


the clean/blues channel sounds very thick, round and warm, the high gain channel has extreme amounts of gain and a second master volume which seems to be after the first master and only active in the dist channel.
that way you can use the master to get more/a different kind of distortion (phase inverter?) and then bring the volume down again with the pot after that (not sure where it really is in the circuit, just decribing what I'm hearing here)....or you could use that second master to bring down the volume of the dist channel to match it to the volume of the clean/crunch channel.



by "load R" I mean the one that is R5 in this schem

I can only assume that this must be one of the very first of those Marshall mods

 

[KHE]

this isn`t the normal marshall-tonestack. It`s copied from the marshall silver jubilee 2550 amp.
http://www.amparchives.com/Amp Arch...Jubilee Series/2555 100W-50W Lead/2555amp.gif
so the TSC-Programm is pretty useless

i will have a closer look tomorrow and try to help

cheers!

 

[Lasse Lammert]

That silver jubilee looks indeed much more like my tone stack!
(minus the fact that mine had a linear mid pot and that the mid pot in mine was pretty much a total pain to use

Bogner did this mod in 89 I think, when did the 2555 come out?

 

[KHE]

~1987 => http://www.drtube.com/marshall.htm

edit: some values are different in the bogner-mod, i will check this later...

 

[Lasse Lammert]

AH, what a cheeky bastard bogner was

 

[Lasse Lammert]

In the 2550 Tonestack the 100k mid pot is log, mine is linear.
I kinda feel like I need a log to stretch what's between 0 and 1 now to 0-8 or something and compress what's now 2-10 to 8-10 or something.
But you say a log would make it even worse?

 

[PaoloJM]

In the 2550 Tonestack the 100k mid pot is log, mine is linear.
I kinda feel like I need a log to stretch what's between 0 and 1 now to 0-8 or something and compress what's now 2-10 to 8-10 or something.
But you say a log would make it even worse?

I said a log pot would make it worse in reference to a standard Marshall or Fender stack.
The stack in your amp is not standard. I'm not familiar with it at all.
If it is supposed to be log then it can't hurt to try.
k.h.e. seems to know this stack better than I do.

 

[KHE]

unfortunalety i have no idea how this tone-stack sounds.
but the sound is also very interactive with the entire preamp-design.

one suggestion: you can try to lower the value of the 500pF-cap (the one between the two resistors near the mid-pot) to say 220pF like the original 2250.
this will lower some of the high-mids and will make the mid-cut a bit wider.
but i i have no idea if it sounds good, because frequency-simulations won`t tell me that

 

[Lasse Lammert]

thanks.

I'm uploading a video right now (upload finished in 90 mins)....
I'm loving the sound of the amp, I just wish the taper on those pots wouldn't be that fucked (you're gonna see it in the vid I think).

I'm gonna try a log pot in 2-3 days (just ordered one).


There aren't too many high mids but a pretty prominent honk between 400 and 1k or something as soon as I set the mid pot above 2.5-3

it's not that the amp isn't usable or anything,a s I said, it sounds fantastic, it'd just be much easier if the pots had a different taper.

Anyways, I'll post the video in a bit

 

[Lasse Lammert]

what I've found out meanwhile (nice guy on RT has a similar amp): this mod is called "snorkeler mod" with an additional "cantrell mod"...
this seems to be the same amp (well, same model and mod etc) that's been used on AIC Facelift.
and I gotta say it nails those tones perfectly

 

[Lasse Lammert]

putting in a log mid put fixed it!

...and the treble pot was log where it should have been lin

 

[Lasse Lammert]

Damn, changing the treb and mid pots fixed ALL the issues...
now it sounds kickass with all the dials on 12:00 and everything is much more controllable!

 

[::XeS::]

Congrats dude! It's very pleasant when a DIY mod sounds good

 

[sprack]

Awesome. The clip of this monster is badass.

putting in a log mid put fixed it!

...and the treble pot was log where it should have been lin

 

[Cryo114]

Speaking of amp mods, Lasse. You mentioned in a thread a long while back about a jcm900 mod guide you had somewhere. You didn't by any chance find it did you?

Thought I'd ask in a current thread rather than bump that dinosaur thread.

 

[greyskull]

Tidy! sounds beastly mate!
Can i Hi-jack this thread a little as there are loads of people in here who MAY know the answer...
My 78 Park 2203 Is a little fucked right now, The Hi-input is completely dead.

When I bought it, Internally it was a total mess, but was working.... okay; I played with it in that way for a while and all was well...

After a while the amp started developing a bizarre sounding crackling/buzzing; and I just shipped it straight off to Marshall In Milton Keynes who did the following...

Fit 5w 1k resistors to output valve bases
Replaced mains socket
Replaced Bias Capacitors
Replaced all pots
Replaced Input/Speaker sockets
Replaced Dead smoothing block and removes incorrectly fitted caps on main PCB
Replaced output valves
Replaced V1 valve base
Replace pre-amp screening cans
Rewired mains transformer for 240 v ( that was quite worrying!)
Replaced Mains and stand by switches
and tidied up wiring/ past repairs

Since then the issue recurred with The high input buzzing and crackling randomly, especially after being on for a while, losing body and eventually now it's faded to nothing.

The low input still works perfectly, understandably as it's a cascaded input; so that leads me to believe that theres an issue with the pre-amp section relating to V1.

Here's what I've tried so far to rectify the issue.

Swapped over Pre-amp Valves with known working ones in case it's one side of the valve that's broken,

Replaced C3 (0.22)

Checked the leaf spring on the low input, and cleaned it with contact cleaner just in case even though it's new.

Checked soldering on V1 base, although it IS a new base


I'm fresh out of ideas! if you could help I'd be eternally greatful.

Here's the gut's post Marshall Fixing, Yes... it's enormous.

 

[KHE]

hmm, hi-input-jack is ok?

 

[greyskull]

The jack is fine, but plugging into it, you get NOTHING

 

[sprack]

Pull either the 1M or 68k resistors on the hi input and how do you still have the issue?

 

[greyskull]

quoi?