planning for Marshall Amp mod/conversion

[BrianGWN]

I'm looking for some input/advice on what to consider, what may be important versus not so important, if I want to convert a Marshall 100 watt Super Bass to a Super Lead.
The amp in question started out life as a Super Bass, made in 1973 with a PCB, and good ole "Rosemary" had her hands in there during it's creation. It would have been a Marshall model 1992, in the Doyle History of Marshall book there are a couple of schematics of model 1992 on page 209, and this one seems to be a bit of a hybrid of those two schems, EL34 output tubes of the model 1992 but the funky extras around the presence control of the 1992 Mark II, with the .68 uF cap and 4.7K resistor.

I was given the general impression that the amp had been previously "converted" to a Super Lead but that seems rather dubious now that I have gone over the guts and checked various components. The pair of coupling caps between the phase inverter and the output tubes have been changed from 0.1 to 0.022 so that much is going in the Super Lead direction, but a whole lot is still left as Super Bass, such as the associated pair of resistors between the grid bias supply and the output tube grids, they are still the original 82K's but should be 220K. The whole bright versus regular channel response looks to be seriously in doubt, the coupling caps from the first stage are still both .022, while in a Super Lead they are .0022 versus .022, etc. I understand there are a few variations on the 100W Super Lead, the schematic I am referencing is the model 1959 at the top of page 205 in the Doyle book.

Also when I was first looking at the output stage area, things were not quite as I expected as I guess I am more used to thinking in terms of the Super Lead 50 such as the Marshall model 1987. For example in the 100W there some extras in there, some 1k power resistors connecting to the output tubes "screens". As well the bias circuit is a bit different because it looks like there is a separate lower voltage tap on the power supply transformer, the 1987 model doesn't have that. So maybe I'd be better to just ignore the Super Lead 50 details...

I guess at this point the fifty cent question would basically be whether anyone has some kind of hitchhiker's guide to the changes required to effectively turn a Super Bass 100 into a Super Lead 100. Is anyone aware of any schematics out there in computer land that might be particularly recommended? The ones I mentioned in the Doyle book have portions which are difficult to see clearly.

I am making a checklist of the various other "stuff" that "we" will also want to take care of, installing the good dual master volume, experimenting with moving the feedback wire from the 4 ohm to either the 8 or 16 ohm taps, experimenting with the value and/or installing a trimpot right on the circuit board for adjusting the feedback resistor (47K or 100K or whatever). Since this amp has the PCB I will have to temporarily remove the front panel pots from the chassis to be able to get at the solder side of the PCB, and I would rather not have to do this more than once , would want to do all required conversion/mod/upgrade work in one shot. Since the amp has already been fiddled with some, I don't think the owner or I have any concerns about diving in there and working it properly into Super Lead 100 shape.

I'm not sure exactly when the "surgery" will proceed, hopefully soon. Any input from any of the amp doctors around here will of course be greatly appreciated.

 

[Marshallhead]

BGWN-If you a brighter more aggressive sound switch it to the lead specs. I personally love the Bass model-mammoth push and its almost identical to the plexi circuit. If it is stock or slightly altered to lead specs it should sound incredible.

Try plexipalace.com if no one comes along to post.

I would think RICH would be able to fill you in no problem. He can field strip an old Marshall and turn it into a high gain monster with a blindfold on,3.00 in parts and a half hour so this should be a quick and easy mod for him to post for you.

Some of those Schems in the Doyle book are not the greatest to read and sometimes at Marshall it was a crapshoot with parts. Basically whatever was laying around they used.

 

[RICH]

(roadtrip..roadtrip...roadtrip)spin

Jeez, 1 thing at a time-

""but a whole lot is still left as Super Bass, such as the associated pair of resistors between the grid bias supply and the output tube grids, they are still the original 82K's but should be 220K. ""

if you're talking about the phase inverter area, on the tube side of the board there would be an 82K / 100K , and on the pot side there should be a 220K / 220K pair, so check this again to make sure THIS is whats going on. if those 220K's are indeed NOW 82K, I couldn't tell you why.


""The whole bright versus regular channel response looks to be seriously in doubt, the coupling caps from the first stage are still both .022, while in a Super Lead they are .0022 versus .022, etc.""

Only ONE of the .022s were switched to a .0022 on the lead model, and that was on the input 1 "bright" side of the first 12ax7, which would be the pin 6 side.


""funky extras around the presence control of the 1992 Mark II, with the .68 uF cap and 4.7K resistor. ""


If thats the case, the superleads were also done this way a little later. The pot values are different If I remember right. The "old" way would have a 5K presence pot, and I "think" they went to a 22K later ( my late 70's head was 22K). Either way, just make sure it's a 5K pot and wire it the old way with the .1 cap instead of the .68 and the 4.7K resistor. On the other hand, fred 58's head was like this, and we left it alone and it sounded good, but if you want to be a purist, wire that the old way. Change the feeback resistor to 100K, and screw the trim pot.

Hitchhikers guide-

Usually the bass models don't have the V1 wired split cathode style, so if you look at V1, you will see a wire jumping from pin 3 across the tube socket to I think pin 8. The wire probably leaves pin 3 and goes to an 820 ohm resistor to ground with a 250 mf (or 330 mf) cap in parallel. You have to cut the jumper wire running across the socket, and then run a new wire from pin 8 to a "new" 2.7 k ohm resistor to ground and put a .68 mf cap in parallel with it. Basically, you're going to do the same circut that comes off pin 3, but with different value parts. If you rewire the presence pot with the .1 mf cap, you'll have the .68 cap you need for this circut.

On V2 you will have to add another .68 mfd cap in parallel with the 820 (or 1K) cathode resistor

coming off the treble pot somewhere there is a 250pf cap, so you'll have to change that to a 500pf. There is another 250pf in there tied into the volume 1 pot too, and that has to be changed to 500pf.


the slope resistor "should" be 56K on the bass model, so it will have to be changed to 33K


Basically look at a schematic for a model 1959 super lead from 1970 and mimic the preamp exactly. there really is nothing to convert in the poweramp section, so unless something is wrong don't even f**k with it.

 

[Marshallhead]

See-look how easy that was. A snip here,a cap there,a resistor over that way-BAM Super lead

BTW You wanna see some nice wiring ck out RICHS' 18 watter:dude Its like a work of art.

I really need to learn electronics someday. I missed it is school because I took drafting instead. Look where that got me:lol

 

[RICH]

""I really need to learn electronics someday. I missed it is school because I took drafting instead. Look where that got me""


yeah, well i had 2 years of it in school. JUST as we were getting into transistors and tubes, the frigging teacher got some weird sickness and was out for 6 WEEKS!! We had a sub who knew NOTHING about electronics and was just a baby sitter. My station was in the back by the window, so I spent 6 weeks blowing 1 hitters out the window while the others sat around playing football with that stupid folded up paper thing, and what I REALLY wanted to know went up in smoke. yeah, there is a hole in my mind, and that gap in the knowledge still F*8ks with me.
:lol2

 

[Marshallhead]

:lol:lol:lol

I have an electronics course from 1972 my stepfather gave me-supposedly you build a stereo amp when done. Its got lab shit and all kinds of goodies. I should dive into that this winter-how bad can house voltage hurt?:lol2

 

[TedB]

...coming off the treble pot somewhere there is a 250pf cap, so you'll have to change that to a 500pf. There is another 250pf in there tied into the volume 1 pot too, and that has to be changed to 500pf.

Just a bit of clarification if it sounds confusing....

A Bass head will have no bright cap across the Ch I volume pot. Super Leads had a .005uF (5000pF) bright cap which basically eliminates all of the usable clean headroom. If you want a more usable range of clean without as much brightness, use the older Super Lead 100pF value.

 

[Scott64]

how bad can house voltage hurt?:lol2

If i recal....house is 110. its not too bad. i got a 220v shock once from a badly wired winch (wench, whatever...the thing with a steel cable that hauls stuff) in a home-made boathouse, and THAT hurt :dead:

P.S. it was a SOLID connectrion....wet skin to metal :wow

 

[Yelly]

Try www.schematicheaven.com

 

[RICH]

Hmmm, I didn't even bring up the bright cap, but there ARE two 250PF caps in there in the general area I mentioned that need to be 500pf. We're talking about BrianGWN here, so I can give him a condensed version, he has an exceptionally large brain, have you seen his posts?.:lol2 :lolspin

 

[BrianGWN]

Thanks all for the responses.
Marshalhead, you took drafting instead of electronics??? :lol Yep you are going to have to do something about that, I think you are going to have to do at least a little hands on mod to be truely worthy of using "Marshalhead" as a handle. If you got to either RICH's or the Dallas gathering I'm sure an amp would be available that you could "touch" the insides to earn your wings.

>RICH: We're talking about BrianGWN here, so I can give him a condensed version<
Hee hee, thanks for that quasi vote of confidence.

>on the tube side of the board there would be an 82K / 100K , and on the pot side there should be a 220K / 220K pair, so check this again to make sure THIS is whats going on. if those 220K's are indeed NOW 82K, I couldn't tell you why<
Yep that pot side pair of resistors are actually 82K not 220K so that is on the list of things that need to be changed. As to why those resistors are 82K, if you check the Super Bass schematic in the Doyle book it appears that is what they are supposed to be. In the schematics with the Super Bass the pair of coupling caps between the PI and output tubes are .1uF and those "grid bias supply/PI coupling cap load" resistors are 82K, while in the Super Lead the corresponding components are .022uF and 220K.

>TedB: A Bass head will have no bright cap across the Ch I volume pot<
Okay cool, once again the amp in question has already been fiddled with, think it has a 470pf added in there that is definitely not original. Thanks, I'll keep the 5000 versus 100 pf issue in mind.

Today I visited a shop that is authorized to do Marshall service, he had some of the blue LCR dual 50uf filter capacitors so I grabbed one of those just in case... After all the amp is 29 years old...

Okay so far so good, I'll have to review this thread when getting the final hit list together of all the required changes. I'll see about getting schematics maybe from one of the online sources.

Oh and Scott64 let me assure you that 120 VAC has plenty of bite if applied across most any body part, no need to go for 220V.

 

[Marshallhead]

BWGN- I do my own biasing and bright cap stuff and have done pots and grid resistors but thats about it. I wasn't even playing guitar in high school:lol


I am ready Jedi masters:lol2

 

[RICH]

Brian, I checked my schematics for the bass marshall, and it DOES show a pair of 82K resistors BUT only on the markII model, which means 6550s. If that amp has EL34s, then it shouldn't be 82K, but should be 220K like the schematic for the EL34 bass model shows. There is the model 1992 (EL34), and there is the model 1992 MarkII (6550).

 

[BrianGWN]

Marshallhead, sorry my misunderstanding I see then that you are already at least a level one Jedi.

>RICH: coming off the treble pot somewhere there is a 250pf cap, so you'll have to change that to a 500pf.<
Thanks, yes the cap that appears to affect the frequency response of the treble control in the tone stack looks to be 250 versus 500 pf in the Bass versus Lead. That is another example of what I was looking for, heads ups about the known differences that hopefully I would eventually notice if I went over the schematics carefully in detail comparing every component.

From the extras around the presence control and the 82K output tube grid resistor pair, I am starting to wonder whether this amp actually started out in life as a model 1992 Mark II and had EL34s stuck in at some later point. Then again maybe it is some kind of transitional mutant. Someone put some stick-on labels on the chassis indicating the tube types, the type where it is a strip that you put in a kind of hand held letter/number punch device. The labels are "ECC83" and "EL34", I would assume that is not original.
By the way that factory label on the chassis with Rosemary in there also has CANADA visable somewhere so maybe Canada was the guinea pig for some of the early PCB style construction amps in 1973.

 

[Marshallhead]

Not even level one yet...

Hey Brian-I'm still getting my ass kicked with that ball of death Luke had on the Millenium Falcon:lol:lol2:lolspin

 

[Sean]

BG, can I make a suggestion?

I agree with Marshallhead's point about bass heads. They rock in their own right. When I got my '72 bass 50w, I commented that it seriously sounded a bit like EVH. TedB pointed out that aside from like one resistor or something, it was a plexi just like the EVH one

Leads are too bright IMO, and the bass has more colour and depth. So it depends what you want from the amp.

My suggestion is, do all the changes one at a time. Like, when I get my half-stack back (long story) I'm going to boost the gain by changing the slope resistor. I already did the neg. feedback resistor, and one or two other things. Oh yeah, the RICH master first and foremost.

My goal is to keep that too-trebley shit out of the amp. You can't dial that out from the guitar or the amp controls once it's in there.

What did the amp sound like with the RICH master in there? :dude

 

[Canuck Lefty]

Sean, if he ever gets it done I'll let you know:lolspin

 

[BrianGWN]

Yes, CanuckL is correct in that we haven't actually got to doing the mods yet. We don't exactly live that close together and time has been limited, also CanuckL has been hosting a visitor from out of the country, etc..., but the main issue/problem is that since the amp is a PCB type construction we agreed we really did not want to be taking this beast apart multiple times (gonna have to remove the front panel pots), and most importantly there are some parts we still need to get. The amp had previously been fiddled with and I think some of the work and replacement parts in there are suspect (to try to get it closer to a Super Lead versus a Super Bass, ie. the previous mod to convert V1 to a split cathode was rather kludgy). I still need to locate some decent 500pf caps.

Once we have everything we might need in terms of parts, we will then likely schedule a full session (like maybe the better portion of a weekend day) to do all the required surgery and mods in one blitz.
We will keep in mind your comments of the Super Lead setup maybe being just a bit too much trebly. From a technical point of view looking at some of what is done in the schematic I might have to agree that could be the end result. We will probably first get things tidyed up in there and then maybe compromise and make a few adjustments so that the bright channel is not so over the top, ultimately it's CanuckL's call... Stay tuned...

 

[RICH]

Hey BrianGWN, and canuck lefty. Keep in mind that ALL of the amps you heard in the deathwall that you were freaking on were superleads, and some were superleads with gain mods, meaning that one or two of them actually have two lead channels jumped together. Basically they are the farthest things from bass marshalls. If you venture off the path and expect the head to sound like what you heard here, it isn't going to, especially if you are going to use the master that I do. In other words, if you like what you heard here, don't screw with the ingredients.spin

 

[Marshallhead]

Im with RICH on that. His amps are SL circuits tweaked. The bass amps have nowhere near the gain in the pre stage and really dont sound great(except for clean tones) until the power tubes are smashed into submission. SBs are great with pedals IMHO or wound up to Jesus volume