Marshall Mods - What are your Favourite Mods?

[Chuck H]

Did you also change the bias feed resistors to 220k?

No. The only change was to the bias supply. And even that was a really rough mod. The common procedure was to simply tack a resistor across the existing shunt resistor for the bias supply voltage divider. Not at all ideal for a PCB amp. I actually learned it here on the forum soon after subscribing over twenty five years ago. Like I said, I was green then.

[Chuck H]

6550's are not a true "audio" tube, originally designed by General Electric for servo amps in B-52 Bombers.
Marshalls UK JMP amp exports fitted with EL-34's weren't surviving shipping by sea arriving in the USA with busted power tubes.
Marshalls USA distributor Unicord came up with the idea of fitting USA JMP's with 6550's here. [in US].
Ive had both EL-34 & 6550 Marshalls. The 6550's are brutal and don't breakup at all.
50 watt amps w/a pair of 6550's work pretty good but a 100 watt with 6550's is just too much power unless you play Wembley stadium.
In my experience guitar amps always sound better with EL-34's because the midrange just turns to butter the more you crank it.

I think we all know the story behind the Marshall switch to 6550's for exports. Essentially you've summed it up. But there's more to a tube than how it performs in any one circumstance. A proper bias for those tubes and maybe some other modifications to the PI or otherwise to control drive voltage may have demonstrated much better results for Marshall with 6550 tubes. 6550's have demonstrated themselves an respectable audio tubes in countless other amps after all. But to judge them based on this one circumstance isn't fair. As far as I know the circuit adjustments were minimal and there were no changes to the transformers. Meaning the transformers were idealized for el34's in these amps and then the 6550 was "plugged in" as a band aid for problems in the north American market. In this light it's hard to asses that the tube model was the problem. Not to say that el34's don't sound great. Of course they do. But let's give the 6550's a fair trial before convictions. I (personally) can't say that I ever have. But I'm open to it. Especially considering the nature of modern el34's available.

[Enzo]

A lot of tubes were intended for other use. There is a whole fandom of dual triodes that began life as computer tubes. Remember the very first SVTs? Used 6146B tubes. Those were designed to be modulator tubes for high power radio transmitters. You never know when a "wrong" tube works out to be cool.

[EL-34/JMP]

We all have out Opine's don't we ?
In my 45 years of JMP experience if you're looking for that warm upper midrange bark of a 60's/70's JMP look no further than EL-34's.
For metal & EMG pup drop tune chuga stuff the 6550's hold together & cut pretty nice but don't break up and can be somewhat sterile sounding.
YMMV.

[Chuck H]

We all have out Opine's don't we ?
In my 45 years of JMP experience if you're looking for that warm upper midrange bark of a 60's/70's JMP look no further than EL-34's.
For metal & EMG pup drop tune chuga stuff the 6550's hold together & cut pretty nice but don't break up and can be somewhat sterile sounding.
YMMV.

Well I don't know that anyone is expressing "opinions". What I see is observations and facts. My own experience with 6550's is similar enough to yours that I wouldn't say we're in disagreement. All I pointed out was that the 6550 tube, as implemented by Marshall, probably never got a fair shake as an overdrive tube because of circuit incompatibilities. I think that the overwhelming opinion that the el34's sound better in Marshall amps is correct (with metal tones perhaps an exception?). But that doesn't mean 6550's couldn't be made to sound good also in a circuit more idealized for that goal.

EDIT: Well I just spent some time looking at the data sheets and what I see is that the way 6550's were implemented in the Marshall amps they were simply not driven hard enough to clip as much as el34's do in those amps. So if a few changes had been made in that regard maybe guitarists would have a different opinion about the 6550's

[nosaj]

Well I don't know that anyone is expressing "opinions". What I see is observations and facts. My own experience with 6550's is similar enough to yours that I wouldn't say we're in disagreement. All I pointed out was that the 6550 tube, as implemented by Marshall, probably never got a fair shake as an overdrive tube because of circuit incompatibilities. I think that the overwhelming opinion that the el34's sound better in Marshall amps is correct (with metal tones perhaps an exception?). But that doesn't mean 6550's couldn't be made to sound good also in a circuit more idealized for that goal.

EDIT: Well I just spent some time looking at the data sheets and what I see is that the way 6550's were implemented in the Marshall amps they were simply not driven hard enough to clip as much as el34's do in those amps. So if a few changes had been made in that regard maybe guitarists would have a different opinion about the 6550's

It's all about setting that governer.(or idle)
nosaj

[EL-34/JMP]

EL-34 is a 25 watts output.
6550 is a 35 watt output.
A 100 watt Marshall with EL-34's can have a peak watts of 175.
Most 100 watt Marshalls i have seem to find the sweet spot between 4 & 6 on the master.
By contrast the JMP 2203 i had with 6550's was outstanding in every way but i could never get the master past 3.5 w/o suffering hearing loss or killing small animals.
Does not mean that amp didn't sound great. It totally did buy there was no way i could maximize its potential w/o modding the amp and it was such a pristine specimen i didn't want to do that so i sold it as is.
The JMP 2204 [50 watt] w/a pair of 6550's i tour with in the 80's ruptured my right eardrum and was plenty loud, also sounded great.
At this point im just trying to preserve what hearing i have left.
I still play with two 100 watt Marshalls both EL-34's. They do the job well.

[Helmholtz]

I asked for Chuck's recommendations regarding technical mods.
Of course that will be based on personal experience and preference.

Please go on Chuck.

[Chuck H]

Please go on Chuck.

Haha. Well, ok.

The Marshalls I had in my arsenal were the two JMP's *(see note), a 1959SLP plexi reissue from the early 90's and a JCM900 2100. We'll skip the 2100

Mods I made to the JMP's were intended for more gain. I wanted that hot rodded sound then and I was using the master volume so all of my mods were to the preamp. I'll post a denoted Marshall schematic to show the changes I ended up with.

*Note regarding the JMP's: There are two 2204 schematics. One for the JMP amps that has the parallel input channels and one for the JCM800 amps that has the channel stack via the input jacks. Though it's not referenced anywhere in Marshall literature nearly all the JMP 2204's made after 1976 had the channels stacked via the input jacks like the JCM800's This channel stacked version is the ones I had. So the denoted schematic will be a JCM800 schematic, but it applies to the amps I owned and modified.

At one point I wired up similar modifications in my 1959 (as yet to be disclosed), but I didn't like it as well. I actually still have that amp and my intention is to return it to stock and perhaps sell it to someone that will love it for what it is. Not sure when I'll get to that. I've been meaning to do it for years already. So...

Schematic to follow.

[Chuck H]

Ok, here it is. I did both amps this way. I loaned one to guy I played with in a band. When I went to collect it he said he couldn't live without it and bought it from me. Of course tone preferences are subjective so YMMV.

[Helmholtz]

Ok, here it is..

Thanks.

That was then. Anything else you would do now?

[Chuck H]

Thanks.

That was then. Anything else you would do now?

Hmmm. Well, sure? I usually hesitate to suggest a circuit I haven't heard. There's the obvious stuff. Like changing the 150k power tube grid loads to 220k if EL34's are in use. I didn't do it then, but it seems a fine suggestion. And I can't tell from this schem if the bias supply might need altering for EL34's. But that would matter. I might change the input resistor to 22k to reduce noise.

I could mess around with Spice and see what might be improvements over where I left off then. But none of that would be ear tested. I'm thinking I'd end up altering gain structures a little if the clipping symmetry is poor and perhaps a grid stop on one or both preamp triodes just for a tighter feel. I probably wouldn't change the power amp. All these amps used the same circuits and both mine had stock transformers. But my early one had 470HV while my later one only had 420HV. I think I reduced the rail resistor to the PI and preamp tubes on the newer one to get operating voltages more similar to the earlier one.

[Chuck H]

6550's are not a true "audio" tube, originally designed by General Electric for servo amps in B-52 Bombers.

From Wiki:

The 6550 is a beam tetrode vacuum tube introduced by Tung-Sol in 1954 for application as an audio frequency power amplifier.

Based on the 6L6 vacuum tube, the 6550 was designed to have higher output power and better stability

This predates the B-52 by just one year. So it's possible this tube was used in the B-52 servo amps, but according to this it was developed as an audio tube by Tung Sol.

[EL-34/JMP]

From Wiki:

The 6550 is a beam tetrode vacuum tube introduced by Tung-Sol in 1954 for application as an audio frequency power amplifier.

Based on the 6L6 vacuum tube, the 6550 was designed to have higher output power and better stability

This predates the B-52 by just one year. So it's possible this tube was used in the B-52 servo amps, but according to this it was developed as an audio tube by Tung Sol.

That's marketing from TS cuz they wanted to sell tubes. Just like a drug company wants to sell drugs & a car company wants to sell cars.
Im quite sure the B-52 was on the Boeing drawing boards in the 1940's
Ive had someone in my family working for McDonnell Douglas / Boeing since the Gemini Space Program early 1960's but most of us are retired.
It takes decades to design / build / & test an aircraft before its ever delivered to the Military.
And like most things we have now they came from Aerospace technology.
In the world of modern invention History guitar amps are pretty low on the tech food chain.

[Chuck H]

Not sure what any of the above is supposed to prove. A tube is a tube. Long having been manufactured. Developing the plane meant they needed one. It seems probable they didn't "commission" the development of a tube any more than the development of rivets or aluminum panels. And there are a lot of references on line indicating that the 5881 was the original tube used in the B-52 servos and was replaced by the more robust 6550 in the LATE 50's. After it had already been introduced by Tung Sol AS AN AUDIO TUBE. But these references are as vague as yours so I'm not inclined to state any of this as fact. If I could find indisputable proof I would post it though. And I'd appreciate if you would do the same or keep such claims to yourself. In this spirit that's my last word on the matter.