Why no 2204 to 1987 conversions?? 1987 to 2204 conversions are common

[Toneboner]

That’s the idea. Mod a 2204 to plexi spec. Plenty of stuff floating around about conversions the other way. Why isn’t this done? Don’t tell me a plexi is just 2204 with the gain knob down lol

Thoughts? Prayers? suggestions?

My considerations so far (besides obvious component changes/de-cascading stages) :
Less filtering/ lower plate V
Nfb to 4ohm tap
Possibly normal/lead blend knob (instead of two volumes)((maybe just lead voiced, scrap the normal??))

 

[ChurchHill]

Why? Not that it couldn't be done, but the reissues are good and there are plenty of alternatives from kits to boutique. More like, if the 2204 isn't working for you, why not just get another amp or build a kit?

Other than that, sounds like you're on the right track. The circuits aren't that different.

 

[FourT6and2]

Those changes will make the amp looser and fizzier. Is that what you want?

 

[Techdeth]

Those changes will make the amp looser and fizzier. Is that what you want?

Not too cool . I’d just by a modded 87

 

[Toneboner]

Why? Not that it couldn't be done, but the reissues are good and there are plenty of alternatives from kits to boutique. More like, if the 2204 isn't working for you, why not just get another amp or build a kit?

Other than that, sounds like you're on the right track. The circuits aren't that different.

Thats a very sensible comment. And I know it seems contrary,, but lets just assume a person had a surplus of 800s and a defecit of plexis. Dont worry,, not originals, perhaps a donor amp with lots of great components tho. And the Will to Create! The idea exactly born from the fact that the circuits are not very different at all and can be put back to original if required.

Those changes will make the amp looser and fizzier. Is that what you want?

I do love the tighter, aggressive 800 sound. Dont get me wrong. But the plexi thing is like the thicc girl with stuff moving around in all these fun places. Dont diss her by saying "loose and fizzy"! I want that joosy, greazy, chewy wild thing

 

[FourT6and2]

I do love the tighter, aggressive 800 sound. Dont get me wrong. But the plexi thing is like the thicc girl with stuff moving around in all these fun places. Dont diss her by saying "loose and fizzy"! I want that joosy, greazy, chewy wild thing

Ok, but again... the changes you mentioned won't get you those things.

Lower plate voltage = looser and fizzier. Real "Plexi" amps had plate voltages much higher than your average 2204, not lower. Most of the ones I've personally had on my bench were north of 500v, some as high as 530v. Most 2204s are in the mid 450v range. Some as low as 385v, some as high as 480v. Lower voltages = looser and fizzier.

Lower filtering: Sure... many Plexi-era 1959 circuits had lower filtering in some places of the circuit. But the lower you go, the more ghosting you'll get, the more AC ripple you'll get, and the looser the low-end attack will get. But hey... good place to experiment.

NFB: same thing. 100K/4-ohm is the more raunchy of the combinations. Not a bad thing, but combined with lower filtering and lower voltages, it might not yield the results you want.

That said, if you have a surplus of 2204s laying around and you want to mod one, go for it It's pretty easy to do and if you don't like it, it's easy to go back and/or tweak the circuit to what you do want. But I recommend keeping your B+ no lower than 470v, targeting 350v at the PI node, and no lower than 160v at V1a plate. Filtering is personal preference, but I would consider doing some research to see what the various combinations were for different years. Some had higher filtering at the screen supply and/or main reservoir supply than others, even higher than what you might find in 2204s. PI filtering and preamp filtering were typically lower.

Marshall also made plenty of actual mistakes. Transition-year amps had mistakes for the B+ droppers and preamp filter wiring where they accidentally bypassed one of the stages/droppers, causing two gain stages to share filtering and leave one half of the filter cap unused. This was later corrected. But it's a reason those specific amps had more grit and fizz. I've seen 3 amps in person with this factory error (it's a documented issue).

So yeah... don't be afraid to experiment. Marshall certainly did!

 

[Anje]

+100% with what @FourT6and2 said.

Also you may clarify what you mean exactly by "Plexi specs"
I usually associate earlier 4-input late 60's "real Plexi era" amps with higher headroom & bolder feel, more often than not higher plate voltages & negative feedback. No master volume indeed.
The late 60's / early 70's amps also had specific key part specs & layout, including the transformers, which bring their own things to the whole package VS the later 2204 / JCM800 amps.
Lots of stuffs are in the details on those old amps IMHO, with many different variations over the years way beyond the "schematics" as again pointed at by FourT6and2.
Overall I'm never a fan of "back dating" things; a good period correct base as a starting point is key to me to really get into an era specifics, sound & feel.
Doesn't mean a later 220x/JCM800 can't sound great modded to earlier NMV specs, but it will be its own thing.

 

[brunocormack]

I think It's all about lead dress and the input layout. It’s always easier to cascade a circuit forward into a master volume setup (1987 to 2204) than it is to cleanly rip apart a cascaded preamp to recreate the parallel, four-input blending network of a true Plexi.