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1976 50watt JMP Marshall - a way to tone it down?

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167
I have a 1976 JMP Marshall 50 watt that is a fire breathing demon. When I bought it, I had envisioned it being more like a "Malcolm Young" sounding amp. Decent breakup, but not a lot of gain. It is an awesome rock and roll machine, but not extremely versatile. I'm not looking to get Fender cleans from it, but I am interested in knowing if there are some alternate preamp tubes I could install to lower the gain to an almost JTM45 level. Thanks in advance
 

Jahn

Listens to Johnny Marr, plays like John Denver
Silver Supporting Member
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29,253
Hmm I know with my Power Station 2 you could just dial it in for the cleans you like, then just use the Power Station to make that clean tone as loud or quiet as you like. Not sure how I'd do it with tubes, since the tone would change along with the gain I bet.
 

Advisable Owl

Silver Supporting Member
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1,203
Not without some invasive work I'd imagine. The post-plexis has stiffer filtering and more gain in general which will require some work on the board to correct.

FWIW I think Malcolm used 45/100s.
 
Messages
167
I was hoping for a different preamp tube suggestion, like a 12au7, or something. Non-invasive and something that could be returned to stock easily. Wouldn't a lower gain preamp tube of some type do the trick? I figured someone would have done this before.
 

pdf64

Member
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8,140
Compared to subbing in different type types, a few minor mods to bring the pre-amp etc back to 5F6A / JTM45 spec would seem to be far more likely to give you the results you want; eg a 12AY7 in V1 would knock the gain down a few dB but will have a more trebly tonal balance.
 

Bandit127

Member
Messages
116
Have you checked that it hasn't been modified? My stock 2204 JCM lives and breathes Angus tones and goes up to Randy Rhoads comfortably but that is about the limit without pedals. If yours has master volume I think you have the 2204. If not you have the 1987 which had less gain.
 

Trebor Renkluaf

I was hit by a parked car, what's your excuse?
Gold Supporting Member
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14,193
Are you jumpering the channels? Blending the low/high inputs can tame a bit of the aggressiveness. The other thing you can try is mod it to 1986 (bass) specs. This is easily reversible and will also tame the amp a bit and give it little bit more of a JTM-45 feel.
 

Vanyu

Member
Messages
738
That depends...

If you have a 2204, plug into the low input, dime the master, and use the preamp volume as your volume knob

If you have a 1987, plug into channel 2

If you have a 1986, you should already be there
 

Geetarpicker

Member
Messages
3,042
Changing the feedback resistor can really change the character of the amp. Most likely it has a 47k or 100k. You could swap it out for a 27k for a cleaner less grainy output stage. Since it's probably a PC board amp (and not quite as easy to cleanly replace parts) you could just carefully tack on a resistor on the top of the board in parallel with what is there. If you have a 47k, you could add another 47k in parallel for a net value of about 27k. If you have a 100k you could put another 100k in parallel for a net of about 47k, or try as low as about 39k or so to net a parallel value of about 27k. My '68 Plexi Superbass 100 has the 27k feedback resistor, and I'm pretty sure the bass spec 1986 and JTM45 amps had the 27k feedback resistors.

Another issue with the later metal panel amps is the taper of the pots in that era came on very quickly. This made for amps that sounded quite grainy even with low knob positions. It kinda added to the one trick pony nature of these amps IMHO. I'm not saying you should change out original pots on a vintage amp, but I've noticed the pot taper was much more gradual in the earlier Plexi era amps. This made it easier to make small but noticeable changes, instead of the all or nothing nature of the controls on the later amps.
 
Last edited:

Laurent Brondel

Silver Supporting Member
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3,324
Since the OP has not said which 50w it is, 1987 or 2204, it's hard to know what to suggest specifically…
For sure both amps have a bright cap across the volume control, and that makes the gain come on pretty fast. Also unless the pot is at least halfway up, the tone is way too trebly, especially with single coils…
On a 1976 it should be a 100k NFB resistor going to the 4ohm tap (1987 and 2204), and IMHO with the heavier filtering they have those amps do not sound as good with a 27k or 47k.
 

MKB

Silver Supporting Member
Messages
8,710
If you plan on keeping the amp forever, a few mods might be in order. Just wanted to mention that if it is fully original and breathes fire, any mods other than tubes would detract from it's eventually significant value.

If it is a NMV with tons of gain, it might have a capacitor across the cathode resistor of V2 (not the 100k, the 820/1k or thereabouts, V2B in some amps). If a cap is there, I'd replace one of the tone or volume controls with a push/pull pot to disconnect that cap. You could also change the resistor from 820/1k to 10k, switch that as well. Going from 820/1k in parallel with a cap to 10k with no cap will really drop the gain. If you have access to a 2204, plug into the low gain input to get an idea of how much will be on tap.

Unsolicited plug in perhaps the wrong forum: IMHO the best sounding power attenuator to run Marshalls at super low volumes is a Kemper. :)
 
Messages
167
Right now this is the only photo I could find (I'm at work already). Probably not enough information for anyone to tell what model it is. And yes, I did have it jumpered in the pic, but that would get plugged and unplugged as the songs needed. I play it into an old early 60s Bassman cab loaded with Scumbacks, and in between the head and cab is a Bad Cat Unleash. I really do love the head, but I find I don't use it as often as I would like to. Right now, I'm stuck on my 62 Brownface Deluxe.

 

Laurent Brondel

Silver Supporting Member
Messages
3,324
That's a 1987 circuit.
Without getting into modifying the amp's board, it's a PCB and IMHO the circuit is perfect as is, there is a 4700pF cap across the Volume I pot, clip one end or replace it with a much lower "Plexi" value like 100pF or 120pF if you like the added brightness.
The 4700pF cap lets through all high mid and high frequencies even close to zero, makes the taper of the pot almost inexistent and brings distortion and volume right away.
It has no effect anymore when the pot is fully open.
Without it you'll regain a useful taper, much fuller tone below 10 and the distortion comes in much later. You can still bridge the 2 channels, as the bass channel (Volume II) adds depth the guitar channel does not have.
 

MKB

Silver Supporting Member
Messages
8,710
That's indeed a NMV. Many of those 1987's are basically on/off distortion machines (extremely tasty ones however). Other than a Vintage Modern with NOS tubes, this would be my favorite Marshall head. The mods I mentioned in post #13 would apply to such a model.
 
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