1967 JMP 50 circult

stblack

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This https://reverb.com/item/15264841-marshall-jmp-50-1967-plexi

Your opinion about this circuit.
This amp has the same preamp section as the JTM, but with a power amp section for guitars.
But the power amp section has a later guitar version. was this present in the early days of JMP?
Was this present in the early days of JMP or has it been changed?
I can't find the circuit for guitar pre&power until mid-68.
 

TAZIN

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Thankyou do you think the JMP Lead circuit released in 1967?
For the 50 watt I'd say yes and no. Yes; in that there was a 1967 'Lead' version (model 1987) which was a Bass spec circuit with an added 100pF bypass cap (bright cap) on the Bright channel volume pot.

No; the 'Lead' spec which utilized a split-cathode for V1, a 0.68uF cathode bypass cap for V2a, a large 0.005uF volume pot bypass cap, and the 0.022uF output coupling caps, didn't start until around the 2nd quarter of 1968.
 

stblack

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This https://reverb.com/item/15264841-marshall-jmp-50-1967-plexi

Your opinion about this circuit.
This amp has the same preamp section as the JTM, but with a power amp section for guitars.
But the power amp section has a later guitar version. was this present in the early days of JMP?
Was this present in the early days of JMP or has it been changed?
I can't find the circuit for guitar pre&power until mid-68.
Thank you for your feedback. I read Marshall's technical book. The book knows a lot about 100w, but not about 50w. When do you think 100pf he will be 5000pf?
 

crossroadsnyc

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Thank you for your feedback. I read Marshall's technical book. The book knows a lot about 100w, but not about 50w. When do you think 100pf he will be 5000pf?

Hey dude … just a friendly heads up, but it looks like you accidentally hit “report” when I assume you were trying to hit “reply”, so a couple of your posts wound up in the reporting system rather than in this thread. There was nothing wrong with them, of course, so please go ahead and give it another go If you’d like.
 

stblack

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Great amp ! I have # 106xx and it still has the rectifier and it's original silver script logo:)

Earliest of the JMP panels and one of my favorite 50 watters !!!!

If I'm not mistaken it's very similar to the Black Flag JTM circuit with the JMP panel. View attachment 115613
Thank you very much. Do you think the power section of guitar amps changed first? I think this amplifier is a very difficult individual to process.
However, this amp does not have a treble cap on the volume. Is it someone's modification? However, I don't think it's an appropriate repair because I'm using discerning parts.
 

tomsvintage

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Thank you very much. Do you think the power section of guitar amps changed first? I think this amplifier is a very difficult individual to process.
However, this amp does not have a treble cap on the volume. Is it someone's modification? However, I don't think it's an appropriate repair because I'm using discerning parts
Not sure. That's a question for the technicians here :)
 
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AndyD

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It's been modified with some 'Lead' spec parts.
Very strange mix of bass/lead parts, namely the addition of a bypass cap on v2 and the output coupling caps reduced to 0.022.
 

stblack

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Very strange mix of bass/lead parts, namely the addition of a bypass cap on v2 and the output coupling caps reduced to 0.022.
Yes
It is strange that Tone is 56k,250pf but only 0.68uf is added to V2.
And 0.022uf in the Power section, I have never seen this combination before.
But if you look closely at each soldered section, they are touching.
 

TAZIN

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Thank you for your feedback. I read Marshall's technical book. The book knows a lot about 100w, but not about 50w. When do you think 100pf he will be 5000pf?
I would say the 5000pF bypass cap on the volume pot was introduced around the same time the 'Lead' circuit switched to the split-cathode arrangement in 1968.
 
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For the 50 watt I'd say yes and no. Yes; in that there was a 1967 'Lead' version (model 1987) which was a Bass spec circuit with an added 100pF bypass cap (bright cap) on the Bright channel volume pot.

No; the 'Lead' spec which utilized a split-cathode for V1, a 0.68uF cathode bypass cap for V2a, a large 0.005uF volume pot bypass cap, and the 0.022uF output coupling caps, didn't start until around the 2nd quarter of 1968.
Is that the organ model? I recently bought and waiting on a 67 early 68 bass 50 watt later 10000 model with the stock 100pf cap. Also a cool cab.
 

TAZIN

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Is that the organ model? I recently bought and waiting on a 67 early 68 bass 50 watt later 10000 model with the stock 100pf cap. Also a cool cab.
Yes, the 'Lead' and the 'Organ' models shared the same circuit as far as I've seen. Marshall's marketing scheme was to associate different speaker cabinets with each model in order to differentiate them further. Their catalogs refer to these as 'setups'.
 

stblack

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I would say the 5000pF bypass cap on the volume pot was introduced around the same time the 'Lead' circuit switched to the split-cathode arrangement in 1968.
thankyou,Adoption of 0.005 uf CAP along with split cathode. Leading. dose it excist 0.1uf (as-JTM) power section with split cathode?
 

stblack

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Is that the organ model? I recently bought and waiting on a 67 early 68 bass 50 watt later 10000 model with the stock 100pf cap. Also a cool cab.
Several 100pf/V1s are listed in "AMP ARCIVES" 67/JTM. These were made before the split cathode was adopted. I don't know how else to tell them apart.
 

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