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1970 Marshall JMP 50w - RESTORATION PART 1
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- Опубликовано: 7 авг 2024
- A 1986 50w Bass spec JMP has come in. The owner purchased it and sent it straight to me for a ‘service’. We are going to restore this timepiece. Part 1 is a deep dive on the existing amp as received and diagnoses of the issues with it.
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0:00 Intro
2:46 At The Bench
35:25 Schematic Review
41:44 Tones
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Definitely looks like EI96 PT, which you are correct in it being smaller than the original EI114... great to see you back on the bench and especially with an absolute specimen in that condition!
Can't get enough of these vids man! You are the reason I modded my own Origin 20, please keep this kind of content comin. Rock on!
I'm always glad to see a new video from you. I learn so much from you every time, thank you so much!
No worries, nice to hear you are digging the content.
Good to have you back! Great video, thanks for the explanation. I noticed the disgust in your voice when you said "pedal platform" haha. Nah its cool I know you like pedals. Just not a fan of a clean amp with 10 drive pedals on the pedalboard lol!
Great amp, nice troubleshooting of the choke. Marshall didn't start using screen resistors in the 50 watt amps until early 1971 or so.
Yeah, I’m sure it was to save a few cents. Contrary to belief Marshall wasn’t using the best components or anything special.
Great to see you up and about mate. This is my kinda amp ♥...I actually love the big open stringy tones of the high and upper mids. Killer.
That is a mint amp! What a dream! Keep on rocking, Jason!
Cool amp, can't wait to see it in top form!
Me too! The B+ line is being completely rebuilt, it should kick ass!
Can't wait to hear it restored
Just found your channel from a comment in Lyle's latest video - have to say I'm very impressed - you definitely got a subscribe from me!
Hey thanks and welcome! Lyle is great.
I think you would have to get it working properly before deciding on modding it.
The power transformer worries me as it may not be regulating as well as it should. Will you check the cap on the shared cathodes?
Good spotting on the choke being faulty, it’s not that common and you could understand someone missing it.
You can get the choke rewound, I’ve used a local guy in Sydney who does a great job. The choke would still look original.
The 5000pF bright cap on the lead amps is pretty crazy, it means a frequency band is operating at full volume.
BRIMAR PreAmp Valves...that's a treasure!
That clip is a capacitor clip where you can clip (large) axials to preserve them from moving around. Commonly found e.g. in old AC30s for screens and preamp B+
Good to see you back mate, was wondering how you were traveling!
Thanks man, getting there
2:30 looks like a 1970s SG - great!
Take care of that knee Jason! I know you will take care of that JMP!
Hey David, hope you are doing great man.
Seeing this now I understand that the Dutch London city amps ( plexi clones) have exactly copied this standard Marshall schematic for their dea100 mkiv amps. I was always wondering why they used different tone slope resistors and had a shared cathode setup since the standard Marshall schematic of a lead is different. It also has the old style presence design with a 5 k pot. So if your potentiometer was dirty you would have no sound anymore. Later they would put a parallel resistor in to prevent that. It seems the first version didn’t have screen resistors, the MK2 version did
I am building from a kit an JMT45 and was thinking of having 2 bias pots fitted but all other amps only have one so not sure which is the best way to go.
Interesting about the choke.
As I understand it the 50 watt Marshalls at that time didn’t have lower screen voltages. I guess they thought they could get away with it cause the plate voltage was lower than the 100 watt version.
Even though my favorite Marshall ever is the the ‘76/‘77 2203 Lead, these “Bass” Marshalls still rank very high with me. Absolutely amazing guitar amplifiers.
How do you like that Hantek LCR meter? I’m looking at picking up a couple diagnostic tools.
The Hantek was not expensive but is all I need, it’s a nice addition to the bench.
Nice work Jason. Is that PT putting out a higher B+ than you'd expect for a JM P 50w?
Depends on the era. Only the late 70’s ones were very low voltage for a while.
@@HeadfirstAmps Thanks Jason. You're right! The JMP50 I just completed with a low B+ of 368v was from 1979. I must admit this '79 sounds sensational with more than enough volume (at 36wRMS before clipping). I'm enjoying your channel :) Chris.
1k ohm 5 watt screen resistors are missing to pins 4 on power tube socket
👍👍
Your neighbours must love you. 🤣
Hahaha, yes very much.
Oh yeah J’s back in black
Yo!
@@HeadfirstAmps should be Oi ! Haha
Hi....do you need an original power transformer? I have one for sale
Look like a Demeter PT (made in uk)
I would guess or say PT is a later Dragnal marshall make. The label fell of ?
Could be!