Need advice from the experts!
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Need advice from the experts!
I bought this Marshall almost twenty years ago. It split its time being played live and in a few studios where it ended up in the shark week theme as well as a few Lobster records' albums.
Unfortunately its spent the last ten years in my closet since it got married and had a kid.
Its time to dust it off and play it again and I just never knew what I really had.
As far as I know, its a 1971 JMP50 with British plug. Its sounds phenomenal but channel one only comes through at full volume. Ive never had it looked at because I never trusted anyone to work on it.
Either way in curious to hear from some experts....
Unfortunately its spent the last ten years in my closet since it got married and had a kid.
Its time to dust it off and play it again and I just never knew what I really had.
As far as I know, its a 1971 JMP50 with British plug. Its sounds phenomenal but channel one only comes through at full volume. Ive never had it looked at because I never trusted anyone to work on it.
Either way in curious to hear from some experts....
We shall not cease from exploration, and the end of all our exploring will be to arrive where we started and know the place for the first time.
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Re: Need advice from the experts!
Updated, higer res pics...
We shall not cease from exploration, and the end of all our exploring will be to arrive where we started and know the place for the first time.
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Re: Need advice from the experts!
looks good to me
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Super 100 amps: 1202-119 & 1202-84
JTM45 RS OT JTM50 JMP50 1959/2203/34/39
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Super 100 amps: 1202-119 & 1202-84
JTM45 RS OT JTM50 JMP50 1959/2203/34/39
- herbvis
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Re: Need advice from the experts!
Is the 220k in the bias circuit a little high of a value?
- bill bokey
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Re: Need advice from the experts!
What do you mean ? It only sounds good at full volume ? Or there's no sound at all unless dimed ?eatpasta wrote:channel one only comes through at full volume.
With a 4n7 bright cap the amp sounds terrible belox 7 or 8 on the volume knob. If this is what you are experiencing then it's pretty normal with split cathode lead spec amps.
- neikeel
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Re: Need advice from the experts!
Pretty much what I said on Marshall forum
Clean the switching jack contacts properly.
As Bill says the big 'bright' cap on vol 1 pot can make the volume knob like a switch rather than a pot.
If you are using new EL34s you nee dto move the screen wires to pin 4 and add 1k resistors between pin4 and pin6 on the output sockets, I would remove the flyback diodes on the sockets but you can leave them.
You say you have not run it for a couple of years, in which case slow fire up on a variac or follow Larry's cap reforming instructions on this forum before you switch it on again.
Clean the switching jack contacts properly.
As Bill says the big 'bright' cap on vol 1 pot can make the volume knob like a switch rather than a pot.
If you are using new EL34s you nee dto move the screen wires to pin 4 and add 1k resistors between pin4 and pin6 on the output sockets, I would remove the flyback diodes on the sockets but you can leave them.
You say you have not run it for a couple of years, in which case slow fire up on a variac or follow Larry's cap reforming instructions on this forum before you switch it on again.
Neil
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Re: Need advice from the experts!
will do - thank you for the advice.
I desperately miss playing this thing, but not only do I just not have the time - its so bloody loud, there's no place to play it unless its in a studio or along with a drummer.
I desperately miss playing this thing, but not only do I just not have the time - its so bloody loud, there's no place to play it unless its in a studio or along with a drummer.
We shall not cease from exploration, and the end of all our exploring will be to arrive where we started and know the place for the first time.
- neikeel
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Re: Need advice from the experts!
A decent attenuator is fantastic with these, just enough to take the top off the volume.eatpasta wrote:will do - thank you for the advice.
I desperately miss playing this thing, but not only do I just not have the time - its so bloody loud, there's no place to play it unless its in a studio or along with a drummer.
Neil
- rgorke
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Re: Need advice from the experts!
I know this is a little bit of an old thread...what's the cap on the V1 socket between the plate and the grid?
Also, looks like someone replaced the output caps and cut them as to not disturb the red on the solder joint.
Also, looks like someone replaced the output caps and cut them as to not disturb the red on the solder joint.
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- chrisom
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Re: Need advice from the experts!
Isn't there supposed to be another red 'dogbone' capacitor across the ends of the 2 'chicklet' output caps?
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Re: Need advice from the experts!
The tiny cap on v1 is there to squelch oscillations and it will also cut a bit of the high end. You will often see it used in cascaded preamp designs like the 2204...http://site.triodestore.com/JCM8002204Stock.pdf
- neikeel
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Re: Need advice from the experts!
500pF 1kv is typical, lots of chatter about this on the SIR thread (values, best position, vs use of hot shield) but it certainly adds stability, particularly if you try to boost gain a lot with bypassed low value cathode resistors on both cascaded stages.danman wrote:The tiny cap on v1 is there to squelch oscillations and it will also cut a bit of the high end. You will often see it used in cascaded preamp designs like the 2204...http://site.triodestore.com/JCM8002204Stock.pdf
Neil
- rgorke
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Re: Need advice from the experts!
neikeel wrote:500pF 1kv is typical, lots of chatter about this on the SIR thread (values, best position, vs use of hot shield) but it certainly adds stability, particularly if you try to boost gain a lot with bypassed low value cathode resistors on both cascaded stages.danman wrote:The tiny cap on v1 is there to squelch oscillations and it will also cut a bit of the high end. You will often see it used in cascaded preamp designs like the 2204...http://site.triodestore.com/JCM8002204Stock.pdf
Do you have a ballpark as to where in that thread? There are over 10k replies....
"If you make a mistake, do it twice and smile and let people think you meant it." Jan Van Halen.
- neikeel
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