Need advice from the experts!

[eatpasta]

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....

[eatpasta]

Updated, higer res pics...

[Roe]

looks good to me

[herbvis]

Is the 220k in the bias circuit a little high of a value?

[bill bokey]

channel one only comes through at full volume.

What do you mean ? It only sounds good at full volume ? Or there's no sound at all unless dimed ?

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]

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.

[eatpasta]

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.

[neikeel]

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.

A decent attenuator is fantastic with these, just enough to take the top off the volume.

[rgorke]

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.

[chrisom]

Isn't there supposed to be another red 'dogbone' capacitor across the ends of the 2 'chicklet' output caps?

[danman]

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]

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

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.

[rgorke]

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

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.

Do you have a ballpark as to where in that thread? There are over 10k replies....

[neikeel]

Ok follow this thread instead:
viewtopic.php?f=3&t=34368&hilit=hot+shield&start=15