Help identifing Marshall JMP?

[Stevewdewitt]

I have a Marshall JMP, I think, on my bench. The first channel seems to work ok (not great). The second channel is not working. Looks like it has had work done to it at some point in its life. All the tubes have power.

Nothing looks burnt.

I have some voltages but it isn't great. It was tough getting voltages, hard to get through the crud, and I will have to do it again I am sure.

Also the Mains was set for 250 volts, I changed this to 120. Any suggestions about where to start would be great. I attached a link to the photos below.

V1 12ax7 - P1 365vdc, P6 367.5vdc
V2 12ax7 - P1 0, P6 369.5vdc (not great)
V3 12ax7 - P1 376.3vdc, P6 370.8vdc
V4 EL34 - P3 386, P4 386, P6 386
V5 EL34 - P3 386, P4 385.8, P6 386
V6 12ax7 - P1 378.3

Link to photos https://drive.google.com/open?id=0Bz1qDUh5_0zrUTZweFFmNHdQbm8

Steve

[PRR]

Since all the tubes show about 370V, either they have no heater voltage or there's a ground connection missing.

What are the Cathode voltages??

[Ritchie200]

This was Marshall's country/surf amp.  Complete with reverb.  Produced between 71-78  Voiced differently, supposedly for a tele - which is why it sounds like crap .  Please see link for appropriate voltages.  The jacks to the reverb tank are gone. How has this been jumpered/removed?  That could have been a good master volume mod with just a jumper.  Reverb only worked on channel one.  Is this the one that works?  Channel 2 is verb-less.  Presence is a master for both channels.  Lots of new parts on the board, maybe someone tried to make it sound better.

When Jim Marshall was asked what his least fav production amp was, he quickly replied "Artiste".  All the bones are there, you will just have to voice it like a normal 50.

Jim

http://el34world.com/charts/Schematics/files/marshall/Marshall_artiste_50w_2040.pdf

[Stevewdewitt]

Ok, I have better voltages, with tubes in, and the second channel is now working. The issue was a bad 100k resistor that connects to V2 pin 1. It was open and not passing any current.

V1 12ax7 - P1 142vdc, P3 1.2 vdc, P6 123.6vdc, P8 .7vdc
V2 12ax7 - P1 144vdc, P3 1.15vdc, P6 111.2vdc, P8 .63vdc
V3 12ax7 - P1 205vdc, P3 1.12vdc, P6 97.5vdc, P8 .52vdc
V4 EL34 - P3 306.5vdc, P4 301.7vdc, P5 -9mV, P6 305vdc
V5 EL34 - P3 305vdc, P4 300vdc, P5 5mV, P6 305
V6 12ax7 - P1 165vdc, P2 17.7vdc, P3 28.7vdc, P6 155vdc, P7 18.3 vdc, P8 28.7vdc

Now that there is sound coming from the 2nd channel, which is the reverb channel, it is really thin and weak still and crackles and fizzes a little.

thanks,
Steve

[Stevewdewitt]

The jacks to the reverb tank are gone. How has this been jumpered/removed? 

The jacks for the reverb are on the top of the chassis between V3 and V4

[PRR]

> V4 EL34 - P5 -9mV
> V5 EL34 - P5 5mV

I smell EL34 burning.

Also why your "420V" seems to be 300V.

The bias supply has failed. Take EL34s out, go through the bias supply (from PT HV to V4 V5 pins 5) until you find NEGative 30V.

[Ritchie200]

Ooops!  Sorry I was looking at this...  When you do get it up and running you can fabricate a RCA jumper for a little more drive to that channel.

Jim

[Stevewdewitt]

OK, I replaced all the bias circuit components and it is way better now. Biased at 33mV and 31mV. This is pretty cool.

V4 P3 - 366vdc, P4 361vdc, P5 -22.28vdc, P6 364.5vdc
V5 P3 - 364vdc, P4 360vdc, P5 -22.22vdc, P6 363.5vdc

The fuzziness is gone from the reverb channel and both channels sound pretty good. I can't really open it up since it is 50 watts

Thanks,
Steve

[Stevewdewitt]

Also why your "420V" seems to be 300V.

So thanks for the help with the bias supply. Why would the plate voltage be so low? 366vdc seems low. Where is my "420"?

The amp sounds pretty good at this point. Nice clarity on the notes, plenty of volume. The reverb channel is bright but sounds good. Especially if you turn it up.

[PRR]

> Why would the plate voltage be so low?

Sometimes because you are sucking a lot of current.

When my well-pump kicks in my house lights dim.

> Where is my "420"?

Looking at the PDF that Ritchie posted: output bottle should have 420V on plates and -30V on G1. You report 366 and -22. (Improved from 300V and ZERO V.) You do not report any current or heat measurements. One interpretation is that "-22V" is insufficient bias, tubes are sucking big current, 420V is sagging.

> sounds pretty good

That may be. However I have heard audio gear sound "really good!" and then it went BOOM from excess power.

If you don't/won't have 1 Ohm cathode resistors to monitor current, at least set your repaired bias-supply closer to -30V. See if 366V comes up closer to 400+V, and it still sounds good.