1972 Marshall JMP 50 Watt Evaluation and possible Modifications

[Platefire]

Every amp seems to have something that's a weak poor design, this must have been Marshall's.

I planning to bias this thing prior to the weekend and if everything looks good, take it to Church and hook it to my Boogie 4-10 cab. I need to get a little taste of how this babe works as a preview of how my Marshall project might sound prior to installing the MV. If he insist, I guess I will go with the lar-mar type 2 PPIMV. Then I will probably need to re-bias again with the re-routing of the bias path. Platefire
 

[sluckey]

You should not have to readjust the bias just because of installing a MV correctly. Still a good idea to check the negative voltage on pin 5 of the output tubes before and after doing anything in the grid circuit.

[Platefire]

Well this is the plan I was intending to go by in attachment. Doing away with the 220K grid resistors and running the bias to the pots with the blue wire. I personally can't see how the bias grid flow can remain consistent running to each ground lug on the pots and coming out the output lug? It seems the change in volume setting on the MV would effect the amount of neg bias voltage going to power tube grids. Apparently I'm not understanding the circuit. Maybe you could explain it to me. Platefire

[sluckey]

There is no "bias grid flow" in a properly operating PP amp. Well, there can be some 'current flow' when the grid is overdriven hard, but you will be setting bias with no signal applied. No grid current flow means there can be no voltage dropped across those 220K resistors or that MV pot. Changing the MV volume setting will not change the grid bias voltage. If it does change, you have a problem. Just try it and see.

[Platefire]

Well it won't be the first thing I put together I didn't understand but still worked

I just got through firing it up for the first time. Voltages are provided/recorded on a scan of that 1987 schematic attached below. Had a rough time at first because I had no sound with a guitar plugged in but could hear the amp driving the speaker. So I pulled out each preamp tube and tried a known good tube and when I got to V1 it started working fine. Boy is this thing loud!!!!

Anyway I'm not familiar with what Marshal voltages are suppose to be since they didn't put the expected voltages on the schematic like fender. It's pretty high on the power tube side(464), but that may be right for this one---you tell me? No red plating detected and the tubes don't feel abnormally hot.

So I need to get down to what the bias mA should be running that plate voltage. According to the Weber bias calculator running AB with EL34's at 464V plate voltage bias should be 37.7 mA. For those why know what the voltages should run on this amp, please check it out and let me know if they are OK. I'm not Marshall knowledgeable but they seem pretty OK, to me.

Also for added fun---I tried VMS's method he pointed out in post #40 for measuring resistance across the two OT primary sides, then measuring the voltage drop and dividing the voltage drop by the resistance---if I understood it right? here goes:
OT primary Ohms 44.4/45.5 Ohms, Voltage reading at center tap=464.7, voltage reading at plates 463.4/463.7, Voltage drop from CT to plates 1.3/1.0.

1.3 divided by 44.4=.029(29mA) and 1.0 divided by 45.5=.021(21mA)

If I did this method/calculation correct, looks like the bias is way cold!

For Even more added Pleasure:  EL34=25 watts divided by 464 Plate voltage = .0538 or 54 milliamps X .7 = 37.8 mA bias---BTW that's pretty close to Webers calculation   
Platefire
 

[Ritchie200]

Hmmm, that's a little high for that one.  Are you connected to the 110v tap?  If so, it needs to go to the 120v tap.  MHO is the EL34 sweet spot is in the 390 range.  The 110v tap is another trick to push the amp a little harder to keep up with the SVT armed bass player....

Jim

[Platefire]

Best I can tell there is a red, yellow, orange, brown and blue coming from the PT on primary side. The Blue is hooked directly to the polarity switch--see bottom pix. The brown appears to be hooked to the bottom of the little narrow PT connection board on the third turret from the end closest to chassis and has a black above board jumper to 2nd turret. Then the bottom of the 2nd turret appears to have a longer black jumper to both legs of bottom of power switch---see first pix. The examples of Marshall PT on Doug's site didn't show any brown primary leads? I hope someone else can tell what's going on with this hookup.

These two new pictures might help identify. Platefire

[Platefire]

Ok, I went ahead and hooked up a borrowed Eurotube bias probe to a socket. The mA reading was very low. I turned the bias pot to drop bias voltage as low as it would go(hotter) which only went to 14mA, the bias voltage changed to -48.8, plate voltage to 453.

So as the bias voltage was set a little bit hotter the plate voltage went down and mA went up. So I'm thinking there is probably no use proceeding until I replace those original 10uF/160v bias caps! Does anybody second this? Platefire

[sluckey]

The mA reading was very low. I turned it up with the bias pot as high as it would go which only went to 14mA

That's even lower than you measured using the OT method. You started with 29mA and 21mA. And all those readings are much lower than 70%. I don't trust one of those readings.

Pull the output tubes. Then measure the voltage on pin 5, first with the bias pot fully CW, then again with the bias pot fully CCW. What have you? You may need to change that 47K resistor in the bias circuit to get a good voltage range. Are you using 6550s or EL34s?

[Platefire]

Thanks, yeah I'm using EL34's. So you want to get an idea of the complete bias range in the circuit. Ok, will do as soon as I go take my walk:>) Platefire

[Platefire]

Bias range is from -49.54 to -66.6. A total coverage range of -17.8 volts. Platefire


BTW---I measured the bias resistors in place a 213.5K, 15.38K and 46.44K

[sluckey]

Bias range is from -49.54 to -66.6.

That's fine for 6550s but it's too high for EL34s. You need a range of about 35 to 50. Change that 47K to a 33K (maybe even 27K) and check the new range. You can do this without the EL34s plugged in.

[Platefire]

I installed a 27K first and got a range from-32 to -51. So that was right in the pocket for EL34 you indicated, so I didn't even try the 33K.

I went ahead and fired it up with power tubes using the bias probe. Set it at 40mA. At that setting plate voltage is 440 and bias voltage is -40. Weber bias calculator says 39.7mA.

Check the Voltage drop method which at OT CT measured 442 and at both plates were 440:

2V/44.4=.045----so it looks like we have 5mA difference in what the bias probe is showing and the voltage drop calculation. Tube is pretty hot to the feel but not red plating.

I do know that I can tell a marked difference/improvement  in tone and response. It sounds good just dry signal.  Platefire

[sluckey]

I installed a 27K first and got a range from -32 to -51... Set it at 40mA. At that setting plate voltage is 440 and bias voltage is -40. Weber bias calculator says 39.7mA.

You in the zone now! Nice work. Now write it on the wall! 

[Platefire]

Well thanks for your help. I appreciate it! Platefire

[Platefire]

I took the Marshall to the Church this morning and hooked it up to my 4-10 Boogie Cab and tweaked the amp and pedals for the new setup. I never played live on a Marshall before, so I can't let this opportunity pass me by. After all I need to test this new bias circuit change to make sure it's holding I'm just really impressed with how good this thing sounds now that the bias is adjusted right---as different as night and day. The clean channel not only sounds good but is touch responsive also with nice harmonics. I usually don't like a dry amp but it even sounds good dry to my ears. I hope the Marshall 2204 I'm building sounds something like this. It's good that I heard this one and will give me something maybe to shoot for on my DIY one.

I mostly been playing though channel 1 for mostly a clean sound but have tried channel 2 and like the way is distorts as you crank it up. I can just imagine playing through this with a cranked in channel 2----some serious raunch. Platefire 

[HotBluePlates]

... The clean channel not only sounds good but is touch responsive also with nice harmonics. ...

I mostly been playing though channel 1 for mostly a clean sound but have tried channel 2 and like the way is distorts as you crank it up. I can just imagine playing through this with a cranked in channel 2----some serious raunch. ...

Both channels are the "clean channel" (unless there has been an unmentioned mod to cascade the two channels. I had this model, but one year later (when Marshall switched to their first p.c. board). I considered the channels "Icepick" & "Mud". Jumper the two channels with a patch cord connecting 1 jack from each channel, and use the two volumes to set your overall sound.

[Ritchie200]

Something is not making sense on this one.  That plate voltage is way high.  They changed from laydown to upright power trannies in 72.  The laydown would run about 415-420 on the 120v tap with wall voltage in that range.  The upright trannies would run in the 390-395 range on the 120v tap with wall voltage in that range.  The trannie looks original, or is it?  Is this an oddball that Drake sent them on the changeover?  The 120v tap is usually brown, so we look good there.  What is your heater voltage?

Jim

[Platefire]

HBP---no mods. I just plugged into channel 2 briefly and could swore it had more gain than channel 1 but I turned it up louder also. So both channels are clean? OK! I take it you were not to happy with your JMP 50?
For some reason, I don't know why, I've always resisted Marshalls. Back in the 70's when I was doing a lot of playing/traveling I always heard reports that they were UN-reliable. To top that off when I was living in Oklahoma City in 1979 I seen first hand Marshalls break downs on stage and the techs were going crazy trying to get it going again. I bought a Peavey Mace because in those days I was a big Skynyrd fan, it smoked and I had to send it back to the factory under warranty for a new board. So sometimes you get crazy ideas in your head.

Jim---You know when I had it opened up the other day, i forgot to check the heater voltage. I've got to take it down again and probably do a three conductor cord and maybe a MV mod, so I will do it then. You want to see if the heater voltage is high too huh? Well if somehow the PT is hooked up wrong, I sure would like to know myself. Maybe I need to check the numbers out more closely on the PT and see what I can find. It would be good to know if its not original. Platefire

[Platefire]

Here is a link to a discussion of Marshall voltages and it seems based on this with a  stand up 1202 164 Drake with SS rectification 435-440 plate voltage is pretty normal. It kinda seems they know what they are talking about, check um out. The Plate voltage did come down off the 464 to 440 when I changed the 47K bias resistor to 27K that adjusted the bias range to properly bias the EL34. Here is the link:

http://forums.vintageamps.com/viewtopic.php?p=58287

I played it in my Church band this morning it was just a real positive experience. The volume was on 2:>)
I did use my Route 66 Compressor and OD and also a VS Route 808(set to clean boost). I even got some compliments on the sound. So it was just all right. Kept and eye on the tubes and touching them are hot but not overly hot--they don't burn your hand, no red plating even once since I set the bias. I think is very close to right. Platefire

[Ritchie200]

Wow Plate, that is interesting.  It looks like the plate voltage was all over the place!  From year to year even!  I think those Drake employees were doing some chemicals!  I was referencing my trusty Marshall book.  It seems that real world is very different!

I would still check those heater voltages....!

Jim

[Ritchie200]

Plate you also need to make a few adjustments next time you play it.....

Jim

[Platefire]

Well I tried to get him to come get it and try it now that it is biased right because it sounds so much better thinking he might forget the MV. He is determined to have the MV. So my plan is to put this version(see pix) in it that requires no more coupling caps. He brought me a kit with the dual 250K pots, two 2.2M resistors, a length of blue hook up wire and also shielded cable. The kit is from ValveStorm. So based on the parts I suppose that's what he wants even though he didn't specifically say the word "Lar-Mar",  the parts he provided match the mod perfectly and he did say Post MV. Platefire 


JIM-with that kind of adjustment my wife would have me kicked out on my ear. Shes always the one that tells me when I too loud

[Platefire]

OK, I'm proceeding on with first the three conductor cord and then the MV.


On the three conductor cord hookup my plan was to leave the polarity switch in place but more or less disabled. That way it would still be holding it's original part in place couldn't get lost being there bolted in. I have been reading previous post and looking at the situation. As shown in the picture the power cord white is tied directly to the polarity two top terms, the hot through the fuse comes by the yellow wire to the polarity switch two bottom terms and of course the two middle terms go through the cap to ground.


The PT primary blue tap is tied to bottom terms of polarity where the yellow hot from fuse/black power cord hot is also tied and PT primary brown tap is tied to the 120VAC tap of the multi tap AC power board and is tied to the bottom terms of the power switch by a black wire to the 120 tap connection board. This is somewhat shown in attached picture.


So I'm thinking I could accomplish what I'm desiring to do by just removing the cap from the center terms of the polarity switch and use the the ground lug that the cap was tied to on chassis to connect my green from the power cord and off course the white would go to  polarity switch top terms and black would go to fuse.


Please check me and tell me if I'm thinking wrong! Platefire


 

[2deaf]

That's what I would do.

[Platefire]

Got the new power cord in but it was a headache

I found out what that inner plate at strain relief was for-----that's the only thing holding the cord. The hole in the chassis is about 1" in Dia so that's no use! You can't tell the hole is that big because it's covered with the face plate on the outside and on the inside has that extra plate---so the main hole in the chassis is covered up. With that plate not being anchored to anything made it really hard to feed the strain relief with cord through the plate. The only way you could anchor the inner plate permanently is drill holes through the chassis & face plate and install a couple of small bolts/nuts. I decided to install with inner plate loose just like it previously was. Had to file the strain relief down some with a round file and clamp the plate down to be able to put some pressure on it but finally got it through and it's in and pretty tight. Fired it up and seems to be working OK. Hardest cord I ever installed. Platefire

[Ritchie200]

Congrates Plate!  You are now officially a Marshall Meister!   You will be rockin' the Praise in no time, smashing guitars and blowing up your amps!  It's a disease, there is no cure!

Jim

[Platefire]

I'm getting to old for this high drama in amplifiers You would think installing a cord was brain surgery 

Well got the post PIMV to go. I'm ready to get done with this job, too much research required to try to overcome the Marshall curve for me! At least this will give me a preview of my own 50 watt Marshall clone. Platefire

[Platefire]

I will be starting the Lar-Mar PPIMV today. One thing I was wondering considering preserving and a possible future reversal of this mod that does away with the two 220K bias resistors. My concern is the grid wires that run under the board to the power tubes. I'm wondering about disconnecting those wires from the power tubes, cap them and abandon them in place? or is that asking for trouble and would be best to remove?

I will be removing the extra speaker jack to install the MV pot in there. Should I preserve those old original leads from main speaker jack to the extra speaker jack? Never done this before, so being a classic I'm trying to be sure about each move for possible restoration to original. Platefire 

[Platefire]

Got all the old parts removed, got new pot in, new wire cut and just need to solder
It up. Decided to take a nice break before I put my solder hat on. Platefire

[2deaf]

Those wires aren't reversed are they?  If they are, it will change the NFB to PFB.

[Platefire]

Yeah thanks 2deaf for reminding me of that, definitely something to consider or will have big squeals. IOW to maintain the same connection I previously had I need to reverse the wires on #5 pins of power tubes sockets where black will be on the inside tube and red outside tube.

Also best I can tell on this one the OT secondaries is Gray 16 Ohm, Green 8 Ohm and yellow 4 Ohms. The NFB is connected to the yellow 4 Ohm tap---same a the 6550 version schematic shows. The only way I was able to determine this color coding is by the readout window on the impedance selector and noting the connections made from the inside. For EL34's on the 1987 schematic  shows the NFB connected to the 8 Ohm tap. So if I'm understanding things right, I need to de-solder NFB from the yellow wire terminal and connect it to the green wire terminal.

I can't help but wonder if switching to the 8 Ohm tap for NFB will get me back in PFB territory again? May need to not solder up both pin 5's until I see what it behaves like powered up. 

I hope some of you Marshall folks are looking in because I'm feeling my way on this one. Platefire

[sluckey]

Don't fret over a mole hill. Plug it in and turn it on. if it squeals, reverse the red and black on the board. Don't mess with the NFB wire or OT wires.

[Paul1453]

Got the new power cord in but it was a headache

I found out what that inner plate at strain relief was for-----that's the only thing holding the cord. The hole in the chassis is about 1" in Dia so that's no use! You can't tell the hole is that big because it's covered with the face plate on the outside and on the inside has that extra plate---so the main hole in the chassis is covered up. With that plate not being anchored to anything made it really hard to feed the strain relief with cord through the plate. The only way you could anchor the inner plate permanently is drill holes through the chassis & face plate and install a couple of small bolts/nuts. I decided to install with inner plate loose just like it previously was. Had to file the strain relief down some with a round file and clamp the plate down to be able to put some pressure on it but finally got it through and it's in and pretty tight. Fired it up and seems to be working OK. Hardest cord I ever installed. Platefire

I try to connect the cord green/gnd wire under a PT lug nut if at all possible.   

[Platefire]

Sluckey---my wife tells me I don't need to exercise because I get plenty making mountains out of molehills, flying off the handle and jumping to conclusions! I also suffer from another serious disease "The Paralysis of Analysis"


paul1453----I thought about those PT bolts but when I seen that little bolt where the death cap attached when I was removing it, I decided that's it! I did see a picture of an old Marshall with that round receptacle with the two prong/conductor socket. I suppose that might have been in it at one time, reason for the big hole? Platefire

[Ritchie200]

Plate,

That big hole was to accommodate the big Bulgin Euro mains plug.  The cover plate you modified were fitted for the US exports.

Jim

[Ritchie200]

Sluckey---my wife tells me I don't need to exercise because I get plenty making mountains out of molehills, flying off the handle and jumping to conclusions! I also suffer from another serious disease "The Paralysis of Analysis"

 
Jim

[Platefire]

Ok, Export to USA got the loose conversion plate I'm wondering about the origin, the customer said it had 6550's in it when he got it. The circuit is straight 1987 for EL34's with the exception of the NFB being on the 4 Ohm tap. The 1987 schematic shows it connected to the 8 Ohm tap. Can't help wonder if it originally had EL34's and at some point someone changed it to 6550's and now I'm changing it back to EL34's. Don't know, makes me wonder?

Also I copped me some 10Ga copper wire tonight. Going to make a "U" for that Ohms selector. I've been testing it with the old selector held on with Scotch tape---can't stand that! Platefire

[Platefire]

This is probably way over-kill but! This is the second or third Marshall I've had to build an Ohms "U" connector for and without the little window on the original handle your kind of flapping in the breeze to know which plug in connection to make to get the right Ohms? So I made a drawing to indicate this. I don't know if it would work for all selectors but for this JMP 50 will work. Platefire

[jojokeo]

Plate, just a fyi once the ppimv is in. I usually find them dark sounding when turned down below 50% & usually need 100pf treble bleeds on both pots, ymmv?

[Platefire]

OK, thanks for sharing your experience. So your talking about a 100pf from input to output terms---right?

I will build it as is first, and if it's apparent it needs it, add them latter. Platefire

[shooter]

I copped me some 10Ga copper wire tonight

Be real careful, at least in MI the cops are cracking down pretty hard on that sorta thing

[Platefire]

Well, I think I will go ahead and turn myself in 

Life is rough on the run 

[Platefire]

Just soldered everything up, double checked everything and fired it up. Best I can tell so far everything is working good. The amp is operating nice and quiet. Only hums when I get too close to the chassis with my strat. Checked the bias voltage and it was holding around -40 or -41 same as it was set before the mod. I hooked a delay up to it and played around with it and I think the MV really helped it! You can really get some good sounds and response at lower volume now. I think the customer is going to be really happy with it.
I really can't tell any loss of highs myself. In fact when I started jamming on it I turned the treble, middle and presence down. The bass was already up about 75%. I was able to get the kind of tone and response I like out of it. I couldn't turn it up like I would like, the wife is home:>) Platefire

[Ritchie200]

Come on Plate, crank it up!  That's rock and roll - taking chances, living on the edge, having women scream at you!

Jim

[Platefire]

Yeah, now that it's got a MV, Rock n Roll at a lot lower level  and still get dinner!

[Willabe]

 

[Platefire]

I decided to double check the bias now that I got the mods done. Guys I need some help, things are not adding up on the bias!

First I've been calculating the bias by the measuring the resistance across the OT primaries and measuring the voltage drop which the resistance is to V4=43.9 Ohms and V5=45.2 Ohms.

The calculations I did with his tubes he sent in the amp shows a vast difference in mA between both tubes. He claims his tubes are supposed to be matched tubes and shouldn't be that far off. The last calculation I did with his tubes were way off. From my past experience usually a big miss match in the tubes had been due to the tubes. So I have a new matched set of EH EL-34's for my Marshall clone project and decided to try those to see if it helped the situation.

One of the problems I'm having using the method is in the amp loaded voltage at the OT center tap is fluctuating by a volt or two continually. So by the time you've made three voltage readings and recorded them, the voltage has changed. Here are my last readings and calculations just to show you what going on as the amp warms up as follows:

1-(A)OT center tap=442.6V/V4 plate=441.2V/V5 plate=441V
   (B) V4 442.5V-441.2V=1.4 Voltage Drop & V5 442.6V-441.0V=1.6 Voltage Drop
   (C) V4 1.4 Voltage drop divided by 43.9 Ohms=.031 & V5 1.6 voltage Drop divided by 45.2 Ohms=.035
   (D) Bias is V4=31mA & V5=35mA

2-(A)OT center tap=440.5V/V4 plate=439.1V/V5 plate=438.6V
   (B) V4 440.5V-439.1V=1.4 Voltage Drop V5 440.5V-438.6V=1.9 Voltage Drop
   (C) V4 1.4 Voltage drop divided by 43.9 Ohms=.031 & V5 1.9 voltage Drop divided by 45.2 Ohms=.042
   (D) Bias is V4=31mA & V5=42mA

3-(A)OT center tap=437.6V/V4 plate=436.5V/V5 plate=436.2V
   (B) V4 437.6V-436.5V=1.1 Voltage Drop V5 437.6V-336.2V=1.4 Voltage Drop
   (C) V4 1.1 Voltage drop divided by 43.9 Ohms=.025 & V5 1.4 voltage Drop divided by 45.2 Ohms=.030
   (D) Bias is V4=25mA & V5=30mA

4-(A)OT center tap=442.4V/V4 plate=441.2/V5 plate=440.2V
   (B) V4 442.5V-441.2V=1.3 Voltage Drop & V5 442.5V-440.7V=1.8 Voltage Drop
   (C) V4 1.3 Voltage drop divided by 43.9 Ohms=.029 & V5 1.8 voltage Drop divided by 45.2 Ohms=.039
   (D) Bias is V4=29mA & V5=39mA

5-(A)OT center tap=440.9V/V4 plate=439.5V/V5 plate=440.1V
   (B) V4 440.9V-439.5V=1.4 Voltage Drop & V5 440.9V-440.1V=.8 Voltage Drop
   (C) V4 1.4 Voltage drop divided by 43.9 Ohms=.031 & V5 .8 voltage Drop divided by 45.2 Ohms=.017
   (D) Bias is V4=31mA & V5=17mA

So 5 separate checks and you see the progression as the amp warms up! It's apparently not the tubes with a new matched set in there. Bias voltage is pretty constant at about -40V. So what could be causing the unbalance to this degree and variation in voltages? I'm not going to be able to properly bias this until I can get to the bottom of this. Kind of at a loss on how to proceed to correct this. Help please  Platefire

[sluckey]

Buy a bag full of 1Ω resistors. You will get much more consistent readings.

[Platefire]

Using the Shunt method with my Fluke 115:

His tubes-V4=42mA   V5=24mA (These 18 mA apart)


My tubes-V4=26mA  V5=34mA (These are 8mA apart)

The plate voltage is fluctuating between 445 to 437 range. It seems to start off high and drift down as the amp warms up. Never seen an amp fluctuate in plate voltage that much?

My tubes are new. 8mA is a pretty big divide for new tubes.  My inclination using my tube is to set V5 at about 40mA and that may bring V5 up to 32mA. Platefire