1972 Marshall JMP 50 Watt Evaluation and possible Modifications

[Platefire]

Got this amp head in today that looks so far in original condition. The customer wants me to modify it install a Master Volume, three conductor cord and change operation of power tubes from 6550's to EL34's.
First I need to figure out what circuit it is and get a schematic. Will take some pics soon and post them. The serial # is SA 1273D Lead. Also on a tag on the chassis says"Lead 1459".  I think he wants a post PPIMV. He provided a kit with a dual 250k pot, shielded wire, hook up wire, shrink wrap and two resistors. So right now I would appreciate some help determining what circuit I got. Thanks, Platefire

BTW-Looking at Doug's schmatics it looks like it may be 1987 Lead Mark II?

[Platefire]

Also here is a pix of what's written on the chassis plus my best guess on the correct schematic.

[Paul1453]

You mean he wants to change output tubes to EL34's, right?   

To change to EL84's is a whole different story.  New sockets, double the number of output tubes, probably a different OT needed too.

If he really wants EL84s, I'd probably say he'd be better off selling this amp and buying the parts for you to build him what he really wants.   

[Willabe]

No way I'd drill a hole in that amps chassis for a MV. 

[DummyLoad]

pardon the directness: oh hell no! that thing is a classic & a jewel of amp as is. putting any holes in that amp would be like hacking the dash on a 64 1/2 mustang convertible to put a CD player in it. ugh! not only that, mods of that sort will substantially reduce the resale value.

sell it. build him one with a MV.

i think you mean EL34's right? EL84s would require a rework of the entire PS.

--pete

[Platefire]

Yes, sorry-----EL34's. As I understand it a re-biased and move the NFB from 4 Ohm to 8 Ohm tap will do it. He wants the MV in one of the input jack holes?? Never seen that before, if it would work, it wouldn't require any drilling. Platefire

[JB]

I'd say that's pretty much the right drawing for it.  It may have shipped with EL34's.  Wasn't there something though to do with the US distributor swapping them on arrival for 6550's as they were more reliable?  i.e. Then the distributor got fewer warranty repairs.  I guess Unicord was that distributor hence they are showing 6550's on their drawing.

Re-bias, not heard the requirement to move the feedback point though?

Fairly common to use one of the 4 inputs for MV.  Looks weird but at least you don't have to drill.  If it's a 250K and a couple of resistors it sounds like the LAR-MAR PPIMV mod. Replaces the 220K bias feed resistors (interestingly they're shown as 150K in that schematic, would be worth finding the equivalent UK EL34 drawing for that amp, plus of course opening up and seeing what's actually installed!).

If it were my amp I'd leave it all stock....

[tubenit]

Platefire,

It looks like you have 3 switches on the front:  polarity, on/off, & standby.  I think I would be inclined to see about using one toggle for the on/off & standby.  IIRC,  Sluckey has a schematic that shows how to do that. 

http://vintageamps.com/plexiboard/viewtopic.php?f=1&t=78112

Then you could put the PPIMV in the standby hole or even more the indicator light into the standby hole and use the empty indicator light hole for the PPIMV (if the indicator light hole is round).

OR you can lose the polarity switch (maybe that's not needed if you have a 3-prong cord) and then move the on/off and standby over ............. leaving an empty hole where the standby was.

With respect, Tubenit

[DummyLoad]

I'd say that's pretty much the right drawing for it.  It may have shipped with EL34's.  Wasn't there something though to do with the US distributor swapping them on arrival for 6550's as they were more reliable?  i.e. Then the distributor got fewer warranty repairs.  I guess Unicord was that distributor hence they are showing 6550's on their drawing.


If it were my amp I'd leave it all stock....

exactly.

perhaps a power soak may be an option?

--pete

[RobBozic]

The usual place for the PPIMV is in the speaker extension jack, as it's less prone to P Oscilliation / closer to the EL34's / away from the sensitive input.


Check out google images


Rob

[Platefire]

Well I'm feeling kind of bad because I agreed to take this job on before I really realized what it is. It's a true classic in real good condition. The customer is very aware of it's value and it's place in amp history. He said he wanted me to save the two conductor cord I guess if it needed to be put back. He also wants me to examine the circuit to determine the originality of it. This guy is a musician that makes his living playing & dealing in new & vintage pedals on the net. So I'm not jumping into this with solder flying. I consider this a consultation period and anything I do will be after considering the best options. Thanks for all your input and suggestions. My next step will be to crack it open and examine the circuit for originality and being my experience it not in Marshalls, I Will need your help. I will post some good pics of the circuit once I open it up. Thanks, Platefire

[HotBluePlates]

The usual place for the PPIMV is in the speaker extension jack, as it's less prone to P Oscilliation / closer to the EL34's / away from the sensitive input.

Yes, do this. I'd strongly advise against putting a master volume in an input jack hole; the circuit wiring would have to cross over itself a couple times, inviting oscillation.

Regarding the switch from 6550's to EL34's: Usually this involves moving the feedback tapping point on the OT; raising the bias-feed resistors to 220kΩ (if they're lower; some 6550 Marshalls are); checking the bias supply for acceptable range of bias voltage adjustment; and changing bias supply resistors to achieve bias voltage adjustment range, if needed.

[tubenit]

Well I'm feeling kind of bad because I agreed to take this job on before I really realized what it is. It's a true classic in real good condition. The customer is very aware of it's value and it's place in amp history

This is just my opinion ...................

I don't see any reason to "feel bad" about the job.  I think it is reasonable to see "your responsibility" being to give him an "informed choice" so that he knows the value of the amp as is and possible de-valuing of the amp IF selling it if
one did irreversible mods. (It sounds like the mods considered are not irreversible if using an existing hole like the speaker extension.)

It's his amp!  He can choose whatever he wishes to do with it. If he enjoys the amp more with the mods & gets more of the tone that he wants, why shouldn't he do so?   You can choose to help accomplish his agenda or bow out.

I've modified an original amp and regretted it and lost $75 in value selling it after playing it for about two years.  And I've modified another original amp and absolutely loved it & when I sold it, I made money! 

Just my 2 cents worth.  I think the key is that he is informed about his choice.   

With respect, Tubenit

[Ritchie200]

Well, there are a few things to consider.  Yes this is a good example.  However, switches look like plastic toggle replacements and pilot light is either broken or not original.  Not original fuse holder in back.  I would guess the inside probably has some fresh solder and maybe some new parts in it.  Plate, I would go through the whole amp and see what has been done.  Any solder point that is not showing red nail polish is new.  A real high dollar collector would probably pass on this one just from what I see.   I would think it would get average pricing

Plate there is a paper tag on the top front corner of the chassis.  You may have to remove the chassis to get a good look.  If its dusty/grimy carefully wipe it off.  That will have the sign-offs by the women who assembled it.  There will be dates.

Jim

[Platefire]

Well taking everything in consideration, if the non original changes on the exterior is any indicator of what lies on the inside, it might not be so original. It does have the white tag with signatures in the upper front left hand corner of chassis. I will also take a close up of it. Thanks for all your insight on this subject and amp. I am a little excited getting to handle this ol Warrior. Hopefully I can get gut shots up soon! Thanks, Platefire

[JB]

Have you fired it up yet? Would be interesting to know how the 6550's and EL34's compare.

Jimi had some of his amps converted in the other direction - from EL34 to 6550, apparently sometimes with bigger PT for higher B+ and hence more headroom. Lots of info on this on the Metro amp forum from an engineer who worked on his kit.

[Platefire]

No I haven't fired it up because the customer sent a new set of JJEL34's in it in anticipation my re-bias. So I haven't got any 6550's, wish I did because I would love to hear it with those. I haven't had time to research what the change over from 6550's to EL34's requires, so I left it alone so far. I first read about a 47K resistor in the bias and the fix was to parallel another 47K beside it and then a lot of the post on this thread indicated something else. So it's a little fuzzy right now what to do.

When I got back from my vacation I found my computer crashed when I tried to boot it up. So I just got it back yesterday from repair and got it fired up. I've been trying to do all my internet stuff on my phone and you know how that is in dealing with schematics:>) So now I can actually print out a hard copy of the schematic and study it. First I've got to go to a funeral of an old friend that passed away. I'm still in recovery mode from my vacation playing  catch up. So I will eventually get to the meat of the issue. Platefire

[Platefire]

Ok here are gut shots. Please explain about the nail polish because I'm not familiar with that. I do see several places with new solder and no nail polish. You Marshall experts, please let me know what you see!

[Platefire]

Gut shots continued.

[Platefire]

Gut shots continued:

[Platefire]

Last but not least are the tags and transformers.The little piece of red tag has an initial ending in m, you don't reckon that could be ?????? Platefire

[shooter]

Please explain about the nail polish

We used it way-back as an enamel coat for corrosion, clean surface with 99% alcohol, add a dab.  OEM's used it to *see* if someone made *unauthorized* repairs on warranty equipment.

[2deaf]

I think there is a pretty good chance that this amp originally had EL34's in it.  It has a 220K dropping resistor in the bias circuit, I can't focus on any 1K/5W screen resistors, and the one original socket has pin 1 tied to pin 8 (although they may have always tied 1 and 8 no matter what tube was going in it).  I would see what the bias range is and if it can do EL34's, no modifications are necessary.

[Platefire]

Thanks for cluing me in on the nail polish. It works pretty good because it easy to see what's changed. Only thing with my Mutli-Meter seems like it hard to make contact sometimes with nail polish acting as an insulator.
I tell you, I'm in the school of Marshall and it's somewhat different from Fender 

But that's OK! A little different perspective.

2deaf, you are right! this thing appears to be biased for EL84's already!

I see some black burn marks on chassis around the replaced octal socket. Also the 50uF/50uf cap can associated with V1 and V2 power supply filtering has been replaced and they just run the new leads above the board(white wires) and just tagged on to the 10K/1W resistor leads. I think the other two filter cans are replacements too. Those are the main changes that caught my eye so far--still looking.

The 1987 Lead Mark II schematic had two 10K/1 W power resistors in series after the choke prior to PI and the latter 2204 has only one 10K/1 W after the choke prior to PI. Both for 6550 power tubes on schematics. The one I got has the two in series like the 1987 Lead Mark II----however---the nail polish is still on the bias circuit completely, so I guess it was originally intended for EL34's regardless of what the customer said because of the 220K bias feed resistor right? Platefire   

[HotBluePlates]

Amp appears to be an exact match for the EL34 version of the 1987 chassis. I presume the Yellow wire on the OT secondary is the 8Ω tap; the purple NFB wire is attached to it, as the schematic shows.

220kΩ dropping resistor in the bias supply, as well as 220kΩ bias feed resistors to the tube grids. All stock EL34-version items.

I see a changed coupling cap in the preamp, one changed output tube socket, a couple of new switches, and filter cap cans you note as being new. Everything else looks essentially bone-stock. And right for an EL34 amp.

It would be very nice if you had an ESR meter for the bias filter caps... They're the original Erie caps from the day the amp was made, but you'd like them to be in good operating condition. An ESR meter would tell you exactly how they're doing, as some Marshall fans get upset if they don't see 100% original parts.

[Ritchie200]

That red Erie (probably a .05) cap on the polarity switch needs to go.  Put it in a bag and give it to your client.

Jim

[Platefire]

Thanks all for putting your eyes on this and pointing things out. I appreciate you sharing your experience and know-how! I think that just about covers everything replaced from original.

HBP---I'm not familiar with an ESR meter, please clue me in. I know two people who are Hospital Maintenance/Electricians who also know tube amps who may have one I could borrow.

Did anyone notice that metal plate on inside of chassis at strain relief where power cord enters chassis??? That don't look stock and wondering what the purpose is. The cord is a little loose in the hole. Does anybody know about this? here is the picture. Platefire

[2deaf]

I figured that the chassis was punched for the round British power receptacle and they used that plate to fill that hole and to mount the strain relief so that the cord wouldn't be pulling on the backplate.  If it is a mod, it sure isn't a recent one. 

[2deaf]

I found a gut shot online that has the same plate, so it is probably original.  There are also a lot of backplates with the exact same setup as yours suggesting that Marshall sent out a large quantity with this format.

[Ed_Chambley]

Just do not cut the chassis.  He probably sent you one of those Master Volume Kits Metro sells and adding a LarMar master only removes 2 resistors from the board.  I have LarMar on my 68 Plexi in the speaker jack.  I do not remove the parts and hand them to the owner.  I print the schematic and cut the part where I made changes and roll those parts in the paper.  Then I put the parts in a baggie and zip tie the baggie to the new power cord inside the amp.


With the larger parts like filter caps I baggie them and give them back.


Nothing wrong with Modding any amp that can be returned to stock if you do neat work.  Now I do have an all original JTM45 and will hate the day I have to make the first new solder joint, but most collectors of Marshall amps expect some rework inside as they were made for road use.


It is the smaller Fender amps where most get picky.

[Platefire]

Thanks 2deaf and Ed

Well if this amp was mine, I think I would restore everything back to original save the filter caps.

I still don't understand why the person that replaced the filter can for the preamp stages didn't feed the leads through the holes that were already in the board to the turrets instead of letting it fly all above the board. Wouldn't have been much extra trouble and looked a whole lot neater. Guess it could still be done as a correction.

Can someone tell me which fuse holder is original. I think the one to the outside end but not sure. I'm getting ready to do a report to the customer and want to get all my info straight.

Another thing, on the impedance selector for Ohms speaker out, the selector handle with copper "U" is so loose that if you tilt the amp if falls out. Any good suggestions on correcting this?? Platefire 

[HotBluePlates]

... HBP---I'm not familiar with an ESR meter, please clue me in. I know two people who are Hospital Maintenance/Electricians who also know tube amps who may have one I could borrow. ...

I've gotten away with not having one for a long time. AnaTek sells an easy to use ESR meter which will give an indication of the health of electrolytic caps easily. Check Youtube for "Uncle Doug" videos, as he has at least one demonstrating that specific meter. IIRC, you can test the caps in-circuit.

The value would be verifying the health of electrolytics in old amps, which are either cumbersome to replace (like some multisection cans), expensive to find an exact replacement part (multisection cans, again) or where the owner wants to change as little as possible due to collector value placed on having original parts.

I mentioned it because to me, it is more critical to have well-functioning caps in the bias supply than perhaps even in the power supply. You'll get hum if either go bad, but you risk burning things up with a bias failure.

[Paul1453]

I thought maybe the SS rectifier diodes were replaced, but the other picture shows the same type so maybe not.   

[2deaf]

Can someone tell me which fuse holder is original. I think the one to the outside end but not sure.

Another thing, on the impedance selector for Ohms speaker out, the selector handle with copper "U" is so loose that if you tilt the amp if falls out. Any good suggestions on correcting this??

Probably neither one.  I remember those things having the screwdriver slot holders.  Frequently replaced because the slot would get trashed.

New impedance selectors are available for this model online.  Apparently a lot of people lost those things because they were removable from the outside.   

[Platefire]

Guess What??? I can't find any 1K screen resistors on this amp. Checking with my MM the yellow lead for screen goes under board directly to pin #4 on both power tubes.


 See pix which shows the 10K/1Watt power resistor after choke. The inside yellow goes to filter cap can. The outside yellow that goes under board is screen voltage and the black is output from choke. This is a change we all missed!

Where are these screen resistors normally attached?? Platefire

[sluckey]

The EL34 version did not have any 1K/5W screen resistors. Just another indication that this is an EL34 amp rather than a 6550 amp.

     http://el34world.com/charts/Schematics/files/marshall/Marshall_jmp_lead_50w_1987.pdf

[Platefire]

Thanks sluckey, all my schematics had a 1K which wasn't exactly the right ones, so I copied that schematic you linked. Good, that's one thing less I'll have to straighten out:>)

[Ritchie200]

Can someone tell me which fuse holder is original. I think the one to the outside end but not sure. I'm getting ready to do a report to the customer and want to get all my info straight.

Another thing, on the impedance selector for Ohms speaker out, the selector handle with copper "U" is so loose that if you tilt the amp if falls out. Any good suggestions on correcting this?? Platefire

My '70 small box 50s had slotted fuse holders.  I've seen '72 Majors with slotted fuse holders.  My '73 has the tapered flower pot heads and it is original.  Like 2deaf said, I don't think either one on your amp is original.

I would put that selector in a bag with the death cap and hand it to your client.  A new plug is not going to fit any tighter unless you replace the entire assy and I still would not trust it.  Those things rattling around in the receptacles were the cause of many unintended smoke effects on stage and a dead amp.  Touring guys either soldered 8 or 16 direct to the jacks OR you can strip a 10 gage solid copper wire, bend it in a flat bottom "U" shape and it fits nice and tight.  If you look at my pic below you can see mine.  I didn't want to solder anything permanent because its all original, so I did the wire trick.  I need a needle nose to pull it out.

Jim

[Platefire]

Well thanks everybody for helping me determine what I got regarding original parts verses replacements. He never indicated any interest in restoring it to original so I don't guess I will worry about that.

Next thing to do is fire it up and check voltages and bias. Voltage is no problem but I'm a little concerned about bias. As is the only way to check mA is the shunt method and you know how critical it is to get it right on MM hookup on that. Best I can tell the choke input and OT B+ hookup is on the .5 amp fuse holder. So I would have to put the MM red lead on the fuse terminal where the choke/OT is connected and the black MM lead on Pin #3 of each tube.

Also i have a friend that has a Eurotubes Octal Bias Probe. The type you plug into the tube socket and plug your tube into the probe and attach you MM leads to the probe leads(I think?)---haven't laid hands on it or its instructions but---I'm thinking this would be a safer way to go. Platefire

Jim---on the Ohms selector, I can't see the picture to clearly. Did you insert the new 10Ga "U" into the old twist handle or left it off?

[2deaf]

I would be really interested in the bias voltage range with the 1/2A fuse removed.  Not only would it give you an idea of the ballpark and the ability to bias EL34's, but it would also let you know which way to turn the pot. to get the coldest bias if it's not obvious by looking at it.

[VMS]

You don't have to use shunt method. I use this method from aiken site:

The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off.  Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer.  Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side.  Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting.  You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube.  Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side.[/color]

https://www.aikenamps.com/the-last-word-on-biasing

[2deaf]

As is the only way to check mA is the shunt method and you know how critical it is to get it right on MM hookup on that.

If the thrill of having two hands close to 500V has worn off, you can always use the OT DC resistance method.  It can still be exciting if careless, but at least it's only one hand with no heart in the path.

Edit:  VMS has posted the method I referred to.

[Platefire]

OK-thanks, I will give that a shot and see how it goes. I also ran across this this morning that is similar but shooting for 70% max plate dispensation. Whatever is safer and accurate.

http://www.marstran.com/50W%20Bias.htm

[tubenit]

How about Hoffman's bias checker?  Works great!  Easy to build and safe to use.

http://el34world.com/charts/BiasChecker3.htm

With respect, Tubenit

[Ritchie200]

Jim---on the Ohms selector, I can't see the picture to clearly. Did you insert the new 10Ga "U" into the old twist handle or left it off?

I left the old twist handle off.  Just make sure it fits tight!

Jim

[Paul1453]

OK-thanks, I will give that a shot and see how it goes. I also ran across this this morning that is similar but shooting for 70% max plate dispensation. Whatever is safer and accurate.

http://www.marstran.com/50W%20Bias.htm

You could also get 1 ohm 1% resistors and put them in between the cathodes and Gnd.
Then you just measure the voltage drop across the resistor to tell you the current flow through the output tube.
This reading includes the screen current, so you just subtract the screen current out to get your bias reading measurement.   

I like being able to quickly and accurately check the bias measurements on fixed bias circuits. 

[Platefire]

tubenit Thanks, I really do need one of those!! Right now things are a little to busy for another project, so I think I will borrow my Friends for right now. I have a 50 watt Marshall project of my own in the works that I kind of laid aside temporarily to mess with this guys Marshall JMP 50. So that will go on a future project list.

Ritchie200---Does that copper "U" have to be attached for the amp to work??? or can I just stick it in a baggie with the handle? I may look around and see if I got some 10Ga copper wire but he hasn't requested it.

Paul1435---I don't usually do the bias resistors to an amp unless it is requested, especially a vintage amp.

Platefire

[Willabe]

Ritchie200---Does that copper "U" have to be attached for the amp to work??? 

Yes!   

It's like a switch to choose the speaker load Z. That piece of U shaped copper is the switching element. 

If it's not in place then the speaker jacks are disconnected from the OT secondary hot wire.

[Platefire]

Thanks, I didn't realize it was part of the switching circuit, I thought it just anchored in the switch to rotate it. So it intended to be pulled out and re-positioned to change Ohms? Again I not to familiar with Marshalls. Platefire

[Willabe]

So it intended to be pulled out and re-positioned to change Ohms?

Yes, it's the ohms 'hot' wire in between the the OT secondary and the speaker jacks.