1972 Marshall 50 watt JMP Head with Problems

[Platefire]

Got a call yesterday on this head that I will be getting latter this week. The guy said it started blowing fuses when he plugged a tube in wrong?? He says on standby is fine but when he switches it on, blows fuses. That's all the details I have at this time. I'm thinking something blown in the power supply.

Trying to get a jump start with a schematic on this repair I'm thinking this would be a JMP 50 watt model 2204. For you more familiar with Marshall's, maybe you can pin it down for me. I pray this Marshall repair goes better than the last. Platefire

[HotBluePlates]

JMP, yes. 2204, no (because that's a JCM 800 series amp).

What you'll see is there are Lead and Bass versions of the JMP. Probably won't matter much in your case, because the differences in those are in the preamp, and you're probably looking for a power supply or bias supply problem.

[birt]

JMP, yes. 2204, no (because that's a JCM 800 series amp).

What you'll see is there are Lead and Bass versions of the JMP. Probably won't matter much in your case, because the differences in those are in the preamp, and you're probably looking for a power supply or bias supply problem.

my JMP is a 2203. after '75 JMP's have the 2204 or 2203 schematic and that model number is on the back. Earlier models don't have a master volume. http://drtube.com/schematics/marshall/78331-1.gif

if he plugged a tube in wrong he can also have blown the tube. and now that he tried it plugged in the right way the tube blows the fuse.

[sluckey]

Could be many things. Just wait until you get the amp and you'll be able to get much better clues, including which model it really is.

[HotBluePlates]

Yeah, sluckey's got a point.

I used to own a '73 50w, in the Bass variety. I wish I hadn't sold it and the 100w basketweave cabinet that I had with it.

[Colas LeGrippa]

I have sold so many things I wish I have kept, starting by a telecaster bought brand new in 71 

[Platefire]

Yelp! Could be the tube and damage to power supply too. Just have to wait and open it up. I will also question him more about which tube and how he did it. Thanks for pointing me the most likely right schematic-downloaded it. Platefire

[shortfuse]

Plugged a tube in wrong?
I had a JMP that when I replaced the tubes once a brand new EL34 ran away and would pop the fuse. 
Like Sluckey said you'll know more when you see it.

[Platefire]

Well I'm hoping and praying it's one of the older ones with a turret or eyelet board before they started using pbc. Platefire

[HotBluePlates]

My '73 was pcb. But I seem to recall that the turret construction was used up til '72.

(If you hadn't figured out, I don't have a deep knowledge of Marshall's amp line, the way I might for Fender amps).

[phsyconoodler]

Those amps have HT fuses that blow when there is a tube issue.With worn tube sockets or sloppy ones like some old ones were,it's not that hard to plug in a tube one notch or more off.
  You may have almost no damage at all,maybe just a tube and a fuse if you are lucky.
Time will tell.

p.s. stop fretting about the circuit boards in those amps Plate.They are easy to work on and make pretty good sense.

[birt]

those early marshall circuit boards are laid out like a turret board and are very easy to work on.

[Platefire]

OK, no more fretting. I'm always excited about getting my hands on an old amp. I'll take some good pixs when I get it. I love History, and this is tube amp history in my hands. Also I know most folks don't put much stock in tube testers---so I got that!! but my Military Surplus TV7 don't have a listing for a EL34 but does have a listing for a 6CA7 which on my research on the net last night told me was the same thing? Plus I looked up a 6L6 setting and it was basicly the same as a EL34 except for the bias setting different a few points 23/6L6 and 25/6CA7 and Minimum value 40/6L6 and 30/6CA7. Also my tester has a light that comes on if a tube is shorted. So it should provide some kind of indicator if the tube is dead or still has some life. No I don't have a extra EL34 on hand. Got to get some or at least one if I'm going to keep foolen with Marshalls. Platefire

[The_Gaz]

You wanna use the 6CA7 setting on the TV7. EL34 is the British designation.

[mac0611]

Got a call yesterday on this head that I will be getting latter this week. The guy said it started blowing fuses when he plugged a tube in wrong?? He says on standby is fine but when he switches it on, blows fuses. That's all the details I have at this time. I'm thinking something blown in the power supply.

Trying to get a jump start with a schematic on this repair I'm thinking this would be a JMP 50 watt model 2204. For you more familiar with Marshall's, maybe you can pin it down for me. I pray this Marshall repair goes better than the last. Platefire

  If it is a hi voltage model, cemf may have ruined one of the power tube sockets, or it could be a bad tube. With today's el34's higher screen resistor values need to be considered to keep hi voltage marshalls from melting down. All speculation, of course. 

[Platefire]

Well here we go, adventure on the high seas! Got the 50 W Marshall Head today and also got a Fender Twin with tremolo problems. I will concentrate on the Marshall for now. I have gone over it to check the basics. A paper tag glued to chassis says "50 W Lead M765". A number 5541D is also stamped into the chassis. That's the only hint of model numbers. The schematic Drawing # 78331-2 don't exactly match as it shows the standby after the diodes and this amp apparently has the standby before the diodes. If you view the pix showing the stanby switch and the black diode box or whatever that is on the board--the two red secondarys from PT connect to the bottom of the standby switch. The blue wires from the SB Switch go to the board turrets and from there red wires to the diode connections on the diode box. The white wire from SB is to bias supply. I fired it up with all tubes removed. When you engage the standby, it provides AC to the diodes which blows the 3 amp fuse. I have 671VAC measureing accross the secondarys on the bottom of standby switch. Also reading 6+ AC heater voltage on the tube sockets. If I'm understanding this correctly, it appears the PT is good but the diode box on the board is bad???
This Marshall power supply arrangement is really new to me and I may be getting something wrong, so please check me out.  

[HotBluePlates]

If you view the pix showing the stanby switch and the black diode box or whatever that is on the board--the two red secondarys from PT connect to the bottom of the standby switch. The blue wires from the SB Switch go to the board turrets and from there red wires to the diode connections on the diode box. The white wire from SB is to bias supply. I fired it up with all tubes removed. When you engage the standby, it provides AC to the diodes which blows the 3 amp fuse. I have 671VAC measureing accross the secondarys on the bottom of standby switch.   

The "diode box" is a bridge rectifier. It is not used as a true bridge rectifier, though, because the "-" output terminal is not used. Therefore, only 2 of the 4 bridge diodes are used, making it electrically the same as the full-wave rectifier shown in the schematic.

Someone has done a fair amount of work, in the not-to-distant past, to the power supply. The filter cap cans have been swapped for newer twistlock type cans. They have them mounted further into the chassis than the original cans; I can't tell from your picture angles, but one of the can's terminals may be uncomfortably close to the fuseholder terminals. Likely, the installer didn't want the non-original look of the fiber mounting washers that normal insulate these cans from the chassis, and which also keep the can held further out of the chassis.

The orange drop and 2 black caps beside the bridge are non-original; the schematic shows two (likely 0.22uF) caps capacitively balancing the output of the PT. I'm not sure those are needed, and I wonder about them possibly failing.

671vac across the secondary likely means the PT and associated wiring is good. Try unsoldering the yellow wire that runs from the + output of the bridge (and added caps) to the first filter cap, and power on to see if the fuse holds. If the fuse blows (lamp limiter glows bright), remove the orange drop and black caps, and try again. If the fuse still blows, leave the added caps out, and replace the diode bridge; use the 1000v 3A bridge Doug sells.

Also, the impedance selector switch (held in by tape)... does it really need the tape to stay in place? If it's not tight, it will cause all sorts of grief.

Use a lamp limiter if you have one (build one if you don't), and use it for future power-on testing to see if the short circuit has been resolved.

See below for pictures of the innards of other '72 50w heads.

Close-ups here

[Platefire]

Thanks HBP

har! The customer thinks the amp is pretty much original except for the MV added at one of the input jacks. Boy have I got news for him!

Ok! from you comments I have a few questions as follows:

1-Regarding the diode box. If it does end up need replacing after testing would I just pull that old box and wire Dougs up in same spot?

2-On the filter cap under fuse holder there is about 1/4" clearance. The filter cap clamp is really tight and is not in danger moving. Appartly has been working good without any problems.

3-The added caps is .22 orange drop and .1 two blacks. They are obviouly not on the correct 1972 pix you provided. I can see they are on the schematic I have Dwg. 78331-2 as C106 & C16. Someone must of thought of it as a modern upgrade?

4-OK-on disconnecting yellow wire from B+ out to first filter cap ? I will build a lamp limiter before doing any more testing. I had already downloaded the plans and figured I would need one sooner or later. So I will go ahead and do that.

5-I removed the tape from the Ohms selector switch and the outter dial lid actually pulls out about 1/4" where I can see a couple of pins in the switch. Is there a way to repair that or would it require replacement? See pix with dial pulled out.

Thanks Again, Platefire

[HotBluePlates]

har! The customer thinks the amp is pretty much original except for the MV added at one of the input jacks. Boy have I got news for him!

Hang on there! If you look over the entire rest of the amp, everything is original. It looks like only the swapped filter caps and the caps next to the diode bridge are non-original; however, they are not "mods" either, because they adhere to the original circuit.

1-Regarding the diode box. If it does end up need replacing after testing would I just pull that old box and wire Dougs up in same spot?

Yes. You don't see it in the catalog pic, but there is a small hole in the center of Doug's bridge. You may want a way to mark the ~ and + terminals, because you will have to mount the bridge face-down, and bend the existing leads to tie with the needed wiring.

2-On the filter cap under fuse holder there is about 1/4" clearance. The filter cap clamp is really tight and is not in danger moving. Appartly has been working good without any problems.

Then my needless worrying was just needless worrying. They'll only need to be bothered if you find one of the cans is shorted internally.

3-The added caps is .22 orange drop and .1 two blacks. They are obviouly not on the correct 1972 pix you provided. I can see they are on the schematic I have Dwg. 78331-2 as C106 & C16. Someone must of thought of it as a modern upgrade?

Note the date of the JMP 50w 1987 schematic is July, 1970. One version shows the caps, the other doesn't. It might then be a modern upgrade to remove those caps. They don't serve any useful purpose that I see (or that can't be done better a different way).

My advice is to remove them and not look back.

5-I removed the tape from the Ohms selector switch and the outter dial lid actually pulls out about 1/4" where I can see a couple of pins in the switch. Is there a way to repair that or would it require replacement? See pix with dial pulled out.

The plug has to be removable, so you can select the different impedances. But, it should also fit snugly when inserted.

If it doesn't, or makes intermittent contact, the OT can essentially lose speaker load while you're playing at high volume. The result is possibly high inductive flyback voltage that damages the interwinding insulation of the OT.

If it is very loose (and the socket can't be retensioned), we may need some advice from folks that have fixed more Marshall's than me. They'll likely know the good fixes, or best replacement impedance selector to buy. I'd clear any change on that selector with the customer first, and then them dictate the direction they want to go in (again, only if replacement is necessary).

[birt]

there are also plate resistors that have been replaced. i wouldn't called that a mod but it's not original either. but who cares?

for the impedance selector, if it's ok for the owner it might be better to go with a more modern replacement. this one will fit: http://cdn.banzaimusic.com/images/P/23347_th.gif

[Willabe]

It looks like the death cap is still wired up to the ground switch.            That should be pulled.

The impedance switch is/was common to go bad on the old Marshalls. I had a 73" (?) 50w years ago and someone just took a piece of 14g copper wire and bent it in a U shape to replace the knob, worked fine. I've heard that was a common fix.

Doug's got a 3 way impedance selector, $9.50, but it's a different type, style wise. Looks sturdy.

                 Brad      

[HotBluePlates]

The impedance switch is/was common to go bad on the old Marshalls. I had a 73" (?) 50w years ago and someone just took a piece of 14g copper wire and bent it in a U shape to replace the knob, worked fine. I've heard that was a common fix.

I can barely remember now, but I think my '73 also used a short hunk of ~14ga copper in place of the actual plug.

[Ritchie200]

+1 on getting rid of the death cap.  If you look close on the pic below, I used to have the 14g wire in the selector!  I have since wired 8 ohms permanent.  Those plug selectors fried more OT's than any operator error ever did.  If he wants to keep it stock you may want to ask if you can wire a permanent setting or add Doug's selector if he is not worried about it.

Jim

[Platefire]

I wondered about that ground switch. Should the switch and cap be disconnected and just abandoned in place to retain originality? Would look bad with a hole in the front with switch removed.

Yes something will have to be done to correct the Ohm selector switch. I would vote for saving the old cap/dial in a sandwitch bag taped to the inside of cab and installing the "U" wire connector. I did notice he had is set on 16 Ohms that I thought was odd but I don't know what kind of cab he's using either. Something I would need to get the best options and present it to him for a decision.

OK, here is the red neck, rough riden, stump jumping, rump thumping, light limiter ready for action. Just built in about an hour and a half out of stuff on hand. Now I got to go clean up my mess from eveything I pulled out to put it together. I may get around to testing the Marshall tonight to find out what is shorted. Platefire

BTW-One more question. The customer ask me about the gound and power switch with the lever being plastic coated---was that original? because the standby lever is all metal. I told him I didn't know but would ask.

[Platefire]

I think I might have found something, you tell me. On the orange drop (in pix below) tied to one of the PT secondarys to ground right next to the diode box, I decided to do a continuity check accross the cap and I'm getting continuity signal from my MM with the leads toutched to both ends of the cap. Don't that mean it's shorted and could be my main problem? I also checked he paralleled black ones next door, but no continuity through those. Platefire

EDIT 1: I lifted the caps off ground to the PT secondarys, fired the amp up with stanby on, then fliped the standby off and the light brigtened up a second and went off but pilot light stayed on.
I checked the B+ on the diode output and have 468DCV unloaded. Thanks HBP, your suspensions were correct!

EDIT 2: From this point, went ahead and installed tubes, fired it up with light limiter and everything came up well. Played my guitar through it some. Checked voltages but they were way low. Turned it off and fired it up without the limiter. Everything seems to be working well and voltages were higher in normal range. I'd say we have discoverd the problem. It's just rather the customer wants to maintain the caps accross secondarys mod or remove them. Also issues about death cap and Ohm selector. Thanks, Platefire---Ye Haw!!!  I feel Good---   Happy New Year!

[HotBluePlates]

Respectfully, I wouldn't give him the option. I'd yank those caps out, because if they are problems now, their replacements probably will be in the future, too.

I decided to do a continuity check accross the cap and I'm getting continuity signal from my MM with the leads toutched to both ends of the cap. Don't that mean it's shorted and could be my main problem? I also checked he paralleled black ones next door, but no continuity through those.

Look at the schematic again. The intended placement of the caps was across half of the secondary for each cap. If you measure continuity, and everything is hooked up "properly" then you could get continuity through the secondary winding.

Okay, the winding might be high enough resistance to not give continuity. Just double-check everything before we declare victory.

And the plastic-levered power switch is probably a replacement. I don't know exactly when, but sometime in the late 70's or early 80's, that type of switch was standard on Marshall amps. The correct switch for the '72 has a metal toggle, like the other switches on that amp.

[Platefire]

I've been jamming through the amp just letting it run listening for any hick-ups, all good so far. The clean sound really reminds me of my Bogen/Bassman. It really has a good clean sound.

I did check the bias this morning using the shunt method and both tubes were at about 22 mA which seems kind of cold to me. Plate voltage is at 432. The bias calculator says it should be 40.5 mA.

On those switches the polarity and power are both plastic covered toggels and the stanby is metal.  

Thanks again for your help on this--hopefully we're there? I might have eventually found it on my own but I never would have found the problem this fast without your help.   Platefire

[Ritchie200]

Plate, all the switches were originally metal toggles.  Yep, that sounds a little cold.  I think you will find a different amp - for the better - once you get them cookin'!  About 3/4 up on the volume and you should get that nice woody Marshall grunt.  It will be loud!

Jim

[Platefire]

Thanks Jim for the answer on those switches. Yelp, I'll have to adjust the bias up and give it a test run.

When I first got it up and running. I plugged my guitar into channel one(I). It was unbearably bright. Seemed that no matter how I adjusted the tone controls it was unbearably bright. When I plugged into channel 2(II) the tone then sounded more like my Bassman conversions with a lot more bottom. I was just wondering if that brightness of channel 1 tone normal? Platefire

[HotBluePlates]

I started to write this earlier, then deleted the post. Guess I oughta re-write it.

The brightness of the bright channel is normal. It is somewhat less bright when the volume is up above half.

When I had a '73 50w head, I always thought the normal channel was mud, and the bright channel was too bright. I plugged in my guitar to the bright channel #1 jack, then connected a short cable from bright #2 jack to the normal #1 jack. This jumpers the channels temporarily. Now use the two volume controls as rough treble/bass controls and to set the volume, then do final tweaks to the sound with the tone controls.

Doing that, you can get very nice balanced sounds. You can get get a little more dark or a little more bright, as needed.

[Platefire]

In my reply #26 above I mentioned that bias from bias calculator With two EL34's AB operation at 437 Plate volts should be 40mA and measuring bias using the shunt method showed 22ma--WRONG  I was measuring off the screen on the wrong pin.  

So now that I'm on the plate pin   MAN! was in the 55mA range--Hot! So I turned the bias down a bit.

So those JJ EL34's are not too well matched. Putting one at 40mA, the other reads about 33mA.
I cranked the hottest one up to 41.5 MA and the others is now at 35.2. I haven't had much experiance with JJ EL34's so---Does this seem like a reasonable balance? Lot cooler than it was!

Also I talked with the Customer Thursday and he agreed on removeing the two .22 caps to ground on the PT secondary's, also to abandon the death cap and polarity switch in place and I made a 16Ga copper "U" to go in the selector switch which he approved of that is a whole lot tighter/solid connection. All that has been done and if this last bias setting is OK, I should be good to go and just put the chassis back in the cab. I will run it a while, jam through it, watch those EL34 Plates, notice sound & response and if all seems good call it a dun deel.   Thanks, Platefire

[bigsbybender]

So those JJ EL34's are not too well matched. Putting one at 40mA, the other reads about 33mA.
I cranked the hottest one up to 41.5 MA and the others is now at 35.2. I haven't had much experiance with JJ EL34's so---Does this seem like a reasonable balance? Lot cooler than it was!

Terribly mismatched bias may cause some audible hum. Otherwise just set them as high as you can before one red-plates and back it down a little from there. That's how I do it anymore. I wouldn't worry too much about the "mismatching" of the tubes... I'm sure they'll sound fine...If there isn't an excessive amount of hum, you're in good shape.


j.

[Willabe]

I'm sure they'll sound fine...If there isn't an excessive amount of hum, you're in good shape.

Yes, but, mismatch can also kill bass end response,( because of standing dc in the OT?)          


             Brad      

[Platefire]

Well I ran the lowest one up to about 40mA and that put the highest one around 46 or 47mA. I'm watching it for any red plateing. It really didn't sound very good at that lowest bias setting and was distorting too early(not enough clean headroom). It's sounding pretty good at the last setting if the tubes hold up. Thanks, Platefire

[Willabe]

I think that most guys here say 4 to 5 mA's for a pair, is good, so, I would say your well in the ball park.


       Brad        

 

[Rev D]

 Mismatched or not, I've never had a problem with low end on a Marshall, most of the ones I've played or owned were pants flappers when you get the volume up..   I've had problems with JJ's el34's, on two pairs one of them died in each pair. While I like the little light show they do, if you got two that work excellent, at least now I have a pair that work haha! I think I remember Dr. Z saying JJ's quality control was pretty much nil, I think that was in a recent post (on another forum my bad..). It's unfortunate, I like they're 12ax7's but fortunately I've never had a problem with one of those, must just be they're octal power amp type tubes. Good job on the fix, outside of that power supply, it still has the mustards in there, I bet its a nice sounding amp now! Good job on the fix!

Regards,

Don

[Platefire]

Thanks Don

This is my first vintage Marshall to work on and a lot better experiance so far than the 1986 JCM 800 I worked on a while back.

The bias on this JMP 50 was was set super hot when I got it up and running.  I've set it in what I would consider a more reasonable range to get more life out of the tubes. I keep wondering that if the customer has got use to the super hot bias sound, he might not be happy with my cooler settings? I'm trying to decide rather to leave it like it is or beef it up a bit to see if the tubes will continue to operate without red-plating. I'm really ready to button it up in the cab and start working on the twin reverb that needs some attention, but that's another post.

The sound/response reminds me of my Bogen/Bassman conversion using the #2 input on the Marshall. I can use and work with that sound. The #1 inputs are just to bright for me--can't bear it. The transformers are monsters size-wise compared to most vintage fenders. This has been a learning experiance for sure. Platefire