Marshall JMP 50 watt model 1987 confirmation please

[Randall]

I have what I believe to be one of the earliest JMP 50 watt heads. Owner tells me it is a model 1987 (not year) built in 1967. It has a GZ34 tube rectifier and has the output selector with 100v marking painted over. The 8 and 16 ohm taps measure 0.7 and 0.9 ohms to ground respectively, and the painted over tap measures 0.5 ohms, so I think we can assume it is a 4 ohm tap.

I cannot find a proper schematic for this amp anywhere. All I can find is the solid state rectifier version issued in 1970. I am replacing the 5 electrolytics on the board, but 3 of the five do not match this drawing. The cathode bypass cap on V2 shows 320 uF, but has a 250 uF in it. And the two 50 uF on the plates of V1 and V2 are 22 uF 450v in it now. If no one knows of a schematic for this version, can someone at least confirm that the values in the drawing are the correct ones?

And also, whats up with the 50 uF filter cap on the outputs with the positive side grounded? it's not that way in the amp. I also have a discolored 1 watt resistor after the choke that reads 8.3K ohms that I would like to confirm that also.

It also has a .05uF 1000v cap across B+ and ground. What is the opinion on that? Seems like if it got leaky bad things would happen. Should I treat like a death cap and clip it?

http://schematicheaven.net/marshalla...d_50w_1987.pdf

[Jazz P Bass]

that may be a JTM45 1987 Lead.

Marshall switched to SS rectifiers around then.
Marshall Schematics

jtm45tr.pdf (Ignore the Tremelo circuit)

Not too sure where you are going with the 'Blacked Out 100V'.
That sounds like the power transformer input tap.

[mhuss]

I'd compare to the JTM45 or 1962 schems. Both were 50 watt amps with valve rectifiers.

[52 Bill]

Not too sure where you are going with the 'Blacked Out 100V'.
That sounds like the power transformer input tap.

The early amps had 100 volt taps for line distribution systems, like the 70 volt taps on commercial power amps.

[g1]

Randall, I think you are using ohms sometimes where you mean uF, for caps. It's a bit confusing, especially when no schematic.

[Enzo]

I might guess the 100v painted over is from a constant voltage output. Thinking marshall used some PA amplifier OT early on, and it had a CV winding, which was useless for guitar.

I am looking at your schematic:
get your terms right. It seems you may be confusing resistors and capacitors. 22uf at 500v is surely a cap, not a resistor. While resistors do have voltage ratings, they are only found on data sheets, not on schematics. The first stage bypass cap is 320uf, which is 320 microfarads, not ohms.

whats up with the 50 ohm filter cap on the outputs with the positive side grounded? it's not that way in the amp

.

And it is not that way in the schematic either. Those 50 MICROFARAD caps are drawn "upside down" for convenience to the draftsman. The upper side is grounded, and the bottom end is the + side, connected to B+.

It also has a .05uF 1000v cap across B+ and ground. What is the opinion on that? Seems like if it got leaky bad things would happen. Should I treat like a death cap and clip it?

And if the filter caps got leaky bad things would happen, or the OT or the tubes or etc. Any part in the amp can go bad and cause trouble. But why the urge to remove it? A bypass cap around the filters is a common circuit element, it reduces noise. I don't know what my spleen does, but I sure wouldn't expect my doctor to snip it out just because.

You are apparently fixing this one up for someone. So if he liked the sound of it before, you might resist the urge to change stuff. replace bad parts, yes, but for example the difference between 320uf and 250uf is not much, especially in terms of tone. I'd rather see 50uf filter caps on the B+, but if the 22uf caps are doing the job, leave that value there.

Look at this drawing:
http://bmamps.com/Schematics/marshal...5_lead_45w.pdf

Did you measuer 8.3k or was that printed on the part? Schematic says 8.2k, and there are your 22uf caps, and your 250uf bypass cap.

[catalin gramada]

67 rectifier tube is a jtm 50 for sure. A direct evolution from 45. You Will not found some precise description as the circuit was tweaked a lot în this period. First you have to be sure what output transformer have to know for what tubes was designed. Usually 784.139 for el34. There cap you asked for should be a 0.05uf conected direct afte rectifier to smooth rectifier noise and also act as snubber to tame the pop noise from stb ht swich when comute. You have to put some pics to know exactly what you have. Basically a 45 but have to guess you have a 10k between v1 and v2 into supply rail and also a dual filter cap on the board.whilst 45 have both v1 and v2 conected from a single supply node.and off course a single filter cap. A 67 jtm50 is a very colectable piece. From its design it is unsafe for amp somehow to use it as it is.some tweakings are absolute necessary as screen grid resistors added and corectly wiring of ht supply to stb swich

[Justin Thomas]

Hey Enzo,

I see the same thing Randall sees - the filter for the screens is a 50uF cap, with what seems to be a tiny "+" sign on the end attached to ground. It's on the first & third pages of the schem he linked. The Piazza drawing is right. I'm just going to call it a typo because I know better. The bias caps are also drawn "+" to ground.

Justin

[Enzo]

I see it now, yes, that is a typo. he just said "on the outputs" or something similar, So I looked elsewhere.

[catalin gramada]

Hey Enzo,

I see the same thing Randall sees - the filter for the screens is a 50uF cap, with what seems to be a tiny "+" sign on the end attached to ground. It's on the first & third pages of the schem he linked. The Piazza drawing is right. I'm just going to call it a typo because I know better. The bias caps are also drawn "+" to ground.

Justin

the cap for the screens should be 16uf if original a dual 16/32 with 32 în pi

[catalin gramada]

Later add. You OT is an 784-128 an Early version of 139 which have no 4ohm winding but 100v output for PA aplication. It was designed for el34 use. You amp is a jtm 50 with no doubt 67 era.Don.t mod .dont do no thing with it if you dont like how it sound. Sell it for big money and buy four 1987 for the price if you like

[Randall]

yep, sorry about that. Fixed.

[Randall]

Hey Enzo, I do know the difference between a cap and a resistor, and an ohm and a uF, I promise. It was just a dumb gaff, I fixed it.

The JTM 45 schem more closely resembles the amp I have, except no screen resistors or that big 1K 2W feeding them. My trannys have no markings AFAIK, and definitely is marked JMP on the front panel. Based on what I see in this schem, The 250uF cathode bypass cap is right, the 8 uF bias caps are right (reverse polarity), and I will go with 22uF 500v for the preamp plate nodes, replacing the 16uFs, because that is what is in there now. The 8.3K measured resistor in question also appears to be correct, even if it is a bit too discolored to read.

I think this must be a very early transition from JTM to JMP, because it seems to have a bit of both. I'm thinking the painted over "100v" marking on the impedance selector was just using up old parts before new ones were made. I should be able to verify this by putting a meter on that tap once it is up and running again and seeing if there is 100v there, correct?

[Jazz P Bass]

I should be able to verify this by putting a meter on that tap once it is up and running again and seeing if there is 100v there, correct?

If it is a Constant Voltage tap, 100Vac will be seen Only at full power.

Guide_to_constant_Voltage_systems_original.pdf

[nosaj]

Hey Enzo, I do know the difference between a cap and a resistor, and an ohm and a uF, I promise. It was just a dumb gaff, I fixed it.

The JTM 45 schem more closely resembles the amp I have, except no screen resistors or that big 1K 2W feeding them. My trannys have no markings AFAIK, and definitely is marked JMP on the front panel. Based on what I see in this schem, The 250uF cathode bypass cap is right, the 8 uF bias caps are right (reverse polarity), and I will go with 22uF 500v for the preamp plate nodes, replacing the 16uFs, because that is what is in there now. The 8.3K measured resistor in question also appears to be correct, even if it is a bit too discolored to read.

I think this must be a very early transition from JTM to JMP, because it seems to have a bit of both. I'm thinking the painted over "100v" marking on the impedance selector was just using up old parts before new ones were made. I should be able to verify this by putting a meter on that tap once it is up and running again and seeing if there is 100v there, correct?

Is there a problem with this amp?

the discolored 8.3k resistor wonder if it is still 8.3k with voltage on it? have to confer with Mr Ohm on that one.

nosaj