'68 JMP50 1986

[TAZIN]

A lot of these 118xx-119xx builds tend to be very late '68 thru very early '69. This one could be from Febraury 1969 since the Hunts 50uF-50uF/450v mains filter cap is date as such. Typically, these builds use a Hunts single element 50uF/450v cap which would explain why only one-half of the 50 - 50 is hooked up. The use of the gray Marcon 40uF-40uF/450v is another "staple" for amps built within the 11500 - 119xx range, and they're typically used for the screens/PI position. Another indicator pointing towards an early '69 build is the use of the brown RS 47pF silver mica across the two plate resistors for V3. This is what you tend to see in early '69 builds vs the gray tubular ceramic cap for '68 builds.

The NFB should be coming off the output solder lug on the Selector, and in the photos it sure looks like it does.

The RS/Dubilier of England Blue caps are paper & foil, whereas the Gray caps are metalized paper.
Overall, it's in really nice condition.

 

[AndyD]

A lot of these 118xx-119xx builds tend to be very late '68 thru very early '69. This one could be from Febraury 1969 since the Hunts 50uF-50uF/450v mains filter cap is date as such. Typically, these builds use a Hunts single element 50uF/450v cap which would explain why only one-half of the 50 - 50 is hooked up. The use of the gray Marcon 40uF-40uF/450v is another "staple" for amps built within the 11500 - 119xx range, and they're typically used for the screens/PI position. Another indicator pointing towards an early '69 build is the use of the brown RS 47pF silver mica across the two plate resistors for V3. This is what you tend to see in early '69 builds vs the gray tubular ceramic cap for '68 builds.

The NFB should be coming off the output solder lug on the Selector, and in the photos it sure looks like it does.

The RS/Dubilier of England Blue caps are paper & foil, whereas the Gray caps are metalized paper.
Overall, it's in really nice condition.

Thank you very much for this Tazin. I was hoping you would chime in. Very interested to read what you had to say....and yes, the NFB is coming off the output solder lug on the selector. Shall we say early '69 then?

 

[AndyD]

Replace every cap in the amp, but keep the originals as they might be wanted if you decide to sell the amp down the line. Personally, I'd also replace the bias pot (again, keeping the original in a little baggy). I'd also look at bypassing the impedence selector internally in a reversible manner to avoid it ruining your day if and when you start playing through it.

As @Marcomel79 stated, the panel is in amazing condition, assuming its original! HNAD!!

Hi, what makes you think that the sliding variable resistor should be of concern?
Thank you for your comment on the front panel. Of course it is original!

 

[michelebis]

Congrats on that beauty! i agree about 69. Lead upon the board, and the use of that dubilier caps, also the preamp filter cap is on the side... all together with what already told indicates first half of 69.

 

[PelliX]

Hi, what makes you think that the sliding variable resistor should be of concern?

They tend to become less reliable over time due to corrosion and various other factors, and they were never really the 'best' design. I'd put in a proper pot for peace of mind. As I suggested, by all means keep the original, of course.

 

[BlueX]

They tend to become less reliable over time due to corrosion and various other factors, and they were never really the 'best' design. I'd put in a proper pot for peace of mind. As I suggested, by all means keep the original, of course.

Hi, what makes you think that the sliding variable resistor should be of concern?

An end-stop resistor in series with the bias pot, at least? In case the pot fails short.

I think it's always a balance between keeping vintage stuff as original as possible for as long as possible, and make sure it's safe to use and won't cause "collateral damage".

 

[BlueX]

Here is my recently acquired JMP 50 bass model which I think from the serial number, S/118xx, and H8 and J8 date codes on the AB potentiometers lead me to think it is a mid to late 1968 production amp. The amp came to me in a bit of a state and not working, but with the enormous help of @playloud and a bit of careful cleaning, we managed to get this amp resurrected. The amp seems to be in incredibly original condition with one or two interesting clues to its past life. The filter caps took 5 days to reform, and even then, the preamp cap is borderline in terms of leakage. I am debating in my mind whether or not to replace these for long term reliability. This will cause some strong opinions, I don't doubt! The main issues in getting the amp restarted were down seized potentiometers (which freed up with some dioxite), a failed rectifier diode (all now replaced with new for now) and oscillation which I eventually cured with added 5K6 grid resistors after going through the usual list of lead dress, grounding etc etc. I understand now that these 27K NFB amps are notoriously unstable. I automatically added some screen resistors. The original power indicator had blown so I have temporarily put in a modern replacement until I can source an original (if possible). The -118 transformer i think is a very early replacement. I think this because of the burn marks on the transformer mounting plate, lack of ink on the selector connections and evidence of the transformer mounting bolts being disturbed. I could be wrong, but this is what I think. I haven't had a chance to play it much yet because I have spent all of my time to date getting it to work, but it does sound very promising!
The bias circuit seems stable, the B+ is 420v - 440v depending on my very variable wall voltage and heaters 6.15 - 6.30V.
Overall I think i have got away with murder and couldn't be happier with the outcome. I welcome any further observations from @TAZIN and @neikeel as long as they are positive!

What size (diameter and length) are the Mains and HT fuses on your amp?

Mine are 5 x 20 mm. The fuse holders are mounted with large washer on both inside and outside of chassis, probably due to holders being smaller. I've seen this on many Swedish Marshalls from this eras, so it might be original for SEMKO version (although it looks cheap).

 

[TAZIN]

Thank you very much for this Tazin. I was hoping you would chime in. Very interested to read what you had to say....and yes, the NFB is coming off the output solder lug on the selector. Shall we say early '69 then?

Yes, I would say February 1969. If it were a slightly later build, then there would different indicators.

 

[TAZIN]

What size (diameter and length) are the Mains and HT fuses on your amp?

Mine are 5 x 20 mm. The fuse holders are mounted with large washer on both inside and outside of chassis, probably due to holders being smaller. I've seen this on many Swedish Marshalls from this eras, so it might be original for SEMKO version (although it looks cheap).

View attachment 156398View attachment 156399

Yes, these small fuse holders were used on Scandinavian market builds. The Belling Lee and SMK fuse holders are like 16mm in diameter for the threaded section.

 

[Brewster]

Congrats on the new head! She looks like a great find and a true keeper. Enjoy

 

[guitarzan2525]

Here is my recently acquired JMP 50 bass model which I think from the serial number, S/118xx, and H8 and J8 date codes on the AB potentiometers lead me to think it is a mid to late 1968 production amp. The amp came to me in a bit of a state and not working, but with the enormous help of @playloud and a bit of careful cleaning, we managed to get this amp resurrected. The amp seems to be in incredibly original condition with one or two interesting clues to its past life. The filter caps took 5 days to reform, and even then, the preamp cap is borderline in terms of leakage. I am debating in my mind whether or not to replace these for long term reliability. This will cause some strong opinions, I don't doubt! The main issues in getting the amp restarted were down seized potentiometers (which freed up with some dioxite), a failed rectifier diode (all now replaced with new for now) and oscillation which I eventually cured with added 5K6 grid resistors after going through the usual list of lead dress, grounding etc etc. I understand now that these 27K NFB amps are notoriously unstable. I automatically added some screen resistors. The original power indicator had blown so I have temporarily put in a modern replacement until I can source an original (if possible). The -118 transformer i think is a very early replacement. I think this because of the burn marks on the transformer mounting plate, lack of ink on the selector connections and evidence of the transformer mounting bolts being disturbed. I could be wrong, but this is what I think. I haven't had a chance to play it much yet because I have spent all of my time to date getting it to work, but it does sound very promising!
The bias circuit seems stable, the B+ is 420v - 440v depending on my very variable wall voltage and heaters 6.15 - 6.30V.
Overall I think i have got away with murder and couldn't be happier with the outcome. I welcome any further observations from @TAZIN and @neikeel as long as they are positive!

Very cool amp Andy.
I have one that is slightly earlier than this one as well..

 

[Ivan H]

Hi, what makes you think that the sliding variable resistor should be of concern?

The sliding variable resistor is the "shunt" resistor of the voltage divider that determines the negative bias voltage. The slider simply grounds out a portion of the resistance, thus lowering its value. Should the slider lose contact with the carbon element, the value of this variable shunt resistor would become it's maximum value (100k), meaning the negative bias voltage would also become the maximum available, thus biasing the output tubes cold.
My opinion, hit it with some contact cleaner & leave it in there. Enjoy that fine amp. Cheers

 

[AndyD]

The sliding variable resistor is the "shunt" resistor of the voltage divider that determines the negative bias voltage. The slider simply grounds out a portion of the resistance, thus lowering its value. Should the slider lose contact with the carbon element, the value of this variable shunt resistor's value would become it's maximum value (100k), meaning the negative bias voltage would also become the maximum available, thus biasing the output tubes cold.
My opinion, hit it with some contact cleaner & leave it in there. Enjoy that fine amp. Cheers

Thank you. I am inclined to agree with you regarding the sliding resistor. I automatically cleaned it with deoxite when I comprehensively serviced the amp and the slider seems to work extremely well. It’s a bit fiddly to adjust, but I think it’s a cool feature. Also, a rotary potentiometer uses a carbon track to adjust the resistance so why would this be any less effective? If anything, the carbon track on the slider looks very substantial.
I promise to enjoy the amp!

 

[AndyD]

Very cool amp Andy.
I have one that is slightly earlier than this one as well..

Cool! I would love to see some photos.

 

[neikeel]

Lovely example - glad the encouragement worked.
Tazin and Ivan have you the skinny. Have you bought a pot of Deoxit Gold 100 (with little brush). Perfect for reconditioning the slider. I also work it into pots once they are clean of dust and debris.
Are you going to take it on the road and gig it 5 nights a week? Are you going to alternate between it and your similar era 1987 or your clones?
If your filter caps did not get to proper spec/test in range they should probably be replaced, same with the bias caps. Keep the old ones safe.
Valve storm do an Erie 32/32 that is to 100% spec so try that first and redo the reforming. If you get under 5v within 3-4 hours you are golden. If not I think F&Ts across the board. It will sound and feel different (maybe better) but you will get used to it after a few months.
No idea what the contributor meant removing all the signal caps, must have been smoking something so I will not name (or suggestion being PIO - only seen in 63 offsets).
However if it will influence you I’ll repeat a previous statement- I’ve never seen a transformer taken out by failed filter cap.
Nice buy.

 

[Sigs]

Re this sliding resistor, is there a method to set an approx value before powering up an amp ? for 4 x KT66's.

 

[knulp]

Lovely example - glad the encouragement worked.
Tazin and Ivan have you the skinny. Have you bought a pot of Deoxit Gold 100 (with little brush). Perfect for reconditioning the slider. I also work it into pots once they are clean of dust and debris.
Are you going to take it on the road and gig it 5 nights a week? Are you going to alternate between it and your similar era 1987 or your clones?
If your filter caps did not get to proper spec/test in range they should probably be replaced, same with the bias caps. Keep the old ones safe.
Valve storm do an Erie 32/32 that is to 100% spec so try that first and redo the reforming. If you get under 5v within 3-4 hours you are golden. If not I think F&Ts across the board. It will sound and feel different (maybe better) but you will get used to it after a few months.
No idea what the contributor meant removing all the signal caps, must have been smoking something so I will not name (or suggestion being PIO - only seen in 63 offsets).
However if it will influence you I’ll repeat a previous statement- I’ve never seen a transformer taken out by failed filter cap.
Nice buy.

I’ve had a filter cap dying in smoke some month ago in a tube preamp while it was open in front of me.
It took away with him a few resistors and others started to go hot and smoking.
If this happens in a output stage..can’t the power tubes go in trouble and transformers with them as a consequence?
I’m not saying that there’s a direct connection between filter caps failing and transformer failure but once components and tubes start to go south one after the other I would not feel comfortable with the idea that usually transformers are safe.

 

[AndyD]

Lovely example - glad the encouragement worked.
Tazin and Ivan have you the skinny. Have you bought a pot of Deoxit Gold 100 (with little brush). Perfect for reconditioning the slider. I also work it into pots once they are clean of dust and debris.
Are you going to take it on the road and gig it 5 nights a week? Are you going to alternate between it and your similar era 1987 or your clones?
If your filter caps did not get to proper spec/test in range they should probably be replaced, same with the bias caps. Keep the old ones safe.
Valve storm do an Erie 32/32 that is to 100% spec so try that first and redo the reforming. If you get under 5v within 3-4 hours you are golden. If not I think F&Ts across the board. It will sound and feel different (maybe better) but you will get used to it after a few months.
No idea what the contributor meant removing all the signal caps, must have been smoking something so I will not name (or suggestion being PIO - only seen in 63 offsets).
However if it will influence you I’ll repeat a previous statement- I’ve never seen a transformer taken out by failed filter cap.
Nice buy.

Great to have your input, as usual, Neil. I always appreciate your comments based on your experience and it is especially nice that you think that the amp is a lovely example. I have been especially lucky with this one.
I have deliberately held back from doing anything further than just getting it up and running until hearing advice from people who have more experience with this sort of thing than me. @playloud has helped me enormously through the cap reforming process and troubleshooting. The consensus from those who I particularly trust (including you, of course!) is to replace the bias caps and the 32/32 cap for now and then try a valvestorm 32/32 cap plus another reform attempt (your suggestion!) which I will gladly do.

 

[AndyD]

Re this sliding resistor, is there a method to set an approx value before powering up an amp ? for 4 x KT66's.

Hi, before you come off standby, set your sliding resistor to achieve the highest negative bias on pin 5 and then bias your valves with whatever method you use after coming off standby. Hope that helps.

 

[BlueX]

I've come back to post #1 several times to look at the nice pictures (and for my current restoration of course). It is indeed a lovely amp, not many solder joints have been touched. Understand the hesitation to replace components. Really good that you were able to get it running again with just minor repair.

Did you install the bypass resistor on the standby switch?