'68 JMP50 1986

[AndyD]

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!

 

[BlueX]

Congrats on that beauty!

Will you keep those bias caps?

I see that there is a sticker with "1 . AMP" under the HT fuse holder. I had a similar sticker on my '69. I removed it and the original text says 500 mA.

 

[BlueX]

oscillation which I eventually cured with added 5K6 grid resistors

How did you install them? Can't see that on the pic's.

 

[Marcomel79]

What a beauty, congratulations! That plexi panel looks immaculate

 

[stickyfinger]

I didnt know the 27k feedback amps were unstable. The only wiring difference I can see is the presence cap is mounted to the back of the bass pot. I remember George Metropoulos recommends in the metro kit to mount it to the presence pot..

The coupling caps I believe are all paper and oil. Id check for leakage of those. All the New old stock ones I bought in the past all measured way out of spec and went straight in the garbage.

If your filter caps are not working to spec do not hesitate and replace them!

 

[PelliX]

I am debating in my mind whether or not to replace these for long term reliability.

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!!

 

[Beryllium-9]

Eye candy for sure! It seems like it would take pedals really well...

 

[knulp]

Congrats!

Don’t think that there could be a debate about changing or not old electrolytes .. they are like old tires for a car.
And if they finally give up you could have damage to the transformers and tubes

Keep it functional and rock it!

 

[AndyD]

Congrats on that beauty!

Will you keep those bias caps?

I see that there is a sticker with "1 . AMP" under the HT fuse holder. I had a similar sticker on my '69. I removed it and the original text says 500 mA.

Thank you! Yes, I will replace them and keep the originals safe. Interested in your comment regarding the 1AMP sticker. I dare not look underneath mine but have put in a 500ma fuse anyway.

 

[AndyD]

How did you install them? Can't see that on the pic's.

Sorry, added a newer photo above showing the grid resistors.

 

[AndyD]

What a beauty, congratulations! That plexi panel looks immaculate

Cheers Marco! i have been looking for a while. yes, the plexiglass panel is in good shape. It shows just the right amount of ageing for me.

 

[AndyD]

I didnt know the 27k feedback amps were unstable. The only wiring difference I can see is the presence cap is mounted to the back of the bass pot. I remember George Metropoulos recommends in the metro kit to mount it to the presence pot..

The coupling caps I believe are all paper and oil. Id check for leakage of those. All the New old stock ones I bought in the past all measured way out of spec and went straight in the garbage.

If your filter caps are not working to spec do not hesitate and replace them!

Yes, the 27k/16ohm tap gives the most NFB. The JTM45 had this arrangement but into an 8K primary OT just like the 5F6. On my amp, the 27k on the 4ohm tap still oscillated. The grid resistors did the trick in the end.
I hadn't noticed that the presence cap was grounded onto the bass pot. Good call! Its a neat way of doing it if not strictly ideal.I have to say that this amp is surprisingly quiet.
All of the pots work without any hint of DC on them so at this stage I don't have any concerns regarding leakage from the coupling caps. I will, however, double check the remaining output couplers. So many things to check!
I was expecting plenty of remarks regarding the filter caps! I hear what you say, but I am in no rush at the moment to remove them just yet. They seem to be doing their job.

 

[AndyD]

Eye candy for sure! It seems like it would take pedals really well...

Bass amps really do!

 

[BlueX]

Sorry, added a newer photo above showing the grid resistors.

Thanks!

Interested in your comment regarding the 1AMP sticker. I dare not look underneath mine but have put in a 500ma fuse anyway.

Here's the text on my early '69, S/A10090. Several stickers on the back panel. Heard or read somewhere that 500 mA should be sufficient for 50W JMP HT fuse.

Looks like your Marcon filter cap is grounded on top. My was grounded at the clamp, by removing plastic wrap.

 

[AndyD]

Thanks!

Here's the text on my early '69, S/A10090. Several stickers on the back panel. Heard or read somewhere that 500 mA should be sufficient for 50W JMP HT fuse.

Looks like your Marcon filter cap is grounded on top. My was grounded at the clamp, by removing plastic wrap.

View attachment 156381View attachment 156382

Thats cool. Thanks for sharing that. I haven't seen grounding of the filter cap like that before

 

[Beryllium-9]

Bass amps really do!

My 1986 is from '72. I like 100K NFB and a .68 on the presence pot. Lots of hair but still cleans up really nice!

 

[AndyD]

My 1986 is from '72. I like 100K NFB and a .68 on the presence pot. Lots of hair but still cleans up really nice!

Agreed! That is a nice combo. I had a '74 Lead and Bass 50 with that combination. It was quite a lively amp, I seem to remember. This '68 will be a very different animal.

 

[Beryllium-9]

Agreed! That is a nice combo. I had a '74 Lead and Bass 50 with that combination. It was quite a lively amp, I seem to remember. This '68 will be a very different animal.

I remember your posts of the L&B! I have one from '76 that sounds like crap in the original combo cab but comes alive with a 412! I have been waiting for a head cab for it to come along.

 

[AndyD]

I remember your posts of the L&B! I have one from '76 that sounds like crap in the original combo cab but comes alive with a 412! I have been waiting for a head cab for it to come along.

Cheers! I have a 2045 with pulsonic G12Hs that I played the L&B through and used low gain pedals for drive. These amps definitely like closed backed cabs.
As a side note, I sometimes put the L&B chassis in either a small box or “micro” box. Get one made!

 

[TAZIN]

I see that there is a sticker with "1 . AMP" under the HT fuse holder. I had a similar sticker on my '69. I removed it and the original text says 500 mA.

They're all screen printed with a 500mA for the H.T. Fuse and 2 Amp for the Mains Fuse. It is interesting that an English build would have a 1 Amp dymo label for the H.T. Fuse.