Marshall JMP 50 breaking into oscillation when input level raised

[mikeydee77]

Would love some inspiration on solving this one.

I am working on a UK ~1975/76 Marshall Lead and Bass with the ST1 circuit board. It came in with a burnt out power transformer.

Transformer has now been replaced and the so have the filter caps.
Checked all the resistors on the Fluke and found that the PI plate resistor (100k) had drifted north tho about 120K as had the 220K bias feed resistor (228k).

Powered it up biased around 32mA and it sounds fine. But, raise the input signal just a tiny bit - not even one on the dial and the amp goes into oscillation very loud - I turn off quickly or back off the volume and then it is ok. Have tried alternative tubes throughout to no avail.

Removing the power tubes and the level can go to ten without any sign of a problem. Checked the pre amp output on the power tubes grid's and it looks nice and clean on the scope so I am assuming that it is caused by the power tubes rather than coming from the pre-amp.

One thing that I did notice is that the two resistors from the power tube grids are 82K (and original) and not 220K as per the attached schematic which is the closest one I can find. I changed those to 220K to see if that was a factor but it makes no difference.

Any ideas?

[Mick Bailey]

My first thought is to disconnect the NFB and re-try to see if you're getting PFB. Has the OT been disturbed and the primary connection flipped?

[Jazz P Bass]

Hah.
I had a Koch Multitone that would go 'Bronk" when in 100 watt mode & the speaker damper switch was thrown to the high position.
Scared the cr*p out of me.

Yup, the output transformer was replaced with a Fender & the brown & blue wires had to be reversed.

Positive Feedback.

[Audiotexan]

I'm guessing you need a schem for model "1964 JMP Lead and Bass, 50W"

Forgive me for shifting your post around, but I thought it'd be easier to follow...(with the thought of hopefully finding the right schem first [if possible], then troubleshooting.)

I am working on a UK ~1975/76 Marshall Lead and Bass with the ST1 circuit board.

A great schem/info site that I've run across lists things chronologically and only shows the following for the dates you list. (he DOES note that its not complete yet...so, there's always a chance this falls in the cracks) but what he does show is:
1967 - 1981 The JMP's.
1973 - 1980 The JMP Solid State Amps.
1971 - 1978 The Artiste's.

One thing that I did notice is that the two resistors from the power tube grids are 82K (and original) and not 220K as per the attached schematic which is the closest one I can find. I changed those to 220K to see if that was a factor but it makes no difference.

The only schem he has under the Artiste series is a 2040, which also shows 220K's...so that rules that out. Since you posted a 50w tube schem, that rules out the SS junk. All that's left is the JMP series, and I've been through literally *every* JMP schem in my collection, and only found one schem that utilizes 82k in the output tubes bias network. The model 1992 JMP Super Bass, MkII, 100W (Unicord '70). The only hitch is, that's a 100w that uses four 6550's.

/speculation: Perhaps it shipped with 6550's, and someone 'subbed' outputs out?...or someone tried to convert it for a preference to 6550's?...or it sat dormant and empty socketed for x-length of time and someone guessed at the tubes that were supposed to be there? Lord only knows until we get a schem, but those are my guesses.

Looking for "Lead & Bass" as keywords, I found:
1973/1974 JMP Lead & Bass, 18W/20W - no
2061 JMP Lead & Bass, 20W - no
2100 JMP Lead & Bass, 50W
2195/2196 JMP Lead & Bass, 100W - no (100w)
2201 JMP Lead & Bass, 30W - no (transistor)

So that really only leaves the 2100 on the list.

But given that the site says "This amp was in production from 1973 up to 1976. It is pretty rare as it was only available through mailorder. This is a two channel amp, one channel is that of an 1987 Lead amp and the other channel is that of an 1986 Bass amp. The 1964 is the head version of this 2100 combo."

Looking back for 1964 yields:
"1964 JMP Lead and Bass, 50W" (so this was missed due to not using "and" but "&" in the search) Anyways...given that this rare bird was a mail-order only amp, I hope that someone here finds it in their stash.

Transformer has now been replaced and the so have the filter caps.
Checked all the resistors on the Fluke and found that the PI plate resistor (100k) had drifted north tho about 120K as had the 220K bias feed resistor (228k).

If these are old carbon comp resistors, these are far from drifted values. CC resistors are famous for 10-20% tolerances. So 100k reading 120 is 20% (on the edge yes, out of spec for the period no) the 220k reading 228k is well within spec.

Removing the power tubes and the level can go to ten without any sign of a problem. Checked the pre amp output on the power tubes grid's and it looks nice and clean on the scope so I am assuming that it is caused by the power tubes rather than coming from the pre-amp.

I'd say "Have you tried swapping outputs?" however, given my curiosity about 6550s...I'll refrain. lol

Definitely an interesting beast, and I'll be interested in seeing what happens. Best of luck!! =)

[Audiotexan]

Couple of pics for ID sake...(from what I can tell, both boards say ST-1, but noticeably different)

1964 JMP Lead & Bass 50 Watt Head (ca'74)
Note the script logo on the 1964 pcb


2100 JMP Lead & Bass, 50W (ca'76)
Note the print logo

[mikeydee77]

My first thought is to disconnect the NFB and re-try to see if you're getting PFB. Has the OT been disturbed and the primary connection flipped?

Thanks Mick, I just tried disconnecting the NFB at the impedance selector but no improvement.

[mikeydee77]

Scared the cr*p out of me.

It really needs to be heard to be believed.

[mikeydee77]

Hi Audiotexan,

It's very good of you to have a hunt around for the schematic. That site is a great Marshall resource and I do feel like I have visited there a lot recently and looked at lots of very similar diagrams lately.

The one I am looking at looks exactly like the one in your picture above (1964 JMP Lead & Bass Head). The filter caps were from week 28'75 hence my dating suggestion. The notable features are
1) there is no voltage selector just the pins on the the right of the board which are connected according to operating voltage.
2) the presence cap is 0.1 on the diagram but in reality it is 680nF and also a 470k resistor on the presence pot.
3) there is the big missing 0.68uF capacitor (3rd component from the left) that is only for the lead units according to the assembly notes in the bottom left hand corner.
4) Probably a few others

Next steps on this I think are to review my repair work so far I must have messed something up somehow.

Thanks for all the help so far everyone and please don't hesitate to suggest anything.

[mikeydee77]

Next steps on this I think are to review my repair work so far I must have messed something up somehow.

It looks like there was a bit of dry joint on one of the 1M resistors on the PI. I have had it working nicely for a little while and it sounds nice.

I'll keep it under observation for a few days and see how it goes - but fingers crossed.

[Audiotexan]

Hi Audiotexan,
It's very good of you to have a hunt around for the schematic.

My pleasure! Happy to *try* to help...it does feel like I'm getting closer though (and besides, I love a good "mystery"! lol)

The one I am looking at looks exactly like the one in your picture above (1964 JMP Lead & Bass Head). The filter caps were from week 28'75 hence my dating suggestion. The notable features are
1) there is no voltage selector just the pins on the the right of the board which are connected according to operating voltage.
2) the presence cap is 0.1 on the diagram but in reality it is 680nF and also a 470k resistor on the presence pot.
3) there is the big missing 0.68uF capacitor (3rd component from the left) that is only for the lead units according to the assembly notes in the bottom left hand corner.

Well, given that tidbit, and a bit more digging...I landed here:
MetroAmp.com Forum ? View topic - 1964 (2100) Lead & Bass demistified -- Finally correct v

Postby Souelle » Sat Feb 04, 2006 4:48 am
<snipped to relevant -AT>
There's 2 editions, the one from 1973 with the yellow PCB and the one from 1974 - 1976 with the green PCB

for all editions, you got the exact same board that is founs on the 1986 Bass amp, even the cap with the slope resistor is 250pf like on a 1986 Bass Amp (not 500pf like you can see on the marstran schematic, that's a mistake)

They all have a 500pf cap on the bright channel pot

The difference is that the 1973 edition with the yellow PCB have a .1UF cap and no resistor on the presence pot, again, it's just like on a 1986 Bass Amp

In 1974 (the green PCB) to 1976 (last year of the Lead & Bass model), there was a 680nf cap on the presence knob (instead of .1uf) and there's a 4.2K resistor (lug 1 to ground) on the presence pot, same thing on the 1986 Bass Amp... Not sure, but i think the pot might be 25k instead of 5K

Another comment leads to a defunct link which I brought up via "wayback": https://web.archive.org/web/20060518...shall-1964.htm

Where it's stated "This is a '73 model 1964 Lead & Bass, they were only available by special order." and "The circuit is the 1986 50 watt bass, except for the 500pF treble pot cap."

Hamilton's site also says:

Again, an interesting configuration - this amp has the
European lay-down mains transformer, and no external
H.T. fuse holder, but has the internal American main's voltage buss.

Back to Metroamp.com with another tasty morsel.

by rscogin » Fri Feb 10, 2006 10:35 am

Just to add some confusion to this thread, my main amp for years has been a Marshall JMP 50W. On the chassis, the tag is dated "1978" and the model is specified as "2100". I know the 2100's were supposed to be combos, but this one is a head, and originally came with 6550's (USA export) which I changed to EL34's. There were a few other anomalies inside the amp so this may have been assembled with leftovers at the end of a production run or something similar. Just don't know but this is the best sounding Marshall I've ever played through.

So..I may not have been far off in my guess of it being slotted with 6550's. Perhaps it was built to special order. Or perhaps a factory worker owned unit...But it's definitely an amp with a story.

Same thread, just a much later reply..

Postby Miracle Man » Thu Apr 12, 2012 7:39 pm

I've researched this model a few years ago and found out it's a model 1986 50W bass amp with a 100k NFB resistor and a .68uf presence cap (two typical 70s features) and a bright cap on the first channel volume pot. Combos had a 5000pf cap and the head version had a 500pf cap. Since these were mail-order there was some variaton, but I haven't seen one with bypass cap on v2.

At any rate, with the above info in consideration, I'll toss up a 1986 for you, and I'd say cross that with the 1992 Super Bass output section since you had 82k's in it that appear stock.

[Audiotexan]

It looks like there was a bit of dry joint on one of the 1M resistors on the PI. I have had it working nicely for a little while and it sounds nice.

I'll keep it under observation for a few days and see how it goes - but fingers crossed.

Excellent!!! Congrats on nailing it down!!!

Looks like one more refresh would have saved me a TON of typing..but then I would have missed the "tasty morsel" in the middle of that long-winded post!

In "rscogin's" post that I quoted above verifies that at least ONE amp shipped with 6550's...it may be worth checking into the power circuit abit further. Just in case your's is "number two". Oi. What a trip!