1971 JMP Marshall need more info

[neskor]

Hi.
I have a great offer for this amp.
Seller told me this is a 1971 JMP 50W but not sure.
Can you please tell me is this '71 or later JMP 50W.
Looking the PCB it looks like 1986 bass amp.
Power transformer is changed and some other parts on the PCB.
Also cabinet is not from that era?

 

[Ned B]

I'd estimate that it is 11/70. Changed power transformer and head cabinet for starters.

 

[TAZIN]

I agree with the early November 1970 (within the first two week) build date. A lot of changes circuit wise so I hope the asking price is reflective of this. @neikeel this one is all yours!

 

[neskor]

it will be less than 2K USD

 

[giam23]

it will be less than 2K USD

i would most certainly grab it for that price, while the replaced PT is a bummer, an quite a few parts have been replaced, theres still heaps in there that is original

 

[TAZIN]

I'll have a go at this one...It looks like a whacked out single channel JTM45 type circuit. The valve rectifier, reduced mains & preamp filtering along with increased B+ voltages as a result of eliminating two of the 10K/1w dropping resistors is what points toward a JTM45 type circuit. The "whacked out" part is the apparent single channel coupled with a 2K7/320uF cathode arrangement for the first preamp valve and a cathode bypass cap for the second preamp valve. Changes & replaced parts:

As mentioned earlier, the PT has been changed and the head cabinet is not original.
Channel 1 (bright channel) appears to be disconnected by way of no input wire at V1 grid.
V1 cathode (Channel 2) has a 320uF cap (normal) but has a 2k7 resistor instead of the normal 820-ohm.
For V2a the cathode resistor is a replacement, and someone added a fairly large (250uF?) bypass cap.
The 100K resistor for V2b cathode is also a replacement.
The preamp filter cap (normally a 50+50uF/450v) has been changed to a single 10uF/450v. Additionally, one of the 10K/1w B+ dropping resistors was eliminated for the preamp section.
The output coupling caps for the phase inverter have been swapped out of two 0.047uF? caps.
Another 10K/1w B+ dropping resistor was eliminated which will allow more voltage to the preamp section.
The bias circuit has new caps and an added trimmer pot for adjustment. Not sure why they performed the update in this fashion...Perhaps the new PT supplies a weird voltage?
Both front panel switches have been replaced along with the Indicator light.
The Presence pot is a replacement.
Added Volume pot bypass caps for both channels (1 &2).
The one speaker jack is a replacement as is Mains Fuse holder.
The Mains/power cord is a replacement.
As mentioned earlier, the diode rectification circuit has been replaced by a valve rectifier.
Added 1K/5w Screen/grid resistors on the output valve sockets.
Also added 100-ohm resistors on the output sockets.

This amp would not require too many components in order to return it to "stock" model 1987 (Lead) specs but it would require a fair amount of labor time-wise.

 

[neskor]

Perfect!
Thank you Tazin.
I don't see that V1 grid is disconnected, check attached picture.
Also there is a small cap on V2 between grid and plate.

 

[neikeel]

Sorry late to party.
Importantly started as lead amp (see angle of the 68k grids on V1a and unused turret for split cathode)
Someone swapped slope resistor for cc 56k (vs 33k). Guess treble cap swapped too. The output couplers would be 22nF but looks like 39 or 47nF for that in between range (bass spec being 100nF).
Board material is typical for late 69 to early 71.
The rest you can see and was pointed out by Tazin
I like these sorts of quick resto projects but I’d go back to stock.

 

[AndyD]

FYI the headshell is from a ‘74 ish Lead and Bass model 1964. I have this amp.

 

[TAZIN]

Perfect!
Thank you Tazin.
I don't see that V1 grid is disconnected, check attached picture.
Also there is a small cap on V2 between grid and plate.

I couldn't make out that jumper wire before so, it is a shared cathode setup for both channels.

 

[TAZIN]

Sorry late to party.
Importantly started as lead amp (see angle of the 68k grids on V1a and unused turret for split cathode)
Someone swapped slope resistor for cc 56k (vs 33k). Guess treble cap swapped too. The output couplers would be 22nF but looks like 39 or 47nF for that in between range (bass spec being 100nF).
Board material is typical for late 69 to early 71.
The rest you can see and was pointed out by Tazin
I like these sorts of quick resto projects but I’d go back to stock.

I overlooked the open turret and the angle on the two 68K grid resistors...Good catch.