1986 & 1987 JMP 50 Watt Head with EL34

Freebird · Post by **Freebird** » Thu May 10, 2007 4:17 am

Soon I'd like to build one Marshall 50 watt head. So far I'm not sure which one I will go for. I prefer old blues rock style as Allman Bros, Lynyrd Skynyrd, Rory Gallagher etc. It could be an assortment from both side of 1986/1987.

There are many different buildings even for 1986 and also for 1987. Probably Marshall assembled what they just had, later some were tweaked and some were repaired and had their specific parts. Some very informative websites together with this forum I found here:
- http://www.drtube.com/marshall.htm#JMP
- http://www.marstran.com/
- http://marshall.redpt.com/clay/marshall ... s_101.html

To start from scratch the key data are according to schematic diagram from G+W Unicord Incorporated as follows:

1986 JMP Bass (scheme: http://www.drtube.com/schematics/marshall/1986u.gif)
Basically 1986 Bass amp has similarities to JTM45!
- shared cathode circuit in 1st stage: 820R/320uF
- 1st to 2nd stage coupling cap on bright channel: 0.022uF
- mixer resistors: 470k
- mixer bypass cap: 500pF (common in reality: 560pF)
- bright cap on bright volume: no
- 2nd stage V2A cathode circuit: 820R
- tone stack, slope resistor/tone cap: 56k/250pF
- PI output coupling caps: 0.1uF
- negative feedback resistor: 47k at 8ohm-tap of output tranny (common in reality: 27k)
- presence control: 5k pot/0.1uF
- screen grid resistors: no
- control grid resistors: no
- power transformer: 1202-118 (common in reality from 1969: 1202-164)
- output transformer: 784-139 (common in reality in 1967: 784-128)

1987 JMP Lead (scheme: http://www.drtube.com/schematics/marshall/1987u.gif)
- split cathode circuit in 1st stage: V1A=820R/320uF, V1B=2.7k/0.68uF
- 1st to 2nd stage coupling cap on bright channel: 0.0022uF
- mixer resistors: 470k
- mixer bypass cap: 500pF (common in reality: 560pF)
- bright cap on bright volume: 0.005uF (common in reality: no cap)
- 2nd stage V2A cathode circuit: 820R/0.68uF
- tone stack, slope resistor/tone cap: 33k/500pF
- PI output coupling caps: 0.022uF
- negative feedback resistor: 47k at 8ohm-tap of output tranny (common in reality: 100k at 4 or 8ohm-tap)
- presence control: 5k pot/0.1uF (common in reality: 25k pot with 4.7k/0.68uf)
- screen grid resistors: no
- control grid resistors: no
- power transformer: 1202-118 (common in reality from 1969: 1202-164)
- output transformer: 784-139 (common in reality in 1967: 784-128)

My questions or what do you recommend for?
- rectifier diodes: 2 or 4, 1N4007 or FRED?
- choke: Is it 3H?
- main filtering: dual 32uF, 40uF or 50uF?
- screen filtering: single/dual 32uF, 40uF or 50 uF?
- preamp filtering: single/dual 32uF, 40uF or 50 uF?

I know above mentioned information can vary from amp to amp of different years and so. In reality the amps have other parts and components in use as shown in standard schematic diagrams. That makes building a 50 watter unclear and could becom a long story to nail it. I'd like to take the bull by the horn and hopefully find the path to the right direction.

Thanks in advance for any advice, recommendation or straight tip.

Daniel

neikeel · Post by **neikeel** » Thu May 10, 2007 1:56 pm

From the top:

I would either use the metal case diodes x4 or the 1N5408 (1kv 3A) diodes x4 to be sure-to be sure.

Std 7H choke (352-114).

Dual 32uF in preamp, 32uF PI, 32uF screens and dual 32uF in power supply (ie x3 32uF caps) 2 on top one horizontal inside.

I like split cathode and a small bright cap, you might prefer shared cathode without one. Not sure there is much difference between 56k/250pF and 33k/500pF tone stack (see calculator).

I prefer 0.022/0.033/0.047uF in the PI rather than 0.1

I remain unconvinced by 0.0022coupler and may switch to 0.018 soon

I prefer 47k NFB onto the speaker jack (ie usually 16ohm).

I used Marstran 139 OT and Metro PT

I would strongly urge you to use 1k grid protectors with new output valves.

What do the rest of you think?

Freebird · Post by **Freebird** » Fri May 11, 2007 3:29 am

Thanks neikeel for your responses!

neikeel wrote:From the top:

I would either use the metal case diodes x4 or the 1N5408 (1kv 3A) diodes x4 to be sure-to be sure.

Metal case diodes? for good looking or sweeter tone? Why not FRED's?

neikeel wrote: Std 7H choke (352-114).

7H I guess is maximum

neikeel wrote: Dual 32uF in preamp, 32uF PI, 32uF screens and dual 32uF in power supply (ie x3 32uF caps) 2 on top one horizontal inside.

Do you mean 2 dual on top and one single inside on the PTP or dual inside chassis wall?

neikeel wrote: I like split cathode and a small bright cap, you might prefer shared cathode without one. Not sure there is much difference between 56k/250pF and 33k/500pF tone stack (see calculator).

cut-off-frequenzy: 56k/250p=11368Hz, 33k/500pf=9645Hz
In theorie it seems not to be a big difference, but soundwise?

neikeel wrote: I prefer 0.022/0.033/0.047uF in the PI rather than 0.1

I remain unconvinced by 0.0022coupler and may switch to 0.018 soon

I prefer 47k NFB onto the speaker jack (ie usually 16ohm).

Lots to experiment with

neikeel wrote: I used Marstran 139 OT and Metro PT

What's the different between Marstran 139 OT and Metroamp 139 OT?

neikeel wrote: I would strongly urge you to use 1k grid protectors with new output valves.

Yes for sure, that's for protection of the output tranny.

Gunner · Post by **Gunner** » Fri May 11, 2007 4:02 am

I agree with almost everything neikeel suggests. I've rebuilt my -83 Marshall JMP 1987 to a ptp.

I dont't know how you are going to build, designing your own board? But go for a 1987. It's a little bit easier to convert it to 1986 spec if you want to, than the other way (from 1998 to 1987). But i also depends on the sound you want to get (I'm a little bit freaked about the 1987 circuit).

I started with four single 47uF filter caps mounted on the board. Let the first cap can remain as it was, double 50uF. (I don't understand why Marshall put the second cap can so far away from the PI. long cabels across the chassi and cloose to the high ac voltage area - a source for hum.) But I felt the sound was to stiff with the 47uF. So I changed. First lowered to 32uF and then to double 16uF (in parallell, 32uF) in the preamp! That's the sound I like! Much better response and feel (my taste).

I prefer the 56k sloperesistor, not 33k. I use it in all my Marshalls, even my modified 6100

But in the JMP I use a 56k/390pF combination, to get a little bit more aggressive in the treble freqs. Just a matter of taste.

I don't like 0,1uF after PI at all. I often use 0,047. In some cases 0,022 will do better, a little bit tighter bass. But it also depends on what power tubes you choose (and probably the OT - I use the nice Marstran). When I use my Mullard xf2 I can push them with much more bass freqs and still the bass is tight.

Here's a pic from the mounted board (before I changed to double 16uF).

Good luck! / G

EDIT: A component you didn't mention is the B+ resistor after the choke. The platevoltage to PI and the preamp is very important for the feel and sound of your amp. Some JMP:s use 22k after the choke (ore double 10k). You should try what's best for you. If you lower it to for example 10k or even lower the preamp might clean up a bit , raising the value can make the sound a little bit more brown

Freebird · Post by **Freebird** » Sun May 13, 2007 2:57 pm

Thanks Gunner! Your recommendations are very informative. I don't know yet whether I will create my own PTP board or take George's 50 watt lead board, because as you mentioned the caps. I have to reconsider if I'd like to have preamp filtering cap on board or inside at chassis wall like in JTM45. Also details on B+ resistors is a good point I have to think over. So I don't want to hurry up but rather I take the time to find a proper specification.

What I already defined is to use split cathode circuit. So I will go for a 1987 with some variations.

Does anybody know, where I could get a PTP board with turrets, which already has the space for preamp filtering?

Your board looks pretty great. You did nice work.

Daniel

Freebird · Post by **Freebird** » Wed May 30, 2007 4:33 pm

Hello there/here!

I'd like to know, which position you do recommend for preamp filtering caps to get least hum:

a) one single cab on board and one dual cap (only half needed) inside the chassis near PT
b) dual cap on top of the chassis near choke
c) dual cap inside the chassis near PT

Thanks in advance for any recommendations or advices!

Daniel

neikeel · Post by **neikeel** » Thu May 31, 2007 4:39 am

If hum is your enemy

Two caps on top by power tranny, one on top near choke all new high quality dual 50s fully formed as per Larry's scheme.

NK

Freebird · Post by **Freebird** » Fri Jun 01, 2007 1:41 pm

neikeel wrote:If hum is your enemy

Is not only mine

, I guess everyone's! It looks like this:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ (100Hz or 120Hz)

Thanks,
Daniel

neikeel · Post by **neikeel** » Fri Jun 01, 2007 2:10 pm