Marshall JMP Plexi 50-watter from '68

Jucciz · 08-10-2011, 02:59 PM

Hi!

I recently acquired another Marshall from the late 60's - a '68 JMP Plexi 50-watter.

Here's a photo gallery:
Dropbox - Photos - Simplify your life

A relatively nice looking piece of gear, despite the missing logo and a little hole drilled on the back panel for a jack, don't you think? Now here's what definitely isn't original:

- The power transformer, even though it has a 1202-118 cover on top of it (I'm willing to purchase an original Drake 1202-118 or a Marstran clone, please drop me a note if you have one for sale!)
- Knobs (same here, drop me a note if you have period-correct parts for sale)
- Standby switch (and again... drop me a note)
- Fuse holders (the amp now takes 5x20mm fuses - I think all Marshalls of the era originally had 6x32mm - also original fuse holders are of interest)

There was a strange mod between one of the input jacks and a not-so-nicely retrofitted jack on the back panel (ouch!) - probably a line out of some kind - see the pictures. The mod has now been removed and I took the amp to a tech for a cap change. I chose TAD Gold Caps, because they had nice values available: I chose two radial 32+32uF cans on top and an axial 32+32uF on the board. The amp had originally quite rare gray 40+40uF Marcon caps on top of the chassis and a third one on the board - of which I couldn't figure out much - still probably a 32+32uF at most. On a tester the old 40+40uF caps measured 36-37uF so I guess 32+32uF should be fine. I do have a '71 JMP 50-watter when I need more filtering.

What are those metal plates behind each potentiometer? Can they be original - has someone seen similar things in amps from the same era?

Can you spot something else from the pictures that isn't original? And now that we're at it, do you think the amp still has some value - not that I would like to part with it.

Any comments and hints are welcome. Thanks for your time again!

- Jucciz -

Jucciz · 08-13-2011, 02:29 PM

Sorry for the bump... anyone?

peterichardz · 08-14-2011, 08:32 AM

Send a PM to Lane Sparber or Jon Wilder and ask them to take a look. That's a very nice amp you have there. There have been a few things changed and I have never seen pots like those.

MajorNut1967 · 08-14-2011, 02:03 PM

oops

MajorNut1967 · 08-14-2011, 02:19 PM

I'm not one the guys recommended, but I own a 1968 Model 1987. There are several thing that just don't jive, for instance: the slope resistor's been changed to what looks like either a 56K or 68K, it should be a 33K someone tried fenderize this amp or its a Bass amp. The 470 K mixer resistor is not bypassed on the high treble Channel and the bright cap Is missing "although sometimes it wasn't even installed on these amps” or its a Bass amp. The 820 ohm resistor (cathode) is not bypassed with a .68 mfd capacitor. There's something weird going on with the indicator light too. There is an extra tag strip in between the main board and the transformer, that I've never seen before ever. Get rid of that mod completely and plug the bullet hole. It generally needs to be gone through. That things kind of butchered! Its not a 68 by the way the parts installed but the steel chassis and the JMP logo says it is. .

MajorNut1967 · 08-14-2011, 03:32 PM

Also is the NFB Wire (purple) on a speaker tap or on the speaker jack it's self?

Jucciz · 08-14-2011, 03:32 PM

Quote:

Originally Posted by MajorNut1967

I'm not one the guys recommended, but I own a 1968 Model 1987. There are several thing that just don't jive, for instance: the slope resistor's been changed to what looks like either a 56K or 68K, it should be a 33K someone tried fenderize this amp or its a Bass amp. The 470 K mixer resistor is not bypassed on the high treble Channel and the bright cap Is missing "although sometimes it wasn't even installed on these amps” or its a Bass amp. The 820 ohm resistor (cathode) is not bypassed with a .68 mfd capacitor. There's something weird going on with the indicator light too. There is an extra tag strip in between the main board and the transformer, that I've never seen before ever. Get rid of that mod completely and plug the bullet hole. It generally needs to be gone through. That things kind of butchered! Its not a 68 by the way the parts installed but the steel chassis and the JMP logo says it is.

Thanks for your reply.

First of all, it indeed IS a bass amp and thus the slope resistor is 56k as you correctly spotted. The 820 ohm cathode resistor IS bypassed with a 320uF electrolytic - common for many other amps of the era - it's a shared cathode construction found on many other '68 JMP's. My '67 JMP 50W (also a bass model) has it that way too. The absence of the mixer bypass cap is also part of the bass version design if my memory serves me right - at least my '67 also came without it and it seems original.

If you want to compare, here's my '67 JMP 50W, bass version:
Dropbox - Photos - Simplify your life

The extra tag strip has probably been installed there when the power transformer was replaced - for whatever reason, I haven't had this amp for more than a couple of months. Also the indicator light has probably been replaced at some point as well as the standby switch - no big deal tone-wise, I hope.

The strange mod seen on the pictures has already been removed and the amp will be recapped tomorrow with TAD Gold Caps. Also, it will be thoroughly inspected.

You're saying it's not 100% certain it's a '68, so might it actually be a '67? I've been trying to get a confirmation on that one, because according to marstran.com historical database, there is an amp by the serial number "S/11429" and it's from year "67,68" so it could be either one. Mine is only 4 numbers apart being S/11425.

As for the amp being butchered, I've personally seen amps in much worse shape - I mean, much more components changed on the board, both transformers replaced, more holes drilled etc. Of course it's not a museum piece either, but the board itself seems to have survived pretty well, don't you think?

Quote:

Originally Posted by MajorNut1967

Also is the NFB Wire (purple) on a speaker tap or on the speaker jack it's self?

Actually I have to check that one when I get the amp back from the tech. I thought I took pictures from just about any angle I could come up with, but there we go: none of the pictures shows clearly enough where the NFB wire goes. It looks like it goes to the center tap of the impedance switch but I'll confirm that one when I get the amp back.

Thanks for your comments!

MajorNut1967 · 08-14-2011, 03:59 PM

Quote:

Originally Posted by Jucciz

Thanks for your reply.

The 820 ohm cathode resistor IS bypassed with a 320uF electrolytic - common for many other amps of the era - it's a shared cathode construction found on many other '68 JMP's. My '67 JMP 50W (also a bass model) has it that way too.

[/SIZE]

Actually I have to check that one when I get the amp back from the tech. I thought I took pictures from just about any angle I could come up with, but there we go: none of the pictures shows clearly enough where the NFB wire goes. It looks like it goes to the center tap of the impedance switch but I'll confirm that one when I get the amp back.

Thanks for your comments!

No the 820 I am talking about is in the second stage not the preamp! Are going to keep it as a 1986?

Jucciz · 08-14-2011, 04:31 PM

Quote:

Originally Posted by MajorNut1967

No the 820 I am talking about is in the second stage not the preamp! Are going to keep it as a 1986?

Oh, I didn't know you meant that. No, there's no cap there - as is the case with my '67 and many other amps of the 67-68 era. Was the V2 cathode bypass cap introduced a bit later or was it there very early on (maybe just on the lead models)?

I'm not quite sure yet if I'm going to keep it as a 1986 or not. Now that it's not that much of a museum piece anyway I could of course mess around with it quite a bit if I felt like it - but at this point I guess I'll keep it as a 1986, recap it, try to find at least another power transformer (Drake, Marstran or Mercury) and see how I like it that way.

The bass models seem to take pedals very nicely and the overall sound of (at least my '67 JMP 1986) is very pleasing so I'm expecting similar results from this one after it's been recapped and otherwise checked thoroughly.

Ydna · 08-14-2011, 05:28 PM

I vote for keeping it a 1986. That's what Duane Allman used.

MajorNut1967 · 08-14-2011, 05:42 PM

Quote:

Originally Posted by Ydna

I vote for keeping it a 1986. That's what Duane Allman used.

Ya that sound like a plan.

Jucciz · 08-15-2011, 03:53 PM

I've got some news to share.

The amp has now been recapped. Also screen grid resistors were installed - better safe than sorry. There was no 320uF electrolytic capacitor in stock for the V1 cathode so we used a 220uF - something that shouldn't make any difference anyway.

The fixed 68k bias resistor was replaced with a 47k resistor and a 22k trimmer pot for a bit more versatility. With a nice pair of old 60's Telefunken EL34 power tubes inside, we got a nice 70% plate dissipation with the trimmer about in the middle - there's plenty of leeway to both directions should the bias be hotter or colder for different tubes in the future. I'm going to use NOS tubes in this one anyway.

The mysterious power tranny puts out almost exactly 400VDC on plates: (around 400-405V). The filament voltage is a bit on the hot side though: it's 7V, which is just about within the recommended +/- 10% tolerance of the normal 6.3V.

Another good news is that I finally got on contact with Brian Wallace from Marstran and he's willing to sell me a Marstran 1202-118 with dual secondary outputs - this way I'd be able to get both 370V and 420V B+, which could nicely offer a bit of both late 60's and 70's tones. I think it would be a great idea - does someone disagree?

I'll post new pictures of the amp probably tomorrow. This week anyway.

Any comments are welcome!

Jucciz · 08-17-2011, 08:01 AM

Hi again!

Here's some pictures I took today:
https://www.dropbox.com/gallery/4302...fter/?h=6b163c

Notice the following changes:
- New filter caps, TAD Gold Caps (all 32+32uF - two on top of the chassis, one on the board)
- New bias caps (Sprague Atom, 8uF/150V)
- New V1 cathode bypass cap (Philips, 220uF/63V)
- bias trimmer pot (22k), in series with a 47k resistor (which replaced the original fixed 68k)
- screen grid resistors (1k), installed vertically

The purple NFB wire goes to the common tap of the speaker impedance selector - the same way as in my '67 JMP.

Just to mention: the amp sounds KILLER now. I might record some clips in the near future, stay tuned.

The next thing I'll probably do is install a Marstran 1202-118 dual secondary power transformer (as soon as Brian ships it over) and an "on-off-on" type standby switch so this baby will be able to offer me a normal and a "lo-vo" amp in one box - i.e. 420V and 370V.

Any comments are welcome!

P.S. I did some rearranging in my photo gallery. The "before" pictures can be found here:
https://www.dropbox.com/gallery/4302...efore?h=bc7fab

Lane Sparber · 08-17-2011, 07:11 PM

Looks great!

The only thing I'd change -if you want to be 100% accurate to the schematic- would be to just move the NFB wire to the 8 ohm tap, as this was Marshall's (USUAL) preferred tap if you're using EL34s, although the other taps would work just as well. The 16 ohm tap gives you the MOST NFB voltage for your loop, and the 4 ohm has the LEAST. OR you can just keep it the way it is. Do what sounds best to you!

Just offering my two cents.

Cheers!

-Lane

MM54 · 08-17-2011, 08:42 PM

Won't switching the HT like that mess with your bias? Or since it pulls bias from the HT tap will it drop down with the B+?

Lane Sparber · 08-17-2011, 08:59 PM

Usually, if it's pulled off of the HT tap it's not so much of a problem Matt. Or, you can design a dual - output bias supply that you set once and forget for each voltage.

-Lane

MajorNut1967 · 08-17-2011, 09:07 PM

Quote:

Originally Posted by Lane Sparber

Umm...what? We're talking about the negative feedback loop...on the OUTPUT transformer's secondary taps, Matt. No B+/HT there.

-Lane

What!

Lane Sparber · 08-17-2011, 09:09 PM

Quote:

Originally Posted by MajorNut1967

What!

I mistook which post that Matt was referring to and adjusted my post once I'd seen my error.

-Lane

MajorNut1967 · 08-17-2011, 09:12 PM

Quote:

Originally Posted by Lane Sparber

I misunderstood what he was referring to and fixed it above.

-Lane

What!

Lane Sparber · 08-17-2011, 09:13 PM

Never mind. See the new post #16 above.

-Lane

MM54 · 08-17-2011, 09:43 PM

Quote:

Originally Posted by Lane Sparber

Usually, if it's pulled off of the HT tap it's not so much of a problem Matt. Or, you can design a dual - output bias supply that you set once and forget for each voltage.

-Lane

Okay, cool

Just wondered

Jucciz · 08-18-2011, 02:17 PM

Quote:

Originally Posted by Lane Sparber

Looks great!

The only thing I'd change -if you want to be 100% accurate to the schematic- would be to just move the NFB wire to the 8 ohm tap, as this was Marshall's (USUAL) preferred tap if you're using EL34s, although the other taps would work just as well. The 16 ohm tap gives you the MOST NFB voltage for your loop, and the 4 ohm has the LEAST. OR you can just keep it the way it is. Do what sounds best to you!

Just offering my two cents.

Cheers!

-Lane

Thanks for your comment. I might try that, but I actually found that in my '67 the NFB wire is on the common tap of the impedance selector - and it's never been changed - and same seems to apply to this '68. At least the amp sounds great now the way it is. If it ain't broke...

MajorNut1967 · 08-18-2011, 02:55 PM

Quote:

Originally Posted by Lane Sparber

Looks great!

The only thing I'd change -if you want to be 100% accurate to the schematic- would be to just move the NFB wire to the 8 ohm tap, as this was Marshall's (USUAL) preferred tap if you're using EL34s, although the other taps would work just as well. The 16 ohm tap gives you the MOST NFB voltage for your loop, and the 4 ohm has the LEAST. OR you can just keep it the way it is. Do what sounds best to you!

Just offering my two cents.

Cheers!

-Lane

Lane, that is absolutely incorrect what you posted! A lot of the late 60s 50 W Marshall amplifiers had the negative feedback wire connected directly to the speaker jack, including mine and most of the ones I've worked on. I don't know what schematic you're referring to, but the service manual for the late 60s 50 W marshalls does not state to use the 8 ohm tap.

Jucciz · 08-18-2011, 03:42 PM

Quote:

Originally Posted by MajorNut1967

Lane, that is absolutely incorrect what you posted! A lot of the late 60s 50 W Marshall amplifiers had the negative feedback wire connected directly to the speaker jack, including mine and most of the ones I've worked on. I don't know what schematic you're referring to, but the service manual for the late 60s 50 W marshalls does not state to use the 8 ohm tap.

Thanks for confirming, as this is indeed the case with both my late 60's Marshalls: '67 and '68.

By the way, any chance of getting a service manual you're referring to? A pdf copy or anything? Would be interesting to see it!

Which model, year and serial number is yours, btw? Any pictures online?

Lane Sparber · 08-18-2011, 04:51 PM

Quote:

Originally Posted by MajorNut1967

Lane, that is absolutely incorrect what you posted! A lot of the late 60s 50 W Marshall amplifiers had the negative feedback wire connected directly to the speaker jack, including mine and most of the ones I've worked on. I don't know what schematic you're referring to, but the service manual for the late 60s 50 W marshalls does not state to use the 8 ohm tap.

At the time, I was looking at the 1986 schematic here:

http://www.drtube.com/schematics/marshall/1986u.gif

and this 1987 schematic from 1970:

http://www.drtube.com/schematics/marshall/1987u.gif

Almost all of the Marshalls I've worked on with EL34s had the NFB wire on the 8 ohm tap. I've never actually seen a schematic with the NFB wire on the common lug of the selector, although I'm sure they're out there. However, that being said, you've worked on many more vintage models than I have, so I defer to your knowledge and experience. I am man enough to admit when I am wrong, and I stand corrected.

-Lane

Jucciz · 08-18-2011, 05:26 PM

Actually I have yet to find a schematic/layout picture that fully matches either of my 50-watters ('67 and '68). They're both bass versions (model 1986) and while they surely don't differ that much component-wise from their lead counterparts - or even from the early 1970's models - it would be nice to see a very early (that is, late 60's) JMP50 Plexi schematic, provided that there even is one online somewhere. Also the service manual MajorNut1967 mentioned about is definitely of interest.

Can someone shed a little light on these matters?

Lane Sparber · 08-18-2011, 05:50 PM

Quote:

Originally Posted by Jucciz

Actually I have yet to find a schematic/layout picture that fully matches either of my 50-watters ('67 and '68). They're both bass versions (model 1986) and while they surely don't differ that much component-wise from their lead counterparts - or even from the early 1970's models - it would be nice to see a very early (that is, late 60's) JMP50 Plexi schematic, provided that there even is one online somewhere. Also the service manual MajorNut1967 mentioned about is definitely of interest.

Can someone shed a little light on these matters?

Yeah...it can definitely be confusing...hence my error.

-Lane

Jucciz · 09-12-2011, 12:28 PM

Okay, I got the amp back from the doctor. Here are some pictures:
Dropbox - Photos - Simplify your life

Here's what has been done:
- new Marstran 1202-118 power transformer
- replaced the Marstran cover with the (perhaps original) Drake 1202-118 cover
- new correct type rectifier diodes, soldered directly to the PT secondaries
- a lot of cable tidying, as you can see - just compare to the previous pics

- the previously installed extra tag strip was removed
- a 470pF Styroflex bright cap was installed in the mixer - more balanced tone and a wider range of tonal choices
- mains switch properly rewired

The use of the "low voltage" option, however, would've required some extra work because of the bias voltage, so we decided to drop the idea for now. Maybe someday, maybe not, we'll see. Anyway, I'm getting a nice 448V on plates now when using the 230V tap. There's a 220V and 240V available if I need a bit more/less B+.

Tell me how you like it!

HoseaGray2011 · 09-13-2011, 07:18 AM

It can definitely be confusing...hence my error

Jucciz · 09-14-2011, 03:01 PM

Any comments on the latest gut shots etc.?

Btw. any fellow Marstran owners?

08-10-2011, 02:59 PM	#1 (permalink)
Jucciz Senior Member Join Date: Apr 2011 Location: Finland Posts: 119	Marshall JMP Plexi 50-watter from '68 Hi! I recently acquired another Marshall from the late 60's - a '68 JMP Plexi 50-watter. Here's a photo gallery: Dropbox - Photos - Simplify your life A relatively nice looking piece of gear, despite the missing logo and a little hole drilled on the back panel for a jack, don't you think? Now here's what definitely isn't original: - The power transformer, even though it has a 1202-118 cover on top of it (I'm willing to purchase an original Drake 1202-118 or a Marstran clone, please drop me a note if you have one for sale!) - Knobs (same here, drop me a note if you have period-correct parts for sale) - Standby switch (and again... drop me a note) - Fuse holders (the amp now takes 5x20mm fuses - I think all Marshalls of the era originally had 6x32mm - also original fuse holders are of interest) There was a strange mod between one of the input jacks and a not-so-nicely retrofitted jack on the back panel (ouch!) - probably a line out of some kind - see the pictures. The mod has now been removed and I took the amp to a tech for a cap change. I chose TAD Gold Caps, because they had nice values available: I chose two radial 32+32uF cans on top and an axial 32+32uF on the board. The amp had originally quite rare gray 40+40uF Marcon caps on top of the chassis and a third one on the board - of which I couldn't figure out much - still probably a 32+32uF at most. On a tester the old 40+40uF caps measured 36-37uF so I guess 32+32uF should be fine. I do have a '71 JMP 50-watter when I need more filtering. What are those metal plates behind each potentiometer? Can they be original - has someone seen similar things in amps from the same era? Can you spot something else from the pictures that isn't original? And now that we're at it, do you think the amp still has some value - not that I would like to part with it. Any comments and hints are welcome. Thanks for your time again! - Jucciz - Last edited by Jucciz; 08-10-2011 at 03:30 PM.

08-13-2011, 02:29 PM	#2 (permalink)
Jucciz Senior Member Join Date: Apr 2011 Location: Finland Posts: 119	Re: Marshall JMP Plexi 50-watter from '68 Sorry for the bump... anyone?

08-14-2011, 08:32 AM	#3 (permalink)
peterichardz Senior Member Join Date: Jul 2010 Location: Willard, Ohio Posts: 647	Re: Marshall JMP Plexi 50-watter from '68 *Send a PM to Lane Sparber or Jon Wilder and ask them to take a look. That's a very nice amp you have there. There have been a few things changed and I have never seen pots like those.* __________________ *The Sneaky Pete Band* *Loud and Proud for 41 years* 2 '77 JMP 50 watt MV's 1 1960AX W G12M's 1 1960A W/ EVM12L's 1 1960A W/ G12 75's 1 1965A Modded & Loaded with 2 EVM12L's 1 112AVT Loaded with EV SRO Seymore Duncan Convertible 100 watt 2 Gibson Les Paul Jr.s Gibson Les Paul Standard Gibson Firebird Epi. Custom Shop Les Paul Jr. Epi. Les Paul Classic Quilt Top Epi. Korina Explorer Epi. Les Paul Chamilien

08-14-2011, 02:03 PM	#4 (permalink)
MajorNut1967 Senior Member Join Date: Jul 2009 Posts: 3,900	Re: Marshall JMP Plexi 50-watter from '68 oops

08-14-2011, 02:19 PM	#5 (permalink)
MajorNut1967 Senior Member Join Date: Jul 2009 Posts: 3,900	Re: Marshall JMP Plexi 50-watter from '68 I'm not one the guys recommended, but I own a 1968 Model 1987. There are several thing that just don't jive, for instance: the slope resistor's been changed to what looks like either a 56K or 68K, it should be a 33K someone tried fenderize this amp or its a Bass amp. The 470 K mixer resistor is not bypassed on the high treble Channel and the bright cap Is missing "although sometimes it wasn't even installed on these amps” or its a Bass amp. The 820 ohm resistor (cathode) is not bypassed with a .68 mfd capacitor. There's something weird going on with the indicator light too. There is an extra tag strip in between the main board and the transformer, that I've never seen before ever. Get rid of that mod completely and plug the bullet hole. It generally needs to be gone through. That things kind of butchered! Its not a 68 by the way the parts installed but the steel chassis and the JMP logo says it is. .

Today
Marshall Amps	Sponsored Links __________________ This advertising will not be shown in this way to registered members. Register your free account today and become a member on Marshall Amp Forum

08-14-2011, 03:32 PM	#6 (permalink)
MajorNut1967 Senior Member Join Date: Jul 2009 Posts: 3,900	Re: Marshall JMP Plexi 50-watter from '68 Also is the NFB Wire (purple) on a speaker tap or on the speaker jack it's self?

08-14-2011, 05:28 PM	#10 (permalink)
Ydna Senior Member Join Date: Sep 2010 Location: Livermore, CA Posts: 1,514	Re: Marshall JMP Plexi 50-watter from '68 I vote for keeping it a 1986. That's what Duane Allman used. Moose Lewis likes this. __________________ Current Amps 1. 1963 Sano Supersonic 2. Fender Dual Showman 1990s 3. Jtm45/100 build finished. Future Amps 1. Lots of plexis. Vintage ones, home built, or Metropoulos. I don't care. 2. A few preCBS fenders.

08-15-2011, 03:53 PM	#12 (permalink)
Jucciz Senior Member Join Date: Apr 2011 Location: Finland Posts: 119	Re: Marshall JMP Plexi 50-watter from '68 I've got some news to share. The amp has now been recapped. Also screen grid resistors were installed - better safe than sorry. There was no 320uF electrolytic capacitor in stock for the V1 cathode so we used a 220uF - something that shouldn't make any difference anyway. The fixed 68k bias resistor was replaced with a 47k resistor and a 22k trimmer pot for a bit more versatility. With a nice pair of old 60's Telefunken EL34 power tubes inside, we got a nice 70% plate dissipation with the trimmer about in the middle - there's plenty of leeway to both directions should the bias be hotter or colder for different tubes in the future. I'm going to use NOS tubes in this one anyway. The mysterious power tranny puts out almost exactly 400VDC on plates: (around 400-405V). The filament voltage is a bit on the hot side though: it's 7V, which is just about within the recommended +/- 10% tolerance of the normal 6.3V. Another good news is that I finally got on contact with Brian Wallace from Marstran and he's willing to sell me a Marstran 1202-118 with dual secondary outputs - this way I'd be able to get both 370V and 420V B+, which could nicely offer a bit of both late 60's and 70's tones. I think it would be a great idea - does someone disagree? I'll post new pictures of the amp probably tomorrow. This week anyway. Any comments are welcome! peterichardz likes this.

08-17-2011, 08:01 AM	#13 (permalink)
Jucciz Senior Member Join Date: Apr 2011 Location: Finland Posts: 119	Re: Marshall JMP Plexi 50-watter from '68 Hi again! Here's some pictures I took today: https://www.dropbox.com/gallery/4302...fter/?h=6b163c Notice the following changes: - New filter caps, TAD Gold Caps (all 32+32uF - two on top of the chassis, one on the board) - New bias caps (Sprague Atom, 8uF/150V) - New V1 cathode bypass cap (Philips, 220uF/63V) - bias trimmer pot (22k), in series with a 47k resistor (which replaced the original fixed 68k) - screen grid resistors (1k), installed vertically The purple NFB wire goes to the common tap of the speaker impedance selector - the same way as in my '67 JMP. Just to mention: the amp sounds KILLER now. I might record some clips in the near future, stay tuned. The next thing I'll probably do is install a Marstran 1202-118 dual secondary power transformer (as soon as Brian ships it over) and an "on-off-on" type standby switch so this baby will be able to offer me a normal and a "lo-vo" amp in one box - i.e. 420V and 370V. Any comments are welcome! P.S. I did some rearranging in my photo gallery. The "before" pictures can be found here: https://www.dropbox.com/gallery/4302...efore?h=bc7fab Last edited by Jucciz; 08-17-2011 at 10:54 AM.

08-17-2011, 07:11 PM	#14 (permalink)
Lane Sparber Senior Member Join Date: Apr 2010 Location: Queens, New York Posts: 2,505	Re: Marshall JMP Plexi 50-watter from '68 Looks great! The only thing I'd change -if you want to be 100% accurate to the schematic- would be to just move the NFB wire to the 8 ohm tap, as this was Marshall's (USUAL) preferred tap if you're using EL34s, although the other taps would work just as well. The 16 ohm tap gives you the MOST NFB voltage for your loop, and the 4 ohm has the LEAST. OR you can just keep it the way it is. Do what sounds best to you! Just offering my two cents. Cheers! -Lane __________________ New York Guitar and Amp Tech...and NOT a guru! WARNING: Tube amps contain LETHAL voltages long after they've been unplugged. If you are not familiar with the safety procedures involved when working on high voltage circuits (such as how to safely drain the filter caps), then PLEASE take it to a tech to stay safe...and alive. Some of what I do (log in to Facebook): http://www.facebook.com/media/set/?s...4798875&type=3

08-17-2011, 08:42 PM	#15 (permalink)
MM54 Senior Member Join Date: Feb 2010 Location: Erie, PA Posts: 7,809	Re: Marshall JMP Plexi 50-watter from '68 Won't switching the HT like that mess with your bias? Or since it pulls bias from the HT tap will it drop down with the B+? __________________ -Matt If you don't understand what I'm saying, assume I'm making a joke. Trust me. Lachesky Amplifiers - Home of the CA10 Orders for 2013 CA10s are now being accepted! "... And I come from a place where they drag your hopes through the mud because their own dreams are all dying"

08-17-2011, 08:59 PM	#16 (permalink)
Lane Sparber Senior Member Join Date: Apr 2010 Location: Queens, New York Posts: 2,505	Re: Marshall JMP Plexi 50-watter from '68 Usually, if it's pulled off of the HT tap it's not so much of a problem Matt. Or, you can design a dual - output bias supply that you set once and forget for each voltage. -Lane __________________ New York Guitar and Amp Tech...and NOT a guru! WARNING: Tube amps contain LETHAL voltages long after they've been unplugged. If you are not familiar with the safety procedures involved when working on high voltage circuits (such as how to safely drain the filter caps), then PLEASE take it to a tech to stay safe...and alive. Some of what I do (log in to Facebook): http://www.facebook.com/media/set/?s...4798875&type=3

08-17-2011, 09:13 PM	#20 (permalink)
Lane Sparber Senior Member Join Date: Apr 2010 Location: Queens, New York Posts: 2,505	Re: Marshall JMP Plexi 50-watter from '68 Never mind. See the new post #16 above. -Lane __________________ New York Guitar and Amp Tech...and NOT a guru! WARNING: Tube amps contain LETHAL voltages long after they've been unplugged. If you are not familiar with the safety procedures involved when working on high voltage circuits (such as how to safely drain the filter caps), then PLEASE take it to a tech to stay safe...and alive. Some of what I do (log in to Facebook): http://www.facebook.com/media/set/?s...4798875&type=3

08-18-2011, 05:26 PM	#26 (permalink)
Jucciz Senior Member Join Date: Apr 2011 Location: Finland Posts: 119	Re: Marshall JMP Plexi 50-watter from '68 Actually I have yet to find a schematic/layout picture that fully matches either of my 50-watters ('67 and '68). They're both bass versions (model 1986) and while they surely don't differ that much component-wise from their lead counterparts - or even from the early 1970's models - it would be nice to see a very early (that is, late 60's) JMP50 Plexi schematic, provided that there even is one online somewhere. Also the service manual MajorNut1967 mentioned about is definitely of interest. Can someone shed a little light on these matters?

09-12-2011, 12:28 PM	#28 (permalink)
Jucciz Senior Member Join Date: Apr 2011 Location: Finland Posts: 119	Re: Marshall JMP Plexi 50-watter from '68 Okay, I got the amp back from the doctor. Here are some pictures: Dropbox - Photos - Simplify your life Here's what has been done: - new Marstran 1202-118 power transformer - replaced the Marstran cover with the (perhaps original) Drake 1202-118 cover - new correct type rectifier diodes, soldered directly to the PT secondaries - a lot of cable tidying, as you can see - just compare to the previous pics - the previously installed extra tag strip was removed - a 470pF Styroflex bright cap was installed in the mixer - more balanced tone and a wider range of tonal choices - mains switch properly rewired The use of the "low voltage" option, however, would've required some extra work because of the bias voltage, so we decided to drop the idea for now. Maybe someday, maybe not, we'll see. Anyway, I'm getting a nice 448V on plates now when using the 230V tap. There's a 220V and 240V available if I need a bit more/less B+. Tell me how you like it! Lane Sparber likes this. Last edited by Jucciz; 09-12-2011 at 02:27 PM.

09-13-2011, 07:18 AM	#29 (permalink)
HoseaGray2011 Junior Member Join Date: Sep 2011 Posts: 7	Re: Marshall JMP Plexi 50-watter from '68 It can definitely be confusing...hence my error

09-14-2011, 03:01 PM	#30 (permalink)
Jucciz Senior Member Join Date: Apr 2011 Location: Finland Posts: 119	Re: Marshall JMP Plexi 50-watter from '68 Any comments on the latest gut shots etc.? Btw. any fellow Marstran owners?

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode