Advice needed: modded 1969 JMP 50

playloud

Well-Known Member
Joined
Dec 28, 2020
Messages
956
Reaction score
1,202
I'm located in the EU so I haven't got as much selection from high quality trannies as my US fellows. The only laydown I could find that promises to be high quality is the one made in Germany by TAD. I spoke with them and I got the info that this PT is capable of 450 V plate voltage.

Here: https://www.tubeampdoctor.com/en/mains-transf.-for-marshall-jtm45-and-50w-plexi-tad-kit-plexi-45

Would you recommend to replace my '74 Drake 1202-164 PT with this TAD laydown PT?

I wouldn't. Those Drake transformers are good iron. The orientation of the transformer seems trivial by comparison. Besides, stand up PTs started in '69, so it's barely anachronistic.
 

neikeel

Well-Known Member
Joined
Feb 12, 2014
Messages
7,339
Reaction score
7,030
Tbh the TAD transformer looks nothing like an original and I would not bother.
Btw not Superbass, just Bass, it is two cylinder not four!!
 

Anje

New Member
Joined
Jun 16, 2020
Messages
17
Reaction score
15
I'm located in the EU so I haven't got as much selection from high quality trannies as my US fellows. The only laydown I could find that promises to be high quality is the one made in Germany by TAD. I spoke with them and I got the info that this PT is capable of 450 V plate voltage.

Here: https://www.tubeampdoctor.com/en/mains-transf.-for-marshall-jtm45-and-50w-plexi-tad-kit-plexi-45

Would you recommend to replace my '74 Drake 1202-164 PT with this TAD laydown PT?

No.
If you really are bothered by the look of the stand-up PT (which does sound like you are from your posts) I’d look into a more accurate Merren or Heyboer / Marstran.
Probably won’t make much resale value difference for the amp compared with the old original ‘74 PT setup IMHO; it’s already a player amp with changed transformers and those old original transformers are good.
Sound & feel difference you’ll have to experience for yourself.
 

LPman

Member
Joined
Apr 3, 2016
Messages
56
Reaction score
61
Thx folks. I'm not personally bothered by the looks of the stand-up PT and I don't plan to sell the amp because I love it, especially now after the tune-up. Sounds like thunder with the master on 7 and preamp at 3. Feels like I could do controlled demolitions of buildings with this killer of an amp, it's insane. I might post a short sound sample later.

I'm just trying to measure up the restoration options if it comes to reselling in a future scenario. I looked up Merren but no pricing on their site and got no reply to inquiry yet.

What I will definitely do is changing the newer caps and resistors to vintage style Mustards.
 

Anje

New Member
Joined
Jun 16, 2020
Messages
17
Reaction score
15
I think the amp deserves the few board component tweaks to make it as period correct as possible.
If you love the amp & feel as is though beware, you may just prefer the way it is as is and I usually don't try to fix something that is not broken, as the old saying is saying. I made a few stupid moves like this in the past trying to make something great sounding better looking and it never was the same again, playing wise.
If you are thinking about future potential resale value, I'd just source the correct original missing components over time as you bump into deals / opportunities and keep them stored somewhere just in case you are selling one day, and enjoy it as is until then.
 

LPman

Member
Joined
Apr 3, 2016
Messages
56
Reaction score
61
Agreed, this is the best way. I wouldn't be able to use the amp as efficiently in its original 4-holer setup at the moment in a rehearsal setting. It's more practical as a master volume setup. Soundwise I can produce the classic Super Lead growl by turning up the master really high and adjusting the preamp vol accordingly. I will source those few correct parts and if it comes to resale I will offer the restoration to the buyer.
 

LPman

Member
Joined
Apr 3, 2016
Messages
56
Reaction score
61
I used many different Marshall amps in the past: late '70s JMP MK2, '80s Silver Jubilee, '80s JCM 800 2205, various DSL's, etc. And many other high quality brands besides Marshall.

This '69 JMP is the king, no contest. The closest was my '78 JMP but it had a more ice-picky tone. Regarding the Jubilee, which I know many love, it was much more closer in sound to my little DSL-15 practice head than to any classic Marshalls. It was a great sounding amp nonetheless. I liked the 2205 the least, it was always lost in the mix.
 

playloud

Well-Known Member
Joined
Dec 28, 2020
Messages
956
Reaction score
1,202
Agreed, this is the best way. I wouldn't be able to use the amp as efficiently in its original 4-holer setup at the moment in a rehearsal setting. It's more practical as a master volume setup. Soundwise I can produce the classic Super Lead growl by turning up the master really high and adjusting the preamp vol accordingly. I will source those few correct parts and if it comes to resale I will offer the restoration to the buyer.

If you were to restore the original 1986 circuit, I would suggest buying or building an attenuator (take a look at the designs by "JohnH" on this forum).

There's something magical about power tube distortion. I went to a local jam/gear meetup on the weekend, and puzzled some other players when I plugged into a new amp and rolled off the gain while cranking the 'master'. They were less puzzled once they heard the sound!
 

LPman

Member
Joined
Apr 3, 2016
Messages
56
Reaction score
61
If you were to restore the original 1986 circuit, I would suggest buying or building an attenuator (take a look at the designs by "JohnH" on this forum).

There's something magical about power tube distortion. I went to a local jam/gear meetup on the weekend, and puzzled some other players when I plugged into a new amp and rolled off the gain while cranking the 'master'. They were less puzzled once they heard the sound!

I'm already using the amp in its current form basically like a 4 holer. I'm running vol 1 (master) at 7 (2 o'clock) and vol 2 (gain) very low. At that point it's all power power tube saturation sweetness. I'm doing this without an attenuator so that the speakers also add their saturation. It's insanely loud but I have a perfectly soundproofed rehearsal room where I can do that until my ears bleed.

I'm wondering how much different the tone would be if I would restore it as 4 holer (lead spec) and use the high input channel cranked? Minuscule difference I would imagine.
 

LPman

Member
Joined
Apr 3, 2016
Messages
56
Reaction score
61
This is exactly tone I'm having (from 0:00 - 0:20). It's a '71 Super Lead in the video. Tight, punchy, bright. I produce this tone by setting vol I. master between 5-7 and vol II. preamp at 3.

After setting the bias at 60%, increasing the plate voltage to 408 V and increasing the preamp voltage by bypassing one of the 10k dropping resistors, my amp's low end is so tight, it sounds like a jackhammer when palm muting. The preamp's plate voltage is 340 V.

 

playloud

Well-Known Member
Joined
Dec 28, 2020
Messages
956
Reaction score
1,202
If you're happy with it, why change it?

I generally prefer bass spec myself, but that recording you posted is a different reference point.
 

Pete Farrington

Well-Known Member
Joined
Jan 18, 2021
Messages
1,636
Reaction score
1,827
Location
Staffordshire UK
Is it basically a Superbass converted to 2204 with more or less stock Superbass configuration?
That seems like a contradiction in terms? The component values in a 2204 are generally closer to a 1987 lead model than 1986 bass.

The preamp's plate voltage is 340 V.
Maybe V2 pin6 anode?
Otherwise, 340V seems more likely to be one of the HT supply nodes for the preamp? I wouldn’t expect any other preamp anode to be that high.
 
Last edited:

neikeel

Well-Known Member
Joined
Feb 12, 2014
Messages
7,339
Reaction score
7,030
jmp2b-jpg.113805

This is the amp in question?
The PI output caps and PI input caps have been changed to what values (originally 0.1uF PI outputs and 0.022uF mustards respectively), there a quite a few components that have been replaced (it's had a hard life!) but nothing irretrievable if you want to go back to OEM but if you can get the sounds you want out of it, play on!
 

LPman

Member
Joined
Apr 3, 2016
Messages
56
Reaction score
61
That seems like a contradiction in terms? The component values in a 2204 are generally closer to a 1987 lead model than 1986 bass.


Maybe V2 pin6 anode?
Otherwise, 340V seems more likely to be one of the HT supply nodes for the preamp? I wouldn’t expect any other preamp anode to be that high.

The phase inverter preamp tube's voltage is at 340V. V1 and V2 are around 250V. My tech said that PI always has the highest voltage and 340V is not too bad (300-330V is the max according to 12ax7 offiical spec) if I can live with maybe a little shortened tube life.
 

LPman

Member
Joined
Apr 3, 2016
Messages
56
Reaction score
61
This is the amp in question?
The PI output caps and PI input caps have been changed to what values (originally 0.1uF PI outputs and 0.022uF mustards respectively), there a quite a few components that have been replaced (it's had a hard life!) but nothing irretrievable if you want to go back to OEM but if you can get the sounds you want out of it, play on!


What I know for sure regarding mods is that the OT and PT are '74 Drake ones (correct 784-139 and 1202-164 codes). The wiring and presumably some component values were changed to a 2204 setup per the previous owner's request (my friend) who wanted a more usable amp in today's environments. I like it this way.
Naturally, the usual weaker components like tube sockets, filter caps, some resistors and caps were replaced too but I suppose that is unavoidable if a 50-year old amp is used as intended. I'm not an expert but the replacement caps look to be high quality nice Mustards.

Again, what I see (I'm just comparing them visually as a layman) is that my board looks about the same as the '69 JMP Bass spec amp that had been sold as a mint condition all-original example by Guitarpoint (see again below).
 

Attachments

  • Marshall72_50B_4921C_10.jpg
    Marshall72_50B_4921C_10.jpg
    232.8 KB · Views: 6
  • jmp chassis.jpg
    jmp chassis.jpg
    844.2 KB · Views: 6
Last edited:

TheOtherEric

Member
Joined
Oct 25, 2020
Messages
91
Reaction score
89
That seems like a contradiction in terms? The component values in a 2204 are generally closer to a 1987 lead model than 1986 bass.
Yeah, very little about this entire thread makes any sense to me. You have a ‘69 bass spec amp with later transformers and modded to 2204 spec (presumably cascaded input stages and component swaps) yet you want to sort of revert SOME aspects to original specs? Full stop. Why?! This thing is a complete bastard, with zero resemblance to its original form. For example, late 70s 2204 in fact used lower plate voltages (370-380v) so why would you want to go to 420-430v? And you rightly note that the 2204 conversion meant several component changes. Oh, and IT’S CASCADED!! A totally different amp than a ‘69.

Maybe I missed something,in which case, I’m sorry, it’s late. But this seems like an amp that you just enjoy the heck out of as a player since it’ll never be a collector or return to its original form without massive investments. Nothing wrong with that! I have a bastard too (a ‘76 modded to ‘77+ specs) and it’s glorious!
 

sct13

Active Member
Gold Supporting Member
Joined
Jun 7, 2009
Messages
808
Reaction score
202
Location
Michigan
Marstran makes a good laydown replacement.....
 

Latest posts



Top