NAD: 1975 JMP 50 Bass Mk2 1986

[tribute60s]

Having given it a bit more though, I've been eyeing a Marshall power brake on the Bay that's a reasonable price. The collector in me want to keep everything as original as possible and I've heard mixed things about ppimv's. So I'm gonna heed your guy's advice and try an attenuator first before I do anything permanent. If I need more preamp gain I can just goose the front end with a overdrive/distortion that'll compliment the amp.
Really appreciate all the advice and perspectives, Thanks guys
Rock on

P.s.
I'm currently overseas atm working, but once I have the amp in hand I'll make sure to do a full post about the restoration, for those of you that may be interested. with lots of sexy high res images.

 

[Trapland]

NAD!!!

Pretty clean looking JMP 50 from 1975. .......

What are peoples thoughts on adding a ppimv to make it a little bit more usable at lower volumes?

If you can do it without irreversible changes, sure. But a PPIMV sounds less like a cranked Marshall than a wide open amp into a Marshall Powerbrake. I have both and don’t willingly use the master. It’s only there in case I show up at a gig and forgot a Powerbrake.

 

[aberry9475]

Yeah I definitely wouldn't add a PPIMV to that, it's too clean. If it was already hacked up it wouldn't really matter, but they're making some excellent attenuator/load boxes these days. Fryette Powerstation, tone king iron man II, ox box, two notes torpedo, etc. I've got a Faustine Phantom I'd be willing to sell if you want to try that too just PM me.

 

[AndyD]

Having given it a bit more though, I've been eyeing a Marshall power brake on the Bay that's a reasonable price. The collector in me want to keep everything as original as possible and I've heard mixed things about ppimv's. So I'm gonna heed your guy's advice and try an attenuator first before I do anything permanent. If I need more preamp gain I can just goose the front end with a overdrive/distortion that'll compliment the amp.
Really appreciate all the advice and perspectives, Thanks guys
Rock on

P.s.
I'm currently overseas atm working, but once I have the amp in hand I'll make sure to do a full post about the restoration, for those of you that may be interested. with lots of sexy high res images.

I’m a big fan of the Marshall power break. As long as it’s not overdone, it’s brilliant.

 

[AndyD]

I have a 1974 50w Lead and Bass. It’s a lovely amp. It breaks up very early probably due to the low (360volts) plate voltage which is normal for this era.
Nice find!

 

[AndyD]

Just looked at the photos. Very clean and original. Piher resistors and mustard caps! Happy days!

 

[Acvox]

A great example of the classic 1986 bass circuit, one of Jim Marshalls finest imho, and a personal favourite.
While a ppimv is an option, Its a solution best considered after less invasive measures have failed to deliver the tones your looking for.

I'll skip the Power break, attenuator , as they've been well covered.

The other method of volume control, and a method I've used for yrs now to good effect is a 'variable transformer' or 'Variac'.
Reducing the incoming voltage, reduces volume whilst altering the amp's response or 'feel' i.e..A little more 'give', and a little more gain. (Eddie Van Halen mastered this characteristic, it worked out quite well for him, don't you think..? RIP).

Given the combined price of a Variac transformer with a Marshall Power Break attenuator, tends to make a ppimv look like the only sane/sensible and financially viable option available. However.. Looking at the circuit of this 1986 bass amp I see an option that may have been missed..

Soldering a 500pf silver mica 'treble bleed' cap to the volume pot with a 140k- 420K resistor in parallel will boost gain enormously. Most of the gain in Marshalls is generated early in the signal chain.

This should provide much of the gain you need, and it's here that either an attenuator or variac provides the appropriate volume control for the venue your playing at without sacrificing tone quality with a master volume.
Any extra gooey high gain madness is the perfect excuse to break out those truly blasphemous fuzz units you've kept chained in the dungeon for just such an occasion.. Have funresponsibly..

 

[neikeel]

A great example of the classic 1986 bass circuit, one of Jim Marshalls finest imho, and a personal favourite.
While a ppimv is an option, Its a solution best considered after less invasive measures have failed to deliver the tones your looking for.
I'll skip the Power break, attenuator , as they've been well covered.
The other method of volume control, and a method I've used for yrs now to good effect is a 'variable transformer' or 'Variac'.
Reducing the incoming voltage, reduces volume whilst altering the amp's response or 'feel' i.e..A little more 'give', and a little more gain. (Eddie Van Halen mastered this characteristic, it worked out quite well for him, don't you think..? RIP).
Given the combined price of a Variac transformer with a Marshall Power Break attenuator, tends to make a ppimv look like the only sane/sensible and financially viable option available. However.. Looking at the circuit of this 1986 bass amp I see an option that may have been missed..
Soldering a 500pf silver mica 'treble bleed' cap to the volume pot with a 140k- 420K resistor in parallel will boost gain enormously. Most of the gain in Marshalls is generated early in the signal chain.
This should provide much of the gain you need, and it's here that either an attenuator or variac provides the appropriate volume control for the venue your playing at without sacrificing tone quality with a master volume.
Any extra gooey high gain madness is the perfect excuse to break out those truly blasphemous fuzz units you've kept chained in the dungeon for just such an occasion.. Have funresponsibly..

Ah, the 'Parkhead Mod' being resusscitated. Wish I knew who to attribute it to. There was a Parkhead that frequented VAF years ago.
https://www.marshallforum.com/threads/parkhead-mod.86918/
I've used it on a couple of vicious Superleads a few years ago and it did what I needed.
I would still go down the Reactive Attenuator route (not keen on reamping regularly) and the PPIMV mod on an amp with a 35year old pcb is not something I would rush into.
YMMV

 

[Acvox]

I heard of this simple gain mod, so many yrs ago that today I have no recollection of its origin or who I first heard it from but if its known as the 'Parkhead Mod' then that's good enough for me

 

[AndyD]

I thought I would share a couple of things I did to my Lead and Bass amp which might interest you. The name “Lead and Bass” is a bit misleading. It should really be called “Bass and slightly brighter Bass”! This is because v1 has a shared cathode capacitor like the typical bass amp and the lead channel just consists of a 100pf bright cap on channel one volume linked to a 470k/500pf grid stopper. What I did was to separate the shared cathode by breaking the link on v1 and adding a new yellow wire (from where the link ended on the valve holder) and attached it to a 0.68uf cap bypassed with a 2k7 resistor.

In the picture you can see the new yellow wire coming from the 6 o’clock position on the valve holder.
I then added a 500pf cap across the left hand 470k resistor.

You can see the 500pf cap tacked across the left hand yellow/purple/yellow resistor above.
Finally, I added a bright cap (I chose another 100pf cap) to volume 2.

You now have a proper bass channel and lead channel!
I have a feeling Kossoff had this done to his bass amps.

 

[junk notes]

offhand, noticing resistors and caps going in both directions, no worries if it sounds right..

 

[AndyD]

offhand, noticing resistors and caps going in both directions, no worries if it sounds right..

Are you referring to the cathode caps/ resistors? If so, they are wired correctly. Both caps share the same connection to earth.

 

[Acvox]

I thought I would share a couple of things I did to my Lead and Bass amp which might interest you. The name “Lead and Bass” is a bit misleading. It should really be called “Bass and slightly brighter Bass”! This is because v1 has a shared cathode capacitor like the typical bass amp and the lead channel just consists of a 100pf bright cap on channel one volume linked to a 470k/500pf grid stopper. What I did was to separate the shared cathode by breaking the link on v1 and adding a new yellow wire (from where the link ended on the valve holder) and attached it to a 0.68uf cap bypassed with a 2k7 resistor.

In the picture you can see the new yellow wire coming from the 6 o’clock position on the valve holder.
I then added a 500pf cap across the left hand 470k resistor.

You can see the 500pf cap tacked across the left hand yellow/purple/yellow resistor above.
Finally, I added a bright cap (I chose another 100pf cap) to volume 2.

You now have a proper bass channel and lead channel!
I have a feeling Kossoff had this done to his bass amps.

Well.. Spooky synchronicity..!! I literally just left a post on another forum where I describe my love for this uber rare 50 watt Marshall classic, and it's unique tonal characteristics. My particular amp retains its shared cathode arrangement as I love its JTM Preamp.

Perhaps we should start a thread dedicated to the 1964 head and 2100 combo Lead & Bass amplifiers 1973-75 ?

 

[AndyD]

Well.. Spooky synchronicity..!! I literally just left a post on another forum where I describe my love for this uber rare 50 watt Marshall classic, and it's unique tonal characteristics. My particular amp retains its shared cathode arrangement as I love its JTM Preamp.

Perhaps we should start a thread dedicated to the 1964 head and 2100 combo Lead & Bass amplifiers 1973-75 ?

Why not!

 

[AndyD]

Well.. Spooky synchronicity..!! I literally just left a post on another forum where I describe my love for this uber rare 50 watt Marshall classic, and it's unique tonal characteristics. My particular amp retains its shared cathode arrangement as I love its JTM Preamp.

Perhaps we should start a thread dedicated to the 1964 head and 2100 combo Lead & Bass amplifiers 1973-75 ?

I hear what you say. The mod I suggested still retains the original voicing and gives you a more realistic “lead”option. It gives the amp a bit more focus and bite. I really recommend doing it!

 

[Acvox]

I hear what you say. The mod I suggested still retains the original voicing and gives you a more realistic “lead”option. It gives the amp a bit more focus and bite. I really recommend doing it!

This kind of mod, although fairly straightforward is quite beyond my competency, but I've no doubt how good my model 1964 Lead&Bass would sound with the mod you've described.

To my way of thinking this amp combines all that's best in Marshalls development of their circuits over the yrs. My own amp is a very early example from 1973 with the cream circuit board containing the same components used in '72. A good thing, however..
The downside is the copper trace, it was Marshalls first attempt and while adequate was too thin. This improved with the move to the green circuit board in 1974 with thicker copper trace.
Regardless these are epic amp's. The combo version is the favorite of Paul Weller, amongst others. It's a Bluesbreaker with EL34's and ss rectifier. What's not to love?
An amp with the tone of '67 but made in 73-77

 

[AndyD]

This kind of mod, although fairly straightforward is quite beyond my competency, but I've no doubt how good my model 1964 Lead&Bass would sound with the mod you've described.

To my way of thinking this amp combines all that's best in Marshalls development of their circuits over the yrs. My own amp is a very early example from 1973 with the cream circuit board containing the same components used in '72. A good thing, however..
The downside is the copper trace, Marshalls first attempt . This improved with the move to the green circuit board in 1974 with thicker copper trace.
Regardless these are epic amp's. The combo version is the favorite of Paul Weller, amongst others. It's a Bluesbreaker with EL34's and ss rectifier. What's not to love?
An amp with the tone of '67 but made in 73-77

You are absolutely right. It is a fantastic amp in stock form. I hope you manage to use it in a band situation before too long. I certainly do!

 

[Acvox]

Amen to that brother

 

[Pave Dog]

I thought I would share a couple of things I did to my Lead and Bass amp which might interest you. The name “Lead and Bass” is a bit misleading. It should really be called “Bass and slightly brighter Bass”! This is because v1 has a shared cathode capacitor like the typical bass amp and the lead channel just consists of a 100pf bright cap on channel one volume linked to a 470k/500pf grid stopper. What I did was to separate the shared cathode by breaking the link on v1 and adding a new yellow wire (from where the link ended on the valve holder) and attached it to a 0.68uf cap bypassed with a 2k7 resistor.

In the picture you can see the new yellow wire coming from the 6 o’clock position on the valve holder.
I then added a 500pf cap across the left hand 470k resistor.

You can see the 500pf cap tacked across the left hand yellow/purple/yellow resistor above.
Finally, I added a bright cap (I chose another 100pf cap) to volume 2.

You now have a proper bass channel and lead channel!
I have a feeling Kossoff had this done to his bass amps.

I was trying to figure out why in some of your other pictures your Lead & Bass circuit looked a bit different than my 74 Lead & Bass. I will try out the stock circuit after I change out the filter and bias caps and replace a nonstandard pot and clean a few other things up.

 

[AndyD]

I was trying to figure out why in some of your other pictures your Lead & Bass circuit looked a bit different than my 74 Lead & Bass. I will try out the stock circuit after I change out the filter and bias caps and replace a nonstandard pot and clean a few other things up.

The stock circuit works very well. I just like tinker ….sometimes unnecessarily!