Tweaking a JMP 1987 build

[Emiel]

Hi all,

I have had this JMP 1987 clone for a couple of years now. It's a great sounding amp but overall just a bit too smooth for the band I mainly use it for (a Thin Lizzy tribute). I'd like to make it just that bit more wild and aggressive sounding. Lizzy used later (post '74) spec JMPs as far as I know.

The current specs of my amp are largely that of a '68 I guess:

5000pf bright cap on bright channel volume pot
V1a cathode: 820R/220uF & 2.7K/0.68uF
V1a coupling cap, 0.022uF & 0.0022uF (bright channel)
V2a cathode: 820R/0.68uF
Tonestack: 33K/500pF
B+ supply resistors: 8K2/10K
47K NFB resistor wired to 8 ohm tap
Filtering: 32+32 preamp, 50+50uF mains, 50+50 screens
Transformers: Drake 789-139 spec OT made by Dagnall, Tube-Town JTM45/1987 style PT, Dagnall C1999 choke

I think it could do with changing the circuit to a later spec, but which? What would you do?

Get rid of the .68uF V2 cathode bypass cap? Change the NFB resistor to 100K / 8 ohm? Change to 10K/10K B+ supply resistors? Upping the pre amp filtering to 50+50uF?

For the record: I've experienced heavily with speakers already, so just looking for amp mods .

Thanks!


A pic of the amp:

 

[johan.b]

Change the nfb to 100k and put it on the 4 ohm tap and try again.. it makes a bigger difference than you'd think...

 

[Pete Farrington]

The current specs of my amp are largely that of a '68 … Tube-Town JTM45/1987 style PT

That may be too saggy; what‘s the resistance of its primary and the HT windings? You’ll have to measure it, only Hammond publish that level of detail.

I've experienced heavily with speakers already

Have you included G12M25 and especially G12H30 in that?
Preferably 70s models with original cones.
I’m somewhat sceptical that speakers from other companies that they describe as being similar / equivalent to those models will have that same fearsome level of (on axis) attack and bite.

 

[Emiel]

Change the nfb to 100k and put it on the 4 ohm tap and try again.. it makes a bigger difference than you'd think...

Thanks, will definitely check it out!

That may be too saggy; what‘s the resistance of its primary and the HT windings? You’ll have to measure it, only Hammond publish that level of detail.


Have you included G12M25 and especially G12H30 in that?
Preferably 70s models with original cones.
I’m somewhat sceptical that speakers from other companies that they describe as being similar / equivalent to those models will have that same fearsome level of (on axis) attack and bite.

The sound is a bit saggy actually. This is the PT. No idea on the resistance readings, how does one measure those?
https://www.tube-town.net/ttstore/tt-powertransformer-jtm45-style.html

I have plenty of experience with Blackback G12M25s, Creamback G12H30s ('70s ones) and various modern incarnations of both of them. It's definitely the amp.

 

[Pete Farrington]

This is the PT. No idea on the resistance readings, how does one measure those?

It’s irritating how such useful relevant info doesn’t get published.
To measure resistance, you’ll need a meter, connect one probe to one red wire from the mains transformer, the other probe to the other red wire. The reading should be below 200 ohms, probably around 120 ohms for a JTM45/50 spec transformer.
The amp needs to be totally disconnected from the mains, it’s a good idea to discharge the HT caps.

 

[playloud]

Good call on the 100k/4ohm NFB.

There's also the presence circuit to consider, if you want to get closer to that era. I'm not au fait with all the variations, but one version had a 0.68uf cap across the 5k pot, in parallel with a 4.7k resistor, as shown here: https://www.drtube.com/schematics/marshall/1987mk2u.gif I believe a 25k pot was also used on some amps.

Removing the V2a cathode bypass cap would be period-correct, but it'll reduce gain.

The coupling cap from V1b is definitely 2.2nF, right? I can't tell from the pic.

Upping the preamp filtering couldn't hurt.

 

[Emiel]

It’s irritating how such useful relevant info doesn’t get published.
To measure resistance, you’ll need a meter, connect one probe to one red wire from the mains transformer, the other probe to the other red wire. The reading should be below 200 ohms, probably around 120 ohms for a JTM45/50 spec transformer.
The amp needs to be totally disconnected from the mains, it’s a good idea to discharge the HT caps.

Is the JTM45 PT really that different from the one in the 1987? As far as I know, they used pretty much the same one in the early 1968-1969 1987s which is why I went for it (intitial idea was to make a '68/'69 clone). Might be wrong here though!

Good call on the 100k/4ohm NFB.

There's also the presence circuit to consider, if you want to get closer to that era. I'm not au fait with all the variations, but one version had a 0.68uf cap across the 5k pot, in parallel with a 4.7k resistor, as shown here: https://www.drtube.com/schematics/marshall/1987mk2u.gif I believe a 25k pot was also used on some amps.

Removing the V2a cathode bypass cap would be period-correct, but it'll reduce gain.

The coupling cap from V1b is definitely 2.2nF, right? I can't tell from the pic.

Upping the preamp filtering couldn't hurt.

The picture is properly outdated, we started with a 22nF V1B coupling cap but this made the amp too bassy and mushy. It is now 2.2nF.

I will definitely try upping the preamp filtering to 50+50uF.

Some say the .68uF V2 cathode bypass cap can make the amp a bit too muddy and compressed. Any experience with removing it as in the later JMPs (post '71)?

Any other suggestions are very welcome too.

 

[playloud]

Is the JTM45 PT really that different from the one in the 1987? As far as I know, they used pretty much the same one in the early 1968-1969 1987s which is why I went for it (intitial idea was to make a '68/'69 clone). Might be wrong here though!

They're different. Afaik, the Drake 1202-55 that you see in KT66-equipped JTM45s was superseded by the 1202-133 (increased current handling) and 1202-118 (lower B+) when they started using EL34s. The 1202-118 was predominant until the (standup) 1202-164 was introduced in '69.

Some say the .68uF V2 cathode bypass cap can make the amp a bit too muddy and compressed. Any experience with removing it as in the later JMPs (post '71)?

Actually, this is a good thing to consider. It shouldn't make the amp muddier (although this is what happens if you replace the 0.68uf cap with a 330uf "fat cap"), but you'll get more compression - especially if you move the NFB to 100k/4ohm. Might be worth raising a leg of the cap and see what you prefer.