Park Lead 50 model 1239 questions

Ossyrocks

Hi chaps,

I'm restoring a damaged Park Lead 50 1x12 combo and I have a couple of questions.

Does anyone know the impedance of the original speaker fitted in these? It had a modern 8 ohm G12H Anniversary in there when I got it, but I'm not sure it's the correct impedance. Does anyone know?

Also, does anyone have a schematic for these? 

Cheers,
Rob

ICBM

I had the bass version ages ago, which was 8 ohm. It’s unlikely the lead one is different, usually the power sections were exactly the same on corresponding models.

I don’t have a schematic but it’s likely to be very similar or identical to the Marshall 2144.

Ossyrocks

ICBM said:

I had the bass version ages ago, which was 8 ohm. It’s unlikely the lead one is different, usually the power sections were exactly the same on corresponding models.

I don’t have a schematic but it’s likely to be very similar or identical to the Marshall 2144.

Cheers for that, I'm going to put a Neo Creamback in it and just wanted to make sure I got the right one.

Does the Marshall 2144 have reverb & boost? EDIT: Yes it does. Got it now.

Rob

ecc83

Didn't the Park range come about because Jim got a bit shafted by the American distributors and his margins on the Marshall brand were cut to the bone? The Park amps were virtually clones of the Marshalls.

Dave.

Ossyrocks

ecc83 said:

Didn't the Park range come about because Jim got a bit shafted by the American distributors and his margins on the Marshall brand were cut to the bone? The Park amps were virtually clones of the Marshalls.

Dave.

Sort of I think. I remember reading somewhere that he signed an exclusive distribution deal with Rose Morris for Marshall amps and that meant some of the smaller retailers couldn't get their hands on them. So, he hatched a cunning plan to build Park amps for those retailers. It was also an opportunity for the designers to let their hair down and produce some unusual amps that differed from the regular Marshall product line.

Either way, this amp never had a Marshall equivalent. As stated above it's almost certainly based on the 2144, but this amp is a 1x12 combo.

ICBM

Yes, it was due to an exclusive distribution deal for Marshall with Rose Morris.

Many Parks were simply Marshalls with slightly changed cosmetics, panel hole/control positions, control names (but usually not functions) etc, but some were a bit more different including the 75W and 150W models with KT88s, and most of the ones with boost and reverb - but they were all fundamentally based on Marshall circuits and components.

Rich210

"150W models with KT88s,"   

Ossyrocks

Rich210 said:

"150W models with KT88s,"   

Had a plexi one in the 90's. Could barely lift it. Made a great bass amp though.

PeteC

Though many were, not all of the Parks were essentially copies of existing Marshall’s.

 The 1210 Rockhead which Steve Grindrod designed  around 79 has a plate driven tone stack and a post phase inverter master volume which has 2 sets of coupling caps.   Quite a radical design for the times.   Some argue that the tone stack is cold and hard compared to normal Marshall circuits but tbh I quite like it.  It’s been done I many other high end amps like Dumble for example.  

I’ve built a couple of replicas of the Rockhead, in both 100 and 50 watt versions and they are very fine rock amps 

Rich210

Ossyrocks said:

Rich210 said:

"150W models with KT88s,"   

Had a plexi one in the 90's. Could barely lift it. Made a great bass amp though.

A roadies nightmare, a stars dream. I love the idea of a huge bass amp lined up to the rafters with milk bottles. Would awesome to put one head to head with my musicman 150

Ossyrocks

Quick update. I emailed Marshall yesterday to see what info they have on this Park combo.

I got a reply today from Joanna at Marshall with information about the speaker and also attaching Steve Grindrod's hand drawn schematic of the Park Model 1239 dated 17th January 1980.

I have trawled the internet looking for this schematic to no avail and it seems Marshall had one all the while.

Great service from Marshall.

If anyone wants a copy of the schematic, drop me a PM with your email. I have JPG and PDF versions now.

Rob.

ICBM

Will do! Marshall are very helpful, yes.

Ossyrocks

ICBM said:

Will do! Marshall are very helpful, yes.

John, I've sent you those schematics, inc the original from Marshall.

It would  be good to hear your thoughts about how it differs, or is similar to a Marshall.

Cheers,
Rob

Ossyrocks

Ok, the cabinet work is done and I’m pleased with how it’s come together.

I’m now looking at the amp itself and trying to decide how far to go. It works fine, as it should, but it’s a little bright. I think I removed the treble bleed cap off the input gain pot on my last one, so I’ll probably do that again. 

But the big question is should I recap it? 

@ICBM , what would you do?

ICBM

Ossyrocks said:

But the big question is should I recap it? 

@ICBM , what would you do?

Are you going to use it professionally? Does it hum more than it should? Are any of the caps leaking at all, either physically or electrically? Does it need any other major work?

If yes to any of those, re-cap it.

If no to all, then personally I wouldn't - yet. But be aware that it will need doing eventually - sooner or later, and quite likely sooner.

My current record holder for an amp with original caps which showed no signs of problems at all - no hum, no physical leakage, no measurable electrical leakage - is 65 years old. But they could fail tomorrow, and in an amp without really effective fusing (which is most, especially any without an HT fuse) there is a risk to other components, including the PT.

Do you feel lucky, punk?

Ossyrocks

@ICBM , cheers for that.

No, not professionally, maybe only a gig a month.
No, it doesn’t hum.
No physical sign of leaking or bubbling, but I don’t have the ability to test them.
No other work required, the mains tranny is already an old replacement.

Hmmmm.

A couple more questions.

The schematic shows a mains fuse at 1amp, the back panel says 2 amp? Should I go 2amp? The one currently there is 3 amp.

Also the schematic shows an internal fuse on the board at 500ma. It has a 2amp in there. Given that the schematic and back panel differ for the mains fuse, what do you think the internal fuse should be?

Cheers,
Rob

ICBM

Ossyrocks said:

The schematic shows a mains fuse at 1amp, the back panel says 2 amp? Should I go 2amp? The one currently there is 3 amp.

Also the schematic shows an internal fuse on the board at 500ma. It has a 2amp in there. Given that the schematic and back panel differ for the mains fuse, what do you think the internal fuse should be?

Mains: 1A - slow-blow, or T - should be sufficient for a 50W amp at 240V, but it's possible that the start-up surge will blow it, in which case you might need 1.5A. 50W Marshalls were fitted with 2A fuses, but that's because they were intended to be able to run at 120V as well. 3A is definitely too high.

HT: no more than 500mA for a 50W amp. Depending on where it is in the circuit you might possibly need a T, but usually an F is fine.

I would probably leave the caps for now. By the late 70s they were becoming much more durable, it's really when you get back to the dark blue Erie/Daly ones or even older plain aluminium ones that it's almost essential to change them.

Ossyrocks

ICBM said:

Ossyrocks said:

The schematic shows a mains fuse at 1amp, the back panel says 2 amp? Should I go 2amp? The one currently there is 3 amp.

Also the schematic shows an internal fuse on the board at 500ma. It has a 2amp in there. Given that the schematic and back panel differ for the mains fuse, what do you think the internal fuse should be?

Mains: 1A - slow-blow, or T - should be sufficient for a 50W amp at 240V, but it's possible that the start-up surge will blow it, in which case you might need 1.5A. 50W Marshalls were fitted with 2A fuses, but that's because they were intended to be able to run at 120V as well. 3A is definitely too high.

HT: no more than 500mA for a 50W amp. Depending on where it is in the circuit you might possibly need a T, but usually an F is fine.

I would probably leave the caps for now. By the late 70s they were becoming much more durable, it's really when you get back to the dark blue Erie/Daly ones or even older plain aluminium ones that it's almost essential to change them.

John, as ever, great advice, thank you. 

These are the ubiquitous Daly caps, but they are the later pale blue variety. 

So, just some new EL34’s to buy and we’ll be cooking on gas as they say. 

Are the JJ’s still what people recommend?

Rob

Ossyrocks

@ICBM ;

One last question. I feel like I’m pestering you now.

Would you add 1k 5w screen grid resistors? It doesn’t have any, which I find odd.
I haven’t done this stuff for years. I think I can just move both wires from pin 4 to pin 6 and then put the resistor between 4 and 6. Am I right?

Rob

ICBM

Ossyrocks said:

Would you add 1k 5w screen grid resistors? It doesn’t have any, which I find odd.

I haven’t done this stuff for years. I think I can just move both wires from pin 4 to pin 6 and then put the resistor between 4 and 6. Am I right?

They're an essential upgrade in my opinion. Marshall didn't fit them to most of the 50W amps in the 1970s, and the valves back then seemed to be able to take it - but it's bad practice, and modern valves are less tolerant.

Yes, that's how to do it.

Ossyrocks

ICBM said:

Ossyrocks said:

Would you add 1k 5w screen grid resistors? It doesn’t have any, which I find odd.

I haven’t done this stuff for years. I think I can just move both wires from pin 4 to pin 6 and then put the resistor between 4 and 6. Am I right?

They're an essential upgrade in my opinion. Marshall didn't fit them to most of the 50W amps in the 1970s, and the valves back then seemed to be able to take it - but it's bad practice, and modern valves are less tolerant.

Yes, that's how to do it.

Great. I’ve ordered some.