My New Marshall 1973 50w Lead - PICS!

OdgeUK · Post by **OdgeUK** » Thu Nov 23, 2006 4:46 am

Hi all.

I recently purchased a JMP 50w 1973 PTP Lead head on eBay and collected at the beginning of the week.

I have posted pictures of the guts here:

http://www.bellinnburwash.co.uk/jmp50w.htm

Apologies to those of you who frequent the Plexi-Palace as you will have
seen this before.

It was sold to me with advice that the Mains Transformer had suffered a
meltdown and been replaced, the mains Power selector had been hardwired
to 240v and a Kettle lead connecter fitted. Other than that I was
assured that the amp is quite original.

The folk over at the Plexi-Palace have poured over the pics and offered
a lot of advice. Consequently I have passed the amp to a local Amp tech
to give it a full service and a report on what work needs to be
undertaken to make the amp safe. I get the amp back on Tuesday. I want
to use this amp to educate myself in Amp building but for it's first
health-check, I want a professional to give it the once-over before I go
diving into the world of Amp maintenance and modification (I think I'd
like a Master Volume in there somewhere).

Some things that have been highlighted by the Plexi Palace peeps:

- The Transformer is badly installed and the poor routing of the
wires will eventually lead to the Transformer shorting out against the
Chassis.

- The Bias caps are charred and may need replacing

- The Rectifier wiring may need inspection in case this is the
cause fo the original Tranny blow-out.

- The Chassis needs a clean and the board needs removing to
inspect for other signs of damage possibly caused by the Tranny
melt-down.

The control pots are also all scratchy and need cleaning/replacing.

I was wondering if you guys thought it'd be a good idea to wait for the
Amp tech's report and then try and take on the remedial work myself?
I've soldered before, but not often and have a very basic understanding
of electronics. You think you guys would be able to guide me through
this project?

PS: Yes, I have played it. Once, for about 20mins. It was very clean, even with the channels patched. Lovely, lovely sound. Didn't break up into distortion though until the VolII was on about 8 and the VolI was just past 2. Expected it to break up earlier than that, but U was putting it through an 8ohm PA speaker just to test it. Full testing and playing once I fix it up! Can't wait!

clarkydaz · Post by **clarkydaz** » Thu Nov 23, 2006 7:15 am

hi odge, if you have a 50 watt with a 5k bright cap and its still clean then you can bet there are a few things not letting it perform fully. your presence cap/pot is toast. if the bias area is damaged this maybe keeping your tubes cold?

the 2 grey caps in the middle of the board look like they read 8.2? should be 10k.

OdgeUK · Post by **OdgeUK** » Thu Nov 23, 2006 8:20 am

clarkydaz wrote:hi odge, if you have a 50 watt with a 5k bright cap and its still clean then you can bet there are a few things not letting it perform fully. your presence cap/pot is toast. if the bias area is damaged this maybe keeping your tubes cold?

the 2 grey caps in the middle of the board look like they read 8.2? should be 10k.

They are 8.2. Added some larger pics to the webpage so you can see all the values on the board clearly. That prescence post I was just hoping is dirty rather than fried?

clarkydaz · Post by **clarkydaz** » Thu Nov 23, 2006 9:02 am

maybe the pics look a lot worse than it is. safety is an issue with that though so you would want the pt wiring tidy, and with some grommets in there. replacing a few suspect pots/caps is easy and not expensive, just make sure its ticking ok first!

Wicksy · Post by **Wicksy** » Thu Nov 23, 2006 9:15 am

Bloody hell mate, i missed that over at the Palace. That has suffered a serious blowout!

A good tech will fix and clean that all and make sure it purrs like a kitten

Flames1950 · Post by **Flames1950** » Thu Nov 23, 2006 9:54 am

At the very least the current power tranny needs to be rotated, the laminations between it and the output tranny shouldn't line up for best hum rejection. I'd go ahead and put in a laydown power tranny like the amp originally had instead of the standup at that point anyway.
And the tech needs to spend some time in there with a toothbrush and cleaner, YUCK!!!!
It's funny how many of these '72-'73 era amps with that block rectifier have blown a PT, as the Plexi Palace guys may have noted already -- they've discussed it before..........

Michael Patrick · Post by **Michael Patrick** » Thu Nov 23, 2006 10:09 am

Once you get it all sorted out, you will have a righteous amp! Even if you have to drop a couple hundred bucks on the repairs, given what you paid for it you'll still wind up with an amp worth about twice your out of pocket expenses.

Michael Patrick · Post by **Michael Patrick** » Thu Nov 23, 2006 10:10 am

Flames1950 wrote:It's funny how many of these '72-'73 era amps with that block rectifier have blown a PT, as the Plexi Palace guys may have noted already -- they've discussed it before..........

True. I'm not sure I've seen one yet with the original PT, including the '72 I used to own. Of course, they don't all flame out as spectacularly as Odge's must have...

neikeel · Post by **neikeel** » Thu Nov 23, 2006 2:37 pm

Hio Odge edited version of pm.
Saw pics of your amp, I have a '69 and a '76 JMP50 and use the 76 all the time.

Yours looks a bit cooked but so have many of this age (why?). It looks like your snubber caps (0.22uF 1000v caps next to the rectifier block) went too which is strange as they are usually robust. Most of the amps of this era (my 69 included

) have replacement PTs. If the diode block were at fault I doubt it would work now (someone correct me if I am wrong).

The PT (1202-324) is actually the correct Drake replacement issued by Marshall since the late 70s until used on the JMP50 and the JCM800 series amps. It looks strange as it has been mounted 90 degrees out (it should be mounted at 90 degrees to the OT not 'in-line'. This tranny has slightly higher B+ voltage and will give more clean headroom and plenty of oomph if you want it (do search on PP for more info).

You will need to check the preamp socket voltages carefully as you only have one dropping resistor (next to the bias caps) - there are ususally two 8.2k resistors in series there (any you have the two odd but probably original 8.2k grey resistors further in the supply line which are usually 10k).

The cable ties and lack of grommets are a bit dodgy to say the least. George sells a bag of bolts and grommets for this purpose and also sells the correct indicator lamp (6.3V) which is connected to the heaters on the out put valves. Otherwise it looks burned but ok.

I am with Clarkydaz et al. Suggest you give it a clean, replace the bias caps clean the pots and remount the tranny (although I think that Flames is right there is a cut out for a laydown PT which was in originally). Get a set of Mullard EL 34s and the MkII PPIMV either the one George sells -it fits easily or use the one Rockstah posted (Fisher) which is a bit more fiddly. You can put the pot on a speaker out hole or in the lo input of Channel 2 (who uses that anyway?).

Try Ampman72 page for a pic of how yours should look (or on Marstrans excellent site).

There is nothing very frightening there and if you hang around here with the correct searches all will be revealed (I had to learn to do all my stuff myself as I could not find a good tech near me and this place has been hugely helpful!)

MacGaden · Post by **MacGaden** » Thu Nov 23, 2006 5:32 pm

Hi Odge:

Wellcome to the board. I answered over at PP too.

I think the comments cover it. Since George has 50 watt kit coming, I think you could follow some of those threads, ask some questions, add some pics, and fix it yourself..

Do any of the more expirienced techs have any idea what happened ?
It

Flames1950 · Post by **Flames1950** » Thu Nov 23, 2006 5:35 pm

Some markets kept the laydown PT for quite a while, such as Canada.

OdgeUK · Post by **OdgeUK** » Fri Nov 24, 2006 4:40 am

Thanks for the comments guys. Some great, great info. That Marstran picture looks amazing! I'd love to get my amp to that condition. How come you can't see the bottom of the Mains transformer though? And why does it have two big-ass capacitors, while mine has the outline of two? You think mine are missing? Or on the other side of the board maybe?

I'll be very interested to see if the Amp tech picks up on the same areas that have been commented on when he gives me the report over the weekend. Should be a good test of his skill and experience before I agree to any work taking place.

What should I expect for a

neikeel · Post by **neikeel** » Fri Nov 24, 2006 6:22 am

You only have a big hole in the chassis with a laydown tranny so that would be original for your amp.

The missing caps as I said were 0.22uF 1,000v rated 'snubber caps'.

The originals were from Dubillier (early) or Hunts(later) I believe (mine on the '69 are Dubillier white/cream two tone). They can safely be removed without ill effect but if you want to replace them I had to order 5 from Radiospares so I have a couple left over.

Sorry (I must need glasses, I read the label as 1202-324 (I have no idea what -304 is

)

OdgeUK · Post by **OdgeUK** » Fri Nov 24, 2006 6:29 am

neikeel wrote:You only have a big hole in the chassis with a laydown tranny so that would be original for your amp.

The missing caps as I said were 0.22uF 1,000v rated 'snubber caps'.

The originals were from Dubillier (early) or Hunts(later) I believe (mine on the '69 are Dubillier white/cream two tone). They can safely be removed without ill effect but if you want to replace them I had to order 5 from Radiospares so I have a couple left over.

That clears it up then! When you said the caps had gone, I thought you meant that there was evidence they had blown and been replaced. Now I realise, they simply 'aint there! LOL

What do they do?

So a laydown it is then. Should I try and track down an old original? Or just get a MM, OEI replacement?

Cheers!

neikeel · Post by **neikeel** » Fri Nov 24, 2006 6:43 am

They are additional smoothing caps in the supply line (also help with pops and bangs when switching on and off). Early amps use a single 0.05uF

I suggest either Georges Heyboer or Marstran clone either will work well.