Finally…Plexi 100W in collection (paging the experts)

shakti

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I picked up this one over the weekend. It has a cool little story, and also some odd little details worthy of discussion.

I got my first Marshall amp back in 1993, a Bluesbreaker reissue. Around 2000 I got my first vintage Marshall, a ‘71 50w tremolo. Over the next few years I got several early 70s metal panel amps - a ‘71 50W Bass, a ‘72 Super Lead and a ‘73 Super Bass. They were all great, but of course I would have preferred a Plexi. Somehow those always eluded me. I probably wasn’t looking too hard as they were already considerably more expensive than metal panels, but in retrospect I could probably have done very well back then with some effort.

Then in early 2006 I decided to do my first amp build which was a Metroamp JTM45. It sounded great and I was completely hooked. Over the next five years I built probably 12 different amps, going ever deeper in detail level. The ones I kept were the most accurate ones using mostly NOS parts and Merren transformers.

I was so taken with this hobby that I completely stopped looking for vintage Marshalls. Why do that when you can build your own, with modern reliability, to the exact specs you want? I built pretty much every iteration of Plexi era Marshall, so every major variation in the 100w and 50w lines. Lots of fun, great sounds and great looks.

Around 2006 I also remember a fellow Marshall friend telling me a story about a kid who just found a ‘67 100W Plexi tucked away in a cupboard in a church. I seem to remember he even asked if I was interested in buying it, so the finder might have looked to sell it. I passed, as I thought I could just build my own.

Over the years though I’ve just come to the realisation that as good as clones are, there’s no substitute for the real thing. Putting them side by side, an accurate clone will come very, very close, and in certain respects I might even prefer the clone, but with vintage tone as the benchmark, the vintage amp will win the shootout every time.

So, slightly reluctantly, I’ve started to replace at least some of my clones with originals. I’ve scored a ‘66 JTM45 and a ‘68 50w Bass over the last couple of years, both through friends, but a 100W has proven very hard to find. Prices are generally also horrific.

Fast forward to Saturday afternoon. I’ve had an auto-search going with the major Norwegian classifieds site for a loooing time. Frankly, I was getting tired of umpteen alerts per day for Marshall Bluetooth speakers and broken Valvestate amps. And after some more little tweaking, I had pretty much decided just to be content with my very decked out clones. I was going to delete the auto-search, but was too busy that day.

Then bam, an alert comes up for a ‘67 Marshall JMP Super Bass 100W Plexi. One glance at the photos reveals that this is, in all likelihood, virtually all original and almost untouched. Price is good (as in really good). This is when I start to get dizzy and palms sweaty. I can almost not hit the right numbers on the phone to dial, and I have to work really hard to contain my excitement on the phone, but a deal is secured, I put a deposit down and the guy takes down the ad (lesson learned long ago!).

When I went to pick it up I asked him if he knew the history of the amp. Pre-68 Plexis are really scarce in Norway, so it’s fun to trace the history of each and every one. He didn’t know much about the previous history, but he found it in a cupboard in a local church back in 2006 when he was still in his teens, and got it for free! So finally, 16 years later I buy the amp I was offered back then via a friend.

Stay tuned for photos as we go over it. I haven’t had a chance to plug it in yet and am away on travel until next weekend, so I can’t tell you yet how it sounds, but it’s looking really promising.
 

shakti

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Head box is structurally in good shape, but missing back panel, handleand mesh, and all tolex and piping is gone. This will need a full restoration.

Transformers are all correct - 1203-80, 1202-132 and 352-114 choke.

Interestingly this is one of those amps with 4 output jacks. Anyone know the story behind those? Strangely 3 of them are Cliffs while one is a Rendar, but I can’t see any immediate signs that it has been replaced. Input jacks are also Cliff.

The serial number is really strange though. There’s no Marshall serial number, instead it has a gold Dymo style label which says 1103. Same label is on the chassis by V3.

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shakti

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The mains filter caps are replaced, but I got the two original Radiospares 100uF ones with it, we’ll see if they can be reformed. Screen grid resistors replaced but again I have 2 or 3 of the original ones. One bias cap replaced. When he found it it didn’t have a Bulgin inlet but a hardwired mains cord dangling loose in that large hole. This is a little odd - typically Norwegian import amps had the Bulging removed and the hole plugged, and often a new (small) hole was drilled for a hardwired mains cord.

The preamp filter cap is also a little odd, being inserted in the unused first preamp tube hole. I’ve seen this on one or two other same era amps so I thought it was original, but then I see the filter cap is an Italian made Ducati with a 7/71 code which I presume is from 1971, so now I am really unsure if that arrangement is original or not. Tazin is the king of these little details, would love to hear his opinion.

Filter arrangement is 50uF mains, 32uF screens, phase inverter, V2 and V1 respectively.

Other than that this looks almost untouched. Mustard date codes are B6 and A7, but judging y the other specs I’d put this at third or fourth quarter of 1967.

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Ned B

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That transformer set is the most desirable in my opinion. Congrats on that great find.
 

shakti

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Does that mean you like it more than the 1202-119?
 

TAZIN

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Nice score. My first impression is that the amp is a later '67 (around the last quarter of '67) based on the usage of the Murata 47pF ceramic disk cap for the plates of the phase inverter. Earlier amps tend to use the silver mica type. In addition, the 32uF - 16uF phase inverter filter cap tends to support a later '67 build also. The rear plexi panel maybe a replacement from/by Marshall which could explain the lack of serial number...Would need to see the rest of the panel to determine which era ('67, '68 or '69) it is from. The preamp filter mounted in the unused preamp valve hole is clearly not original. I've seen a half-dozen amps setup that way and none of them were factory setups. Besides, you can see that there is solder in the turret that was used to ground the original preamp filter when it was on the phenolic board. The use of four speaker jacks started in 1969 and I don't recall ever seeing a '67 amp with the four speaker outputs which was a 'factory' install. Also, Marshall would not use the Tremolo foot switch hole to install a speaker jack...They always put the second pair of jacks on the other side of the hole.
 
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Cool version. Amps from that time period sound really good. I have been looking around for this era of super bass’s. That has the 4 ohm tap on the transformer not the 100v ?
 

Matthews Guitars

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Nice amp. Congratulations!

I interpret the speaker jack anomaly as being a simple matter of there being four speaker jack holes punched in the chassis already, which Marshall added to all 100 watt chassis as a matter of standardization, and someone decided to make use of them and drill out the plexi back panel and install two jacks he had lying around. They were evidently added, while the first two are likely originals.

(Edit: No, I've revised my opinion. Apparently this never had four speaker jacks in the chassis to start, since one of the added jacks is apparently in the hole that would be for the tremolo footswitch if so equipped. So the fourth jack hole must have been drilled thru the chassis.)

I note that the front panel hole in the chassis for the US market polarity switch is also present, but not in the front panel. That confirms it.

Those chassis were made to be universal to any market. Just cover up the extra unneeded holes with front and back panels that don't have those features punched or marked.

Today I saw a first: A 50 watter, pre-1974, where the back panel had all the legends for the voltage selector but the voltage selector cutout had never been punched.
 
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Ned B

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Does that mean you like it more than the 1202-119?
Yes due to the 4ohm tap though sonic differences would be more about that 2X8K2 dropping resister set up that started around the same time. I'm not taking anything away from the -119 OT, just that your setup is one of 2 holy grail amps to me.
 

Ned B

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Cool version. Amps from that time period sound really good. I have been looking around for this era of super bass’s. That has the 4 ohm tap on the transformer not the 100v ?
Yes, 4 / 8 /16.
 

DragonCrestPC

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Congrats! I love my 68, i've never played a 67, i'm sure its amazing cranked!
 

shakti

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Thanks for the feedback to everyone. I am really excited and looking forward to testing it in a few days.

I was secretly hoping to find this exact spec amp, so to have lucked out like that is incredible. Not that I would have turned up my nose at a dual rectifier/1202-119 circuit, but those are fairly loose sounding. It was really a 66–67 I was looking for, and a KT66-based one is really rare these days, so of all the amps I could realistically hope to find this would be at the top of my list. It is actually the only 100w Plexi spec I have never built or tried (with this filter scheme, single rectifier, 8.2+10k dropper and 1202-132 output transformer).
 

shakti

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BTW, I will be looking for a few different parts to put this fully back to original spec if I can. If anyone is sitting on this stuff so would be interested:
- tolex, gold piping (both thick and thin)
- mesh for vent and back panel
- handle
- 32+32 axial filter cap. Radiospares, Erie or Hunts
- 1k screen resistor (I think the original ones were DCC labelled block types similar to these)
- light blue bias caps (I think they’re Erie, can’t remember)
 
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Congratulations! 👍 This era is type with the ot transformer are great. I think one of the best types. Depending on what you want sonically.

I would not say the earlier 67 dual rectifier el34 version with a 1202-119 is loose however. I would say mine is tighter than a old kt66 45/100 66-67 Marshall. However this is one is middle of 8000‘s serial number 1967 white panel super 100 with the 1202-119 dual rectifier el34 i am talking about . A aluminum block end one. However the era you have right now i really like how they can sound and set up. The lead or bass model.

They are all different though. That one I wrote about is quite the opposite of loose though . The ones with slightly latter different output transformer and the other changes have slightly different classic character To them.

Super loud,percussive ,tight and bell like clean to body shaking volumes. When cranked it is tight and no ghosting . Sounds like Hendrixs axis and after albums with Fender guitars if you want it to . Nice amount of very articulate gain cranked with a Lester. You feel this amp in your junk and bowels lol.

Very exciting stuff and enjoy it ! Ned B’s description above is a good one . Cool amps and classic sounds .
 
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shakti

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Finally had time to test this. I tried reforming the filter caps with a 100k 2w resistor in line before first filter stage, but I forgot to remove the PT centre tap so not sure if I did it right. It stalled at a considerable voltage drop across the resistor, but I also wasn’t getting full B+ voltage down the line because the centre tap was still in place. Had it running for many hours like that, so at that point I just decided to give it a try. The mains filter caps had already been replaced with Sozo Royals, and those see the most stress anyway. After extensive testing the amp sounds nice and quiet, no hum or other noises, and very little ghosting (virtually none I would say). The caps don’t bulge and they don’t get hot so I am hoping to keep them.

The amp sounds simply glorious. I absolutely as good as I hoped and then some. It is later period Cream to a T (Crossroads, Royal Albert Hall farewell gig). Just amazing really, and that’s even before tube tweaking. It matched particularly well with a tall bottom pinstripe cab with the rare 25W T1281s.

I’ll go over the little things that need to be corrected bit by bit and took some more photos. The preamp filter cap definitely isn’t original. You can see a shade from where there used to be a round preamp hole cover. I’ll transplant an Erie 32+32 from one of my other amps.

The extra speaker outputs I am still undecided about. Perhaps TAZIN can have a look at the rear panel again - does it look replaced? It looks alright to my. But the serial number is really odd. Perhaps something put in there by the importer? I found quite a number of same era amps (late 67), mostly Super PA ones, which had 3 speaker outputs, but didn’t see any with 4. All of those had the extra speaker in the DIN socket hole and used a Rendar jack socket there, presumably because it’s larger and covers the (larger) hole better. I’ll probably leave all 4 jacks in there anyway rather than expose any holes.

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