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

shakti

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Oh, and negative feedback wire is hooked up to the 8 ohm tap currently. I am pretty sure it should be at 16, and it looks to me like it has been moved, but wanted to check with the experts. Most of the photos at Amparchives are too grainy to make out where negative feedback is taken from on this era amp.
 

shakti

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I found this one at Amparchives which must be very close in date to mine:

The two signatures on the inspection tag that are visible on mine are the same on this one (SB/10423). The filter caps (all grey RS on the board, screens are 2 x 32+32) appear to be identical. Some preamp board resistors and the phase inverter Murata cap are different but otherwise very, very similar. Finding good photos of amps with 1202-132 output transformer is difficult, but on this one it actually appears to have the NFB wire at 8 ohms like mine. I thought perhaps it had been moved but I am not certain. I tried it at 16 and I liked it better with Strat and fuzz like that, but the gain and fatness with humbuckers and NFB at 8 ohm was really cool.
 

TAZIN

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The rear panel does seem to be of the correct era from what I can see. The dymo label for the fuses is not your 'typical' factory label. Aside from being crooked, Marshall tends to not abbreviate 'AMPS' with just the letter 'A' as seen on your label. I'm still thinking possibly a factory replacement panel done very early on.

As mentioned before, I've seen other 100w amps that had an additional speaker jack mounted in the DIN switch hole for the tremolo switch but they tend to not look factory installed. It wasn't very common for Marshall to build custom features into their amp heads and the majority of their custom builds revolved around speaker cabinets or combo amps. Any custom builds appear to be well thought out so I find it odd that Marshall would install a speaker jack in an over-sized chassis cut out which would rely mainly on a fiber washer and the thin rear panel as holding power.
 

shakti

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I agree with all your points. It’s very difficult to say whether the back panel itself is original. Here’s a thought: perhaps the original back panel was damaged in shipping, and the importer requested a new back panel? This would explain why they could get a replacement panel that was accurate right down to the Super Bass and SB designation. If this had been a repair job even just two years after it was new, chances are that Marshall would have been unable to supply a Plexi rear panel like this. But maybe the replacement panel didn’t come with a serial number or fuse rating info, so the importer put their own Dymo labels on it with a makeshift serial number for reference. Maybe extra jacks were even added at that time.

What can you say about the negative feedback setup?
 

TAZIN

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I agree with all your points. It’s very difficult to say whether the back panel itself is original. Here’s a thought: perhaps the original back panel was damaged in shipping, and the importer requested a new back panel? This would explain why they could get a replacement panel that was accurate right down to the Super Bass and SB designation. If this had been a repair job even just two years after it was new, chances are that Marshall would have been unable to supply a Plexi rear panel like this. But maybe the replacement panel didn’t come with a serial number or fuse rating info, so the importer put their own Dymo labels on it with a makeshift serial number for reference. Maybe extra jacks were even added at that time.

What can you say about the negative feedback setup?
That was exactly what I was thinking when I wrote my response...Damaged during shipping which warranted a new rear panel. That would explain all the anomalies.

The NFB from the 8-ohm tap could be as simple as the assembler being use to hooking up OT's with the 100v tap which meant the NFB wire connected to the middle solder lug on the selector which represented the 16-ohm tap. I've seen other amps around the transition period of the 8, 16, 100v to the 4, 8, 16-ohm OT which also have the NFB on the 8-ohm tap. To me, this simple 'oversight' makes sense. I've even seen amps with the 8, 16, 100v selector but running a 4, 8, 16-ohm OT.
 

shakti

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Thanks, that was exactly what I was thinking too. I’ve made that mistake when looking at the earlier amps with 1202-84 or 1202-119. The 16 ohm tap is in fact the middle one since the selector was 8 - 16 - 100V. Many times I’ve looked at photos and thought there was an oddity with the NFB wire on the 8 ohm tap thinking that was the one in the middle when it’s not. I’ll go with what my ears tell me then, and probably stick with 8 since that may also have been original.
 

AndyD

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Not much to add other than to congratulate you on a cool find!. If it sounds anything like that Youtube clip, it will be amazing! Enjoy.
 

shakti

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I tried reforming the Radiospares 100uF mains caps. On my cheapo LCR and capacitance meter they both checked out perfect with very low ESR and capacitance right above 100uF for each.
Maybe I did something wrong again because with the center tap disconnected and a 100k in series before the first cap I got zero voltage drop immediately. I’ve never experienced that before, even with brand new caps there is a period of gradually less voltage drop across the resistor.
In any case, everything seemed fine, so I was a but miffed to find I liked the sound with the Sozo Royals better. There was more ghosting with the Radiospares and it seemed overall a little muddier and not as gainy. So even though they seem to technically check out fine I am not going to sacrifice sound for originality. The Sozos will go back in.

Furthermore I hadn’t looked much at the tubes but found to my delight that there were 3 x xf2 and one RFT. The idle dissipation was all over the place though so I may have to put in something else. The hottest tube is on the verge of red plating while the coldest was dissipating just 8-9 mA at that setting.
 

shakti

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Doh! I had overseen an under board jumper, so the 100k resistor was paralleled/short circuited. No wonder there was no voltage drop. I measured the resistance across the resistor to make sure it was right and it came in right at 100k so I thought all was good. Apparently this meter is too smart, I thought it would show 0 ohm if there was a jumper/short circuit there.

According to all I’ve heard before I blew my chance at reforming the caps since I already fired them up at full voltage then, but decided to give it a go nevertheless. Voltage drop is going steadily down and well under 1V now. Is there any chance if improving their performance then? At least they seem safe with low ESR and very little voltage drop as well as no heat.
 

Derrick111

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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.
My supertremolo has both a tubular ceramic Murata and a large disk Murata cap. When were they used? I always assumed the amp to be 2nd half of '67 or early '68.

Really nice amp, Shakti!!
 

TAZIN

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My supertremolo has both a tubular ceramic Murata and a large disk Murata cap. When were they used? I always assumed the amp to be 2nd half of '67 or early '68.

Really nice amp, Shakti!!
Super Tremolo amps can be a bit more difficult to narrow down a build date since they tend to be built with left over part sometimes...Same goes for P.A. amps. Generally, if it's equipped with Dagnall transformers and Murata caps; 500pF tubular ceramic, 250pF ceramic disk, 47pF phase inverter ceramic disk, then your into the last quarter of '67 through the first quarter of '68.
 

Derrick111

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Super Tremolo amps can be a bit more difficult to narrow down a build date since they tend to be built with left over part sometimes...Same goes for P.A. amps. Generally, if it's equipped with Dagnall transformers and Murata caps; 500pF tubular ceramic, 250pF ceramic disk, 47pF phase inverter ceramic disk, then your into the last quarter of '67 through the first quarter of '68.
Excellent. I don't want to steal Shakti's post, but this will help him as well... do you know how to decipher the date code on Philips mustard caps? Shakti and I could check these to narrow down our amps...
 

shakti

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I don’t mind! Always wanting to learn from the best. For simplicity I tend to think of Dagnall as 1968 and on but they may have started late 67, as so many changes in the Marshall line (new preamp for the SLs, transition to basket weave, change to Dagnalls).
 

TAZIN

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Excellent. I don't want to steal Shakti's post, but this will help him as well... do you know how to decipher the date code on Philips mustard caps? Shakti and I could check these to narrow down our amps...
The Philips/Mullard 'mustard' caps carry a three digit code (e.g. D7W). The first letter represents which quarter of the year (e.g. A, B, C, D). The second number is the year (e.g. 6 = 1966, 7 = 1967, 8 = 1968, etc.). The last letter indicates location of manufacture (e.g. W, N, S, etc.)
 

Derrick111

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The Philips/Mullard 'mustard' caps carry a three digit code (e.g. D7W). The first letter represents which quarter of the year (e.g. A, B, C, D). The second number is the year (e.g. 6 = 1966, 7 = 1967, 8 = 1968, etc.). The last letter indicates location of manufacture (e.g. W, N, S, etc.)
Thank you, sir... I have some Marshall dating to do. Shakti, tell us what you find out too.
 

Matthews Guitars

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I have no reason to suspect that your amp's rear panel is not authentic, if not original to the chassis. If it's a replacement, then it was replaced with a genuine Marshall panel and it was done early in the amp's life when the original spec panels were still being made.

One thing that gives it away is the gold color of the piece. It looks correct. Believe me, I know how hard it is to duplicate that specific shade of gold. It'd have to have come from the shop that originally did the painting or a shop that at least had been given specific information on the gold paint formula. Even modern Marshall 1959HWs don't have "correct" gold panels. Either they lost the original gold formula or choose not to use it.

That all the markings are offset visibly from where they should be centered up on the jacks and switches is absolutely par for the course for Marshall. They'd ship amps with panels that were as much as 1/4 inch away from being properly registered. Their philosophy would seem to be, "if it's close enough that people understand it, it's close enough."
 

shakti

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Speaking of panels…I recently noticed that the front panel on my ‘68 50watter looks different to other Plexi JMP panels. The gold colour is slightly different, and the font is slightly different. It’s subtle, but the giveaway is the ‘M’ where the sides are slanting inwards like on the metal panel era amps.

I thought it was a replacement, but then I saw several amps at Amparchives in the same serial number range (mine is 119xx I think) with what looks like a similar panel. Again, TAZIN is the one I’d ask. It doesn’t matter one way or the other to me, that one is a player’s amp with replaced power transformer, but it would be cool to know.
 

TAZIN

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Speaking of panels…I recently noticed that the front panel on my ‘68 50watter looks different to other Plexi JMP panels. The gold colour is slightly different, and the font is slightly different. It’s subtle, but the giveaway is the ‘M’ where the sides are slanting inwards like on the metal panel era amps.

I thought it was a replacement, but then I saw several amps at Amparchives in the same serial number range (mine is 119xx I think) with what looks like a similar panel. Again, TAZIN is the one I’d ask. It doesn’t matter one way or the other to me, that one is a player’s amp with replaced power transformer, but it would be cool to know.
Marshall did use a front panel that had the Futura font on some of the 50 watt amps in the 11800 - 11999 range.

EDIT: You also see the use of the Futura font front panel on some Super Leads & Super Bass amps in the 122xx range and 50 watt amps in the 11140 - 11200 range.
 
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shakti

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I’d buy one too. Tazin’s depth of knowledge never ceases to amaze me.
 


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