1968 Plexi component values?

[psychepool]

Hello.
I have always considered the Marshall sound to be my absolute favorite, yet I realized that despite building various amplifiers over the years, I had never built a Super Lead Plexi.
So, I decided to modify an existing 6V6 Blackface DIY amp to create a 6V6 Plexi.
I incorporated 5F6A/JTM45 modes into the preamp using various switches, but for the SLP mode, I based the circuit on the commonly found reissue schematics or Mark Huss's Plexi 6V6 design.

While researching the build, I discovered that the '68 Plexi specifications are considered the "Holy Grail" of tone.
I tried to find details on the 1968 specifications, but I couldn't find any reliable information.
I understand that, unlike today, manufacturing wasn't strictly standardized back then, but I would still like to know the typical circuit configuration that defines a '68 Plexi.

Could you provide information on the component values ​​typically used during that year for the following items?

- V1 cathode configuration (split vs. shared)
- Bright channel coupling capacitor value
- Bright channel bright capacitor value
- Presence of V2 cathode bypass
- NFB (Negative Feedback) value

Alternatively, if there is a well-known schematic for the '68 Plexi, please let me know.

Also, if possible, I would greatly appreciate it if you could provide the values ​​for the 1967 specifications as well.

[martin manning]

Look here: https://www.drtube.com/marshall-jmp/

See JMP 1959 Super Lead schematics.

[Roe]

- V1 cathode configuration (split vs. shared)
- Bright channel coupling capacitor value
- Bright channel bright capacitor value
- Presence of V2 cathode bypass
- NFB (Negative Feedback) value

split 820r/330uf - 820r/.68uf
2n2
5n
?
47k

a 1967 by contrast would have
shared 820r/330uf
22n
100pf or 5n
?
27k

[Fischerman]

- V1 cathode configuration (split vs. shared)
- Bright channel coupling capacitor value
- Bright channel bright capacitor value
- Presence of V2 cathode bypass
- NFB (Negative Feedback) value

split 820r/330uf - 820r/.68uf
2n2
5n
?
47k

a 1967 by contrast would have
shared 820r/330uf
22n
100pf or 5n
?
27k

What was the timeline of the treble peaker and the treble cap? It was 560pF early on I think. Some were Mica some were ceramic dogbones and ???

[martin manning]

The Super Lead schematic at the link above marked early, ca. 1967 doesn’t have any treble peaker caps, while the Unicord schematic dated 1970 has one of the channels shown with a 500pF and a bright cap on the volume.

[pdf64]

If only someone had already written this up
www.amparchives.com/2019/10/12/the-evol ... uperleads/

[Fischerman]

The Super Lead schematic at the link above marked early, ca. 1967 doesn’t have any treble peaker caps, while the Unicord schematic dated 1970 has one of the channels shown with a 500pF and a bright cap on the volume.

Yep and what was actually in the amps was another thing sometimes. The earlier JTM 45/100 had 270k resistors and a 560pF cap but they had the Fender Bassman/Twin tonestack with a 250pF treble cap. At some point the resistors became 470k and the cap changed. By '69 or so it was 470k resistors and 500pF caps in both places. But I'm not sure about those transition years.

[Adrien]

I went down this rabbit hole too, not long ago.

The article from Roe is excellent, also see the massive amount of historical data here: https://marstran.com/Historic%20Data%20Base.htm I am not sure why, but the database only contains 50W models. Still, it should give you a pretty good idea of typical values.

The Marstran site also has schematics for a bunch of Plexi variants here: https://marstran.com/SCHEMATICS.html

[psychepool]

Thank you. Looking at the materials you shared, I realize that the component values ​​I had in mind for the SLP circuit were exactly those of the '68 model.
Since I built it referencing the Mark Huss Plexi 6V6 and the 1959/1987x reissue schematics, it seems I effectively built it to '68 specifications from the start.
I had experimented with changing a few component values ​​after looking at various versions of the circuit found online, but I ultimately ended up reverting to the '68 or reissue specifications.

Below is the schematic for the amp I am currently building. I am repurposing a single-channel Deluxe Reverb clone that I built about 15 years ago.
I’ve incorporated switches to toggle between Bassman, JTM, and SLP modes.

KakaoTalk_20261007_155553151.jpg

After checking the links, I have a clear understanding of the overall evolution of the circuit. Thank you for sharing such valuable information.

[Fischerman]

I went down this rabbit hole too, not long ago.

The article from Roe is excellent, also see the massive amount of historical data here: https://marstran.com/Historic%20Data%20Base.htm I am not sure why, but the database only contains 50W models. Still, it should give you a pretty good idea of typical values.

The Marstran site also has schematics for a bunch of Plexi variants here: https://marstran.com/SCHEMATICS.html

Looking at S/A 10512 and 10513; which appears to be early-mid '68, those both 50W have the 33k/560pF stack and 560pF treble peaker and also a 0.68uF cathode bypass on V2 and also a 5n bright cap. I only bring it up because of the notion that "the '68s in particular sound best". If so it could be due to a unique combination of values that the later ones often didn't have. And since the 560pF was getting phased out right around this time, maybe that is part of the magic '68s.

[Roe]

I went down this rabbit hole too, not long ago.

The article from Roe is excellent, also see the massive amount of historical data here: https://marstran.com/Historic%20Data%20Base.htm I am not sure why, but the database only contains 50W models. Still, it should give you a pretty good idea of typical values.

The Marstran site also has schematics for a bunch of Plexi variants here: https://marstran.com/SCHEMATICS.html

Looking at S/A 10512 and 10513; which appears to be early-mid '68, those both 50W have the 33k/560pF stack and 560pF treble peaker and also a 0.68uF cathode bypass on V2 and also a 5n bright cap. I only bring it up because of the notion that "the '68s in particular sound best". If so it could be due to a unique combination of values that the later ones often didn't have. And since the 560pF was getting phased out right around this time, maybe that is part of the magic '68s.

whether 560pf sounds better than 470pf really depends on the overall balance. Sometimes 470pf is better either in the tonestack or in the channel mixer circuit

[Fischerman]

I went down this rabbit hole too, not long ago.

The article from Roe is excellent, also see the massive amount of historical data here: https://marstran.com/Historic%20Data%20Base.htm I am not sure why, but the database only contains 50W models. Still, it should give you a pretty good idea of typical values.

The Marstran site also has schematics for a bunch of Plexi variants here: https://marstran.com/SCHEMATICS.html

Looking at S/A 10512 and 10513; which appears to be early-mid '68, those both 50W have the 33k/560pF stack and 560pF treble peaker and also a 0.68uF cathode bypass on V2 and also a 5n bright cap. I only bring it up because of the notion that "the '68s in particular sound best". If so it could be due to a unique combination of values that the later ones often didn't have. And since the 560pF was getting phased out right around this time, maybe that is part of the magic '68s.

whether 560pf sounds better than 470pf really depends on the overall balance. Sometimes 470pf is better either in the tonestack or in the channel mixer circuit

I had always associated the 500pF cap with the Lead tonestack and the 560pF with the mixer on earlier 56k/250pF amps. So I was surprised to see those two amps with 560pF in both places. Plus the .68u on V2 and 5n bright cap...in a 50W. That's a lot of mids!

[Adrien]

Yeah those two amps in particular are interesting. Yet another odd thing about them is the split cathodes on V1 - the cathode resistors are both 820R instead of the usual 820R normal / 2.7K bright. If I am not mistaken, this would provide more gain at low frequencies than the usual 2.7k on the bright channel.

[Roe]

anyway, there are no magic values. it is about finding the right mix overall - for a particular player.

I like my amps brighter and more unforgiving than many other do (and my hearing is still good)

[Adrien]

Yes I absolutely agree - my interest in this is to find all the different variations and try stuff to hear for myself what difference they make and what works best for me. Still a work in progress, and it depends on the day, guitar, etc etc....