Help me understand differences between 50 watt and 100 watt plexi

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Sep 1, 2019.

  1. itsGiusto

    itsGiusto Tele-Meister

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    Hey, just researching, wondering if those in the know could help me understand what the differences are, and how they affect each's sound.

    Here are some differences I noticed when looking at the schematics below:

    1. It looks like the 100 watt has a 100uf cap between ground and the center tap of the power transformer. Why would that be there? What purpose does it serve?

    2. There's a more complex RC network leading from the tube screens to ground.
    Of note, not all schematics I've looked at even connect the screens to ground at all (like this one: http://byocelectronics.com/brit50schematic.pdf). What's the purpose of doing this, and the variations? [edit: as Tom points out below, I was incorrect here. This BYOC schematic still does have the cap to ground, it's just shown differently in the schematic, which caused me to overlook it]

    3. Another variation I've seen (between various schematics, not just between 50w and 100w) is that sometimes the transformer secondary leads are connected to the center tap via .22uf caps, pre-rectifier. What purpose does this have?

    50 watt:
    https://drtube.com/schematics/marshall/1987u.gif
    100 watt:
    https://drtube.com/schematics/marshall/1992u.gif
     
    Last edited: Sep 1, 2019
  2. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    1. I've got no idea. I hope to learn from the other answers.

    2. If I understand you correctly, it is not that different.
    - The 50W has just a 50uF capacitor
    - The 50W pdf version does have 'a RC network'. A 32uF capacitor is located a bit below (near the B 480VDC)
    - The 100W version has two 100uF capacitors in series with two resistors in series in parallel and also connected in the middle. Two 100uF capacitors in series are equivalent to a single 50uF capacitor. They double their voltage rating though. The resistors make sure the voltage is evenly shared between the capactors and implement a bleed resistor. (see also: http://www.valvewizard.co.uk/smoothing.html)
    3. I would guess some sort of line filtering. But I'm not sure. [edit: They are a snubbing/slugging capacitor.]
     
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  3. Chris4189

    Chris4189 TDPRI Member

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    The 50 watter will have more mids, breakup quicker and have less head room then a 100. That it’s.
     
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  4. sds1

    sds1 Tele-Afflicted

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    I think what you have there is 2x100uF in series for the first res cap for effective capacitance of 50uF. Then, rather than referenced to chassis, the HT center tap is referencing the voltage between those 2x100uF's. It must be just a convenience thing I dunno, I can't see how that reference voltage can be anything other than 0VDC. Nothing else is referenced there. Maybe there is a noise suppression benefit to it.

    Agree with Tom, this should be for sinking noise to ground.

    Noise filters are the kind of revisions you see in later schematics, a result of the amp being in production and lessons learned. It adds up I think.
     
  5. sds1

    sds1 Tele-Afflicted

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    Now thinking there's more to it than that, the 100uF's in series would need balancing resistors in series to create an effective capacitance of 50uF, as in Fender power supplies. Something more to this configuration... Look forward to finding out what it is.
     
  6. Phrygian77

    Phrygian77 Tele-Afflicted

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    A 1959 has a bipolar rectifier, while the 1987 has a two phase rectifier. If were talking about >1970s models with 50+50uF can caps. The 1987 has one 50+50uF can in parallel for the first filter, so 100uF. The 1959 has two 50+50uF cans, 100uF, for each two phase rectifier in the bipolar supply. People mistakenly think these are in series, and that they equal 50uF, claming that a 1959 has less filtering.
     
    Last edited: Sep 1, 2019
  7. dan40

    dan40 Tele-Afflicted

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    As Phrygian pointed out, the 100 watters used a voltage doubler rectifier arrangement. You will notice that the PT secondaries are only rated at 175-0-175vac whereas the 50 watt PT is almost double that. This rectifier arrangement is the reason that the bias circuit runs from a dedicated tap in the 100 watters.

    Many of the older 50 watters used the .22uf caps as smoothing caps in the rectifier circuit. These seem to only show up for a few years IIRC. This sight has a wealth of info on old Marshalls as well as some great transformers...http://www.marstran.com/Historic Data Base.htm

    http://www.marstran.com/Marshall Pictorial Page.htm
     
  8. stantheman

    stantheman Doctor of Teleocity

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    WHAT???!!! That's a 50 Watt Plexi.

    WHAT!!!!???????????????? That's a 100 Watt Plexi.

    W-H-A-T!!!!!!!!!!!!!???????????????? That's a Marshall Major.
     
  9. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    It seems worth noting that neither of the linked schematics are Plexi Marshalls.
    There were at least four different Plexi 100 circuits within the same model designation, multiplied by four model designations for sixteen Plexi 100 circuits.
    Safe enough to call it four and forget the Bass/ PA/ tremolo versions.
    So the plexi 100 changed each year and can be seen as the most primitive '65 and fairly similar '66, then the new and different '67, changed again for the '68. After '68 it is supposed to be basically the same for '69- '72, yet they do not sound the same.
    Those schematics are dated 1970, and I've had several 1970 model year Marshalls in both 50 and 100w.
    That particular year compared to a '66 or '67 Marshall sounded more like a Fender Super Twin with massive clean headroom and almost no natural breakup. The several '69 model year 100s I've had sounded exactly like the '70 model year.
    By '71 they seemed to get more distorted again.
    I've had at least five from '69 and '70 and those are the cleanest loudest Marshalls IME.

    If you want to understand the Plexi Marshall you need to NOT study the 1970 schematics.
     
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  10. Phrygian77

    Phrygian77 Tele-Afflicted

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    Ok, so I'm wrong, Bruce Egnater says...

    "Bipolar means you would have a zero reference in the center and positive and negative supply voltages. The “stacked” capacitors are used in series to increase the voltage rating. If you place two 100u/350V caps in series, you essentially have the equivalent of a single 50u cap with a 700V rating. More common is to see a resistor in parallel with each cap to keep the voltages equal. Alternately as in this Marshall the center tap does the job of the two resistors. FYI, this arrangement is often mistaken for a voltage doubler, which it is not."
     
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  11. Phrygian77

    Phrygian77 Tele-Afflicted

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    This is absolutely correct concerning the 'Plexi' era circuits, but people seem to use that therm even for 70s era Marshalls, which at that point, the circuits had pretty much become standardized. The later lead circuits had more filtering and less NFB than the Plexi era amps.
     
  12. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    True about pop false info.
    Annoys the hell out of me and is far more meaningful than misplaced apostrophe's!
     
  13. itsGiusto

    itsGiusto Tele-Meister

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    Hmm, this is interesting me now because I planned, in an entirely separate amp's (my Princeton Reverb) bias circuit, to put two 50uf/50v caps in series to function as a 25uf/100v cap. Do you actually need to put resistors in parallel with them for this to work?
     
  14. Phrygian77

    Phrygian77 Tele-Afflicted

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    I doubt you would have a problem with the 50VDC rating being exceeded on one of the caps.

    https://www.illinoiscapacitor.com/pdf/Papers/voltage_balancing_resistors.pdf
     
  15. dan40

    dan40 Tele-Afflicted

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  16. Chunkocaster

    Chunkocaster Poster Extraordinaire

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    I remember seeing some videos of the difference in components/ schematics between various plexi's on this channel.


     
    Last edited: Sep 2, 2019
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  17. 39martind18

    39martind18 Tele-Holic

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    Just a wild guess here: 50 watts?:lol:
     
  18. 24 track

    24 track Doctor of Teleocity Silver Supporter

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    the traynor YB1 amp is actually a small handfull of components away from being turned in to a plexi

    Scanb.jpeg
     
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  19. tubedood

    tubedood Tele-Meister

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    Man Chunko that video was very informative.

    I had forgotten how much I liked the ol' Plexi's till you reminded me!
     
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  20. Chunkocaster

    Chunkocaster Poster Extraordinaire

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    His channel is great, always diming old Marshalls and giving everyone gas for one.:)
    I've got a Ceriatone 68 jmp 50w lead clone that's my favorite amp.
    Although not Marshall Ceriatone give a very basic description of their idea of the differences between 67,68,69 etc plexi amps on their website with 5 versions of each of the 50w and 100w plexi lead and many others. There is a layout for each listed under documentation.

    http://www.ceriatone.com/british-style-plexi50-lead/

    http://www.ceriatone.com/british-style-plexi100-super-lead/
     
    Last edited: Sep 3, 2019
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