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Using attenuator to play 50w amp through 20w speaker?

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Aug 9, 2018.

  1. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    I agree with your reasoning and what you’re trying to do. To your original question...
    To see how much power your speaker is getting, so you can set the attenuator correctly, just put an ac voltmeter across the speaker terminals. For a 16 ohm speaker, 25w would be 20vac. Play some power chords and slowly turn the levels up.

    I have a 50w plexi and a cab with two greenbacks in it, and a Weber mass (the Mass is new, haven’t really had time to test it yet). I have a good master vol. on the plexi which lets me play at reasonable volumes. Check out Lar-Mar PPIMV mod for your plexi, they are wonderful, and pretty easy to do. They sound amazingly good even at lower levels.

    And imo, running an output tube at higher output levels doesn’t shorten the lifespan significantly... the idle bias is typically set at 70 to 80% of max tube dissipation.
     
    Last edited: Aug 10, 2018 at 4:24 AM
    itsGiusto likes this.

  2. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    A bit of an exaggeration... :p
    They were no more so than most other amps in production really. The schematic didn’t change all that much afaik, but I’m no plexi historian. They did play a bit with cap values, especially the bright cap, which is the first thing I changed on mine (a 1987x 50w plexi), lowering it quite a bit.

    Mine sounds just like what I’d expected... the low throaty growl of the normal channel blended with the bite of the high treble channel. I really love amps that allow you to blend the channels with a patch cable.
     

  3. itsGiusto

    itsGiusto TDPRI Member

    Age:
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    Dec 31, 2017
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    I am interested in the concept of this. Is there a schematic you could point me to?
     

  4. Zeonoid

    Zeonoid Tele-Meister

    Age:
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    Slovakia, EU
    I think Celestion Vintage 30 would be a nice accompanion to Greenback 25, thats what I often find in 4x12 vintage boxes paired with Plexi heads
     

  5. Larry F

    Larry F Doctor of Teleocity Vendor Member

    Nov 5, 2006
    Iowa City, IA
    I sympathize with you. I don't like it myself when I ask a question and get told to take an entirely different approach. I try to write my posts so that stuff is covered, but no cigar (or few cigars). I think attenuators are useful tools, used with half a brain. It sounds like you have more experience than some people are giving you credit for.
     
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  6. I solved the problem you are having by picking up a Fargen Mini-Plex. :D
     

  7. telemnemonics

    telemnemonics Doctor of Teleocity

    Age:
    58
    Mar 2, 2010
    Maine
    Well this subject is too broad for me to address clearly without doing some research to clarify my memory, but suffice to say I've owned a dozen vintage 50 and 100w Marshalls with the same basic circuits in question and they are IME far less consistent sounding than all the BF Fender amps I've owned.

    At this point though there has been more standardization of those old circuits, largely because Marshall and a couple of builders came up with their versions of the "Plexi circuit".
    The 1987x was a conglomeration of several amps designs, not one actual amp, so it's a reissue of an amp that never before existed. When it came out, players with vintage Marshalls felt it sounded like a metal panel.

    Those two Ceriatone layouts (JMP '68 Plexi 50 and circa '66 JTM 50) are almost the same amp except for the rectifier, with the same power section filtering. The different PT would hopefully play a big role in the difference. I'd think originals were more different.

    I'm more familiar with the 100's than the 50's, but IME between '65 and late '67 Marshall kept tightening up the power supply to get more clean punch out of the squishy power sections, which involved adding more and more filtering.
    I guess the 50 didn't get as much tightening up over this period though, because it was the small amp, and if you wanted tight punch you bought a 100.
    I prefer the 100 sound so am maybe not the best judge of the 50 sounds.

    One thing that they seemed to keep changing was the voltage from the PT.
    Even between the same transformer supplier in the same model amp, the voltage went higher and lower by a good margin, as Marshall tried to satisfy players, reliability, and just tried to dial in the circuit.
    The voltage increase did not run parallel with the filtering increase, and went down after going up. Almost seemed like mistakes were made ordering stock.

    My favorite vintage Marshall circuits have inadequate filtering which brings hum and ghosting, but with that we get the squishy liquidy sustain that many of us love.
    In '69- '70, the power section was so tight with filtering and high voltage that both the 50 and 100 stayed clean all the way up, if no boost was used.
    By '71- '72 they fixed that and the amps distorted more and sooner, but I'm not sure if it was lower power section voltage or more preamp gain, likely a combo of the two.
    In contrast, a '73 50w I has was distorted at 2, while a '69 50w was clean at 10.

    It's funny- I was trying to figure out if I could get a '67 sound out of a '69 Super Bass 100 (maybe 20 years ago) and called up Mercury Magnetics to see if they had a '67 spec OT, because the 100 OT changed each year from '66- '68.
    They had to run me past several reps and a tech but could not tell me what year plexi the "Plexi 100" OT was blueprinted after, and they only offered a single OT design where Marshall clearly used several in that period.
    So manufacturers have over the past 20 years standardized "The Plexi" into a salable product. Just as Marshall came up with a conglomerate circuit for the 1987x "Plexi RI" that was more metal panel than Plexi, they did the same for the SLP 100 RI, creating a whole new circuit that they felt would satisfy the largest number of buyers, while being reliable and not having the pesky hum of the vintage circuits with inadequate filtering and that liquid sound and response.

    Now most players who never owned a pre '68 Marshall simply accept that what Marshall created later is "the Plexi Marshall", which of course it is because you can buy one and it's called a Plexi.

    After I got nuthin from Mercury Magnetics, I called up Ken Fischer who actually answered and talked to me for quite a while, which was a heluva surprise, given that he is roughly equivalent to Jim Marshall and Leo Fender in the gear world.
    Never really expected to be granted an audience.
    He detailed a long list of changes that he felt would make my '69 Super Bass 100 (same circuit as the '68 Plexi) sound like a '67 100.
    Off the top of his head he went on for 5-10 minutes detailing the changes, which I could not write down or keep track of as fast as he spoke, and I was kind of too stunned to tell him i wasn't getting it all.

    WRT building a 50w capable of sounding like both a '66 and a '68, I'd say the design will be based on a techs choice of which aspects of those amps sounds he wants to prioritize as representative of those circuits, because there is not a single or even two switches that can make a 50 sound like both those amps.

    A switchable tube rectifier would be a good start, as dropping maybe 50v in the power supply will up the sag considerably.

    Funny though, the tube rectified JTM50 I had was a Park version from probably '65, based on it having the earlier hand bent aluminum chassis, and that amp was another example of a deviant Marshall that didn't sound like "a Plexi", instead being clean and loud as hell, partly due to over 500v on the plates, even with the tube rectifier!

    At this point the guitar community has redefined the Plexi Marshall and that's what we've got, so players are free to build amps that do stuff they like without actually building any amp that ever existed before the aftermarket created products to rep the mythology.
    Many of the original circuits benefit from the improvements.

    I bought a RI SLP100 and played a couple of 1987x.
    Returned the SLP100 after a few weeks of disappointment...
     
    Last edited: Aug 10, 2018 at 9:15 PM
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  8. telemnemonics

    telemnemonics Doctor of Teleocity

    Age:
    58
    Mar 2, 2010
    Maine
    @Dacious covered the Gold pretty well, sounds clearer to me and has less congested mids, which I think of as nasal. It's as close as Celestion could get to a Blue but with more power handling, a really great speaker that can be dialed in for many sounds, where the 20 and 25 GB are more stuck in their sort of muddy bottom thick crunch sound.

    Thick mids or congested mids are very much a part of what we associate with Plexi tone, because all the Plexi era amps were run through greenback (or the gray painted Celestion Blue) speakers with the classic thick mids and mushy crunchy bass response, though the G12H30 had tighter bass and a little more open tone, less nasal.
    It's been a while since i used a G12H30 75hz though, I only have the 55hz Heritage version right now, though I have an original G12H30 that's been reconed and sounds wrong, so not representative of that speaker.
    I recently picked up an original Jensen P12N alnico and have been running it with a Gold.
    I'm really liking the more open midrange with this combo.
    Scooped mid American style speakers are not favorites, but this Jensen is not as scooped and has the sweet alnico thing.
    Really, a lot of Plexi sounds were made with the Celestion Blue as shipped to Marshall through circa 1965 or so, and some even into '66 AFAIK.
    Some players kept using the older cabs or speakers with newer Plexi amps.
    Those alnicos sound more open when played clean and then get thicker as the amp distorts, where the early ceramics were pretty much thick and congested even when played clean- sort of...

    Not sure about the AC15 as I never bonded with Vox amps.

    I'll say that I really don't like the original version of the 18w marshall as built in the 1974x, because the fixed eq with a single tone knob lacks the range to dial in the sounds i can get from my two TMB versions.
    Even the TMB though does not sound like the 50 and 100 circuits.
    I attribute that more to the preamp circuit though, not really to the el84 power tubes.

    WRT building a 20w Plexi with el34 power tubes, AFAIK the Origin gets distortion from the preamp, and running a pair of el34 to an output of 20w will not bring the el34 distorted sound, because the power section isn't likely to distort.
    Not sure about that circuit though, and designers get very good sounds now creating distortion in the preamp and PI.
     
    Last edited: Aug 10, 2018 at 12:18 PM

  9. itsGiusto

    itsGiusto TDPRI Member

    Age:
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    88
    Dec 31, 2017
    02141
    Thanks for the info. I don't know this level of detail about Marshall history and changes, just a sense of the sound I want. I'm generally more interested in the "squishy liquidy sustain", the kind of sound that sounds more distorted than it actually is. I'm far less interested in metal-panel chainsaw kinda distorted sound. If you have any advice on how I should achieve this sound, please let me know.

    I was planning on starting out as a basis with the BYOC kit (schematic here) and make it switchable to bass-spec, and possibly black flag JTM50 spec in the future. Do you think this is a good place to start for achieving the sound I want? Partly, I trust BYOC's choice of components and especially transformers more than Ceriatone's, which is why I'm going with their kit. Partly also, the shipping costs for Ceriatone is ridiculous.
     

  10. telemnemonics

    telemnemonics Doctor of Teleocity

    Age:
    58
    Mar 2, 2010
    Maine
    Well I didn't know BYOC sold kits but I'd say that if you're looking for the earlier sound you might want to start with a JTM45 kit and add mods for the later SS rectified circuits.

    The 1968 Plexi circuit is actually the same as the metal panel circuit.
    So if you don't really want the metal panel sound you might aim for the earlier amps.
    It seems like players want the more aggressive snarling as yousay, chainsaw sound of the later amps, but players also like the name "Plexi".
    You actually wanting the older plexi sound might put you in the monority!

    Starting with a chassis punched for three octal sockets, whether it be called "Black Flag" or JTM45 will at least not require punching or drilling later to add the GZ34.

    I'd guess that the JTM50 PT would be different from the JTM45 PT, but there was really only one basic tube rectified JTM50, where there were numerous changes during the JTM45 era.

    Could be you need to go with a scratch build, I'm just not familiar with all the kit offerings. As long as a JTM50 PT fits the opening in a JTM45 chassis, that should work.
    OTOH you might be happier with a JTM45 PT for lower voltage in the power section, which tends to make it more squishy/ liquidy, and also starts you off with less wattage to have to attenuate.
    A JTM45 was in many cases a 35w amp, and I gotta say, the Park JTM50 I had was almost as loud as a '69 Super Bass 100 I had at the same time!
     
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  11. telemnemonics

    telemnemonics Doctor of Teleocity

    Age:
    58
    Mar 2, 2010
    Maine
    I also had a '69 metal panel 50w small box with the laydown PT, same basic circuit as the '68 Plexi, and that was a brutally loud amp that stayed clean most of the way up (with Fender single coils).
     

  12. itsGiusto

    itsGiusto TDPRI Member

    Age:
    30
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    Dec 31, 2017
    02141
    While I'm interested in doing a scratch-build, I've been told the economy you get from starting with a kit, and the hassle of trying to source everything you need if you build from scratch, makes starting with a kit much better. I'll definitely look into whether I could use the BYOC JTM45, but change it to JTM50 specs easily.
     
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