Making an 18 watt sound more like a Marshall, less like a Vox?

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Aug 17, 2019.

  1. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    Calling the 18w a classic correlates to many players loving it and buying it.

    Same as the 5e3 Deluxe is a classic despite not sounding like a 5f6a Bassman.
    These amps are not supposed to sound the same, they are supposed to be different.

    Again, if you want the sound of a 50w Plexi but smaller and less loud, I'd strongly suggest building the same circuit as a 50w Plexi but subbing 6v6 power tubes and the suitable transformers for that power section.

    The reason I'd suggest going with the 6v6 is that there are tons of JTM45/ JMP50 chassis' for sale with the octal tube socket holes in the right places. The JTM45 chassis has the extra tube rec hole so you can either use that, put a filter cap there, or cover it.

    Of course if half your goal is to come up with a newer circuit of your own design then forget my suggestions to go with the path more traveled. I just want to play through amps more than tweak them.

    We've had 6v6 Plexi build threads here and also 6v6 JCM800 build threads.
    Seems to be the shortest path to a mini plexi or a mini 800.

    The 18w variants are a path to a different destination.
     
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  2. bftfender

    bftfender Friend of Leo's

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    What guitar & pickups are you hitting it with ?
     
  3. Asmith

    Asmith Friend of Leo's

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    You can simulate a valve rectifier with a resistor to introduce Sag but not the other way around. A choke stores energy, as its essentially an electromagnet. It passes dc current though and it generates a magnetic field, when the voltage tries to dip (due to a demand for more current) the choke provides the extra omph in order to reduce sag. You can try and replicate it with bigger filter caps but a PT can only take so much and isnt something I would advise.

    Its not really fair to say a lack of something is a design element, they both have no reverb (or other modulation) on the channels in question, they both lack a cathode follower, etc.

    As you fairly said they are both cathode-biased EL84 output stage amps with the architecture of single preamp stage into PI into power stage but that pretty much makes it a barebones amp. Its the minimum parts for a preamp plus the V and T controls, minimum parts for a phase inverter with enough gain to appropriately drive the output stage which is as @David Barnett put

    Calculations for components in a cathode bias output are fairly simple and if you want 18 watts out of one then theres only one correct way to do it.

    Anyway because its such a simple circuit, the low value caps are the only thing that are going to be doing anything significant (cutting bass) other than the tone control which you can tweak how you like. So all your really getting is the pickup tone minus some bass, the distortion characteristic or the hot biased output and the speaker.

    I never really said it was a marshall plexi (very much the opposite), it is a different beast but an 18 watter is as much a marshall as a bassman (or various champs and tweeds) is a fender. There are other tones and circuits that people may think of when you say fender amps (twins) but its still a classic fender sound.

    Well you may aswell sack off the entire tremolo channel and just clone a plexi preamp and PI with the 3 preamp valves. Modify the output so it has a presence control. If you want as close to a plexi as possible with reusing the major components thats what Id advise.
     
  4. Asmith

    Asmith Friend of Leo's

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    I just wanted to go more indepth with my reply to this one but my last reply already felt like an essay.

    My thinking was you wanted to keep the trem so that would mean youd have to do with 1 preamp valve (2 triodes) for the normal channel preamp. The cascasing preamp is essential for the drive character of a plexi, so if something had to go it would be the cathode follower. The cathode follower basically reduces signal loss in a tone stack and the distortion that would be caused by the loading effect of the tone stack on a valve plate. It does its job but you could get close to a marshall plexi sound without it. So thats why an earlier reply to the question of getting the amp to sound more like a plexi was,

    If you want the cathode follower more than the trem LFO then whats the point in the trem channel? Itd be wiser to just remove the channel all together and use the other triode as your second input on the plexi channel which basically makes my advise build a plexi preamp. Thats why I said its rebuild, its not really going to be tweak some component values, add a tone stack and a presence.

    Youre going to have to gut both channels, completely rebuild a new preamp with a tone stack. Rebuild the phase inverter because again all the component values are different. The plexi PI looks like its attempting to balance its outputs and the presence control feeds back into this part of the amp too so it'd at least modifiying to accept that and have an effect to the correct degree.

    You could leave the power amp as is but since everything else is being rewired you may as well convert it to fixed bias.

    So my advise is to build a plexi clone with el84 output valves. The good news is that at least you can still reuse all of the expensive components.
     
  5. itsGiusto

    itsGiusto Tele-Meister

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    Yeah, I'm still unsure what I want to do. I'll probably build a new amp rather than mod the 18 watt, but I'm unsure now about two things:

    1) should I make it more like a jtm45 or a later 50-watt amp, like a 1972 plexi. I might just make it a jtm45 but include a switchable diode rectifier, along with a separate switchable bias pot, so I can bias the tubes correctly in each configuration. (I know there are more differences between the 45 and later plexis than just the tube/diode rectifier, like coupling cap size, filter cap size, etc, and I'll have to decide between each of them or make each of them switchable)
    2) should I actually build a 6v6-plexi? Is that worth it just to drop 3db? Or should I just build the classic circuit, which would be easier to find a kit for, and then I'll just use an attenuator to drop as many decibels as I want?
     
  6. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    OK I wasn't getting that you have an 18w and want to mod it, seemed like you had played an 18w and wanted to build an 18w based amp that sounded like a 50/100 "Plexi".

    As far as building a smaller 6v6 Plexi JMP or JTM 50 circuit or just going for the full size stock amp, it's not just for the few dbs less of volume that we build smaller amps these days. There is the way they distort at those lower volumes, plus the fact that an amp put through an attenuator is not the same sound as an amp running just speakers.
    Then there is the amount of gear you have to carry if you choose an amp twice as big as you need and then bring an attenuator to make it smaller.

    If you don't mind hauling bigger gear I'd say build the full size JMP 50, and choose carefully which one!

    Note that there was no "1972 Plexi".
    The last Plexi was built in 1968, and the 1969 and later JMP era were metal panel amps.
    Then in the later '70s we got the Master Model MKII which was the beginning of the 800 series.
    The '66- '68 Plexi would be a little softer and more creamy sounding, where the '72 would be brighter, more punchy, louder, and more snarling than creamy. A metal panel can do creamy too, but it had more filtering, more treble, and more gain.
    Probably higher voltages too, but there was variation that isn't always by year.
    Actually the 1968 Plexi Marshalls were exactly the same circuit as the 1969 metal panels, so you will find schematics that lump those two eras into one amp type or model.
    The more true Plexi would be an earlier version from maybe '66 or '67.
    I preferred the 100 watters when i had vintage Marshalls and in particular the '67 was my favorite while the changes for '68 changed the sweet creamy liquid sustain into louder crunchy grind and rasp.
    Comparatively speaking.

    I guess you might like the JTM45, but I'd suggest the next gen 50w if you want a Plexi sound and are not looking for a JTM45 Bluesbreaker sound.

    @bftfender has I think each of the 45 and early 50 models, and might comment on which circuit is closest to the amp you want.
    They really are different amps, and all the pre JCM 50w often get lumped together on internet chat forums as if they were all the same amp.
     
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  7. bftfender

    bftfender Friend of Leo's

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    Jtm45 6l6 fender power
    jtm50 el34 ss/tube rec
    jmp50 1987 now 2204

    very cool topic gents:

    My take & a quick overview with a twist at end..i didnt see coming.

    Been stuck recording cause of accidents..so time on hands led to tons of recording & really focusing in.
    The reason i have these amps & most others are gone is...its the early progression to me of what over the pond did. I got them in reverse order from jmp to the 45 & each progression back in time..I heard my tone fatten up..not in the crunch way bit in that pick attack-guitar tone shine thru. At this time i was coming down from years of heavier stuff..DSL& 900 SLX player. My observatons are 3 very distimcive amps but similarities in the mids..especially with greenback(even better on 4x12)

    jmp50 quintessential rock amp...the punch the leads...gibson-fender..the norm-treb allows the fine balance. Boost TS gain off vol dimed to bring up if needed

    JTM50..this is when i started to make my shift in tone...the tube rec side..it sagged but no mid drop..now strats were coming out more..ringing out chords..gain started to come down cause i could hear & feel the nuances on guitars..flip switch to SS rec & tighten er right up,,but not into JMP type

    jtm45..6l6..fender power..shrugged my shoulders at it..finally put it on greenback cab..ok now..a new sag..a new grit...not the gain the others had ..but a fullness...then all them pedals i never needed..instant anything now...i now got the bassman thing & the fender attitude

    key thing..all these needed volume for optimum results..did add trainwreck 3 to all of em..usable from barely on to loud as needed

    Now the twist...i stumbled on a 1974 fender Super Reverb on the cheap..did tons research..tech modded normal chan to be aggressive & break a lil earlier -with jupiter sc10 10's..i have a monster & the 3 d clean ..the mids are there like am used to..the tilt of combo makes the room very manageable..got a RI with jensens & 73 Pro reverb with Weber f150....i sorta moved over mid CD this year to fender..i love both..but am seeing more of my guitars character shine thru & the mix..my wife is not fighting my Guitar...its always a quest..but those are my keeper amps...they see a lot of studio use. The clones can be made so good & are so tweak able..ok...
     
    Last edited: Aug 19, 2019
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  8. itsGiusto

    itsGiusto Tele-Meister

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    Well, I'm not going to get around using an attenuator, no matter what I do. As much as I'd like to play straight into speakers, I use an attenuator even when I play just my 18 watt or 15 watt amps. I know it's not "the same", but honestly, I think my Weber Mass attenuator is damn good, especially with the +3db treble switch kicked in. At the very least, it's no worse than a master volume!

    What exactly do you mean by this?
    "There is the way they distort at those lower volumes"
    Running it with a 6v6, it'd distort at roughly 3db less volume than an EL34, right? Meaning ultimately, I'm saving myself 3db by using 6v6s.
    Regardless, I'd like to try running with 6v6 at some point and see if it's cool. Is that the sort of thing you need an entirely separate build for, or can you just swap 6v6s in and rebias to get them to work?

    Well, I'm just really unsure which one to make, cause I haven't had that much experience playing any of them! I don't come by amps like this every day, unfortunately.

    I'm torn, and here's my thought process:
    Firstoff, I love both bluesy and rock sounds.
    The JTM45 is the sort of sound and characteristics I think I tend to like and associate with strongly with my own playing (mid 60s, tube-rectifier, bluesy, bassy, squishy). I really like Clapton and Hendrix, both of whom used JTM45s.
    However I'm also interested in getting something more like an early-70s plexi (metal-panel) because maybe I already have enough bluesy, tube-rectifier-based amps already. Also several modern players I really really like play metal-panels.
    But then again, I'm somewhat bored by the chainsaw-Zeppelin sound featured on some of Zepp's later albums. Maybe that speaks more to the mixing though?
    I'm not sure which one to get as a result.

    That said, when looking at people like this do comparisons between different types of plexis, they sometimes end up sounding much the same:


    But I figured I'd start with a JTM45 kit, cause then I have an extra slot for the tube rectifier, and I can make that switchable. Easier than starting with a plexi kit and having to drill a spot for the extra tube to go.

    Really, almost anything can be made switchable, so I don't see why not to just try to build as much as I can of both amps in one - that's what's fun about doing it DIY! I count these as the changes between the two "canonical versions" of the circuits:
    • JTM45 uses a GZ34 tube rectifier, early 70s plexi uses a diode bridge.
    • Plexi splits the cathode of the first preamp tube stage, using different bias for each triode.
    • Plexi uses EL34s instead of KT66s, and has higher value and higher wattage screen resistors (1k instead of 470r)
    • Plexi has much higher filtering, using three 50+50ufs, instead of a 32+32uf and two 16 ufs in the JTM45
    • Some difference in NFB which I haven't fully worked out yet (plexi has 100k coming from 4ohm tap, instead of 27k coming from 16ohm tap)
    • Lower PPI coupling caps in plexi, 22nf vs 100nf in the JTM45. This definitely would limit the bass and create a tighter, but less bassy sound in the plexi.
    • Plexi has different output transformer, I think higher-rated for wattage, and also with a smaller primary, something like 4k instead of 8k, I think.
    • I think the power transformer is different too, but I don't know how exactly.
    • Some plexis also seem to have V2's cathode bypassed with a cap, probably to make it higher gain and brighter.
    I mean, I doubt I'm going to go including all the different caps and make the actual filtering sections switchable, but I could if I wanted to!
    And I've definitely made coupling caps and NFB switchable in the past on previous builds, so why not here?
     
    Last edited: Aug 19, 2019
  9. bftfender

    bftfender Friend of Leo's

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    simple answer jtm50 black flag spec'd el 34 with tube rec for spongy & SS for the tighter..its an aggressive amp but does have some jtm 45 tendencies..it literally is the in between progression towards the plexi..it is awesome with gibson & fender..blues from the door..hard rock..can go jtm thick or very tight...
     
  10. Asmith

    Asmith Friend of Leo's

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    Id just pick a favourite, maybe have a have a switch for the tube rectifier vs SS and that will give you most of the stiffness you'd want and would probably provide the biggest change in tone.

    Also if an 18 watter can sound like a vox ac15, with its different power supply, tube bias, volume tone control, consistently smaller caps and the other details previously discussed then maybe all the changes arent a very big deal. When the amp is in the room at least, especially when youll have a full tone stack to compensate for the some of the EQ differences or making your tone sound tighter.

    And a lot of the changes have the potential to be difficult while others being impossible with a standard chassis.
     
  11. itsGiusto

    itsGiusto Tele-Meister

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    Which changes are difficult? It'd be good for me to plan upfront, if that involves getting a different chassis or something.
     
  12. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    I gotta make dinner and get some sleep, lots to cover here.
    I'd strongly consider @bftfender suggestion for a "Black Flag JTM50 to cover the most useful range of Marshall sounds in your stated area.

    A couple of comments:

    As far as the theoretical 3db difference between an 18w and a 50w, that theory really doesn't cover the playing experience.
    For one thing a cathode biased 18w has less headroom so the clean volume is more than 3db different.
    Too much other range though to try to describe in any way that will help you make your choices.
    Noting of course that as Johan shows, they (JTM45/ JTM50/ JMP50 Plexi/ JMP50 metal panel) are not drastically different from one another on recordings.
    They feel more different though when playing them, and in a band.
    A JMP50 metal panel is called a 50w amp yet is sometimes louder than an 85w Fender Twin Reverb becuas the wattage is rated at cl;ipping, meaning maximum clean volume, but gets a lot louder than 50w above clipping.

    AS far as Clapton and Hendrix using the JTM45, Clapton only on maybe one of two early albums before choosing bigger and louder Marshalls, and Hendrix I never saw reference to JTM45 but the first 100w amps were called JTM45 100.
    So don't build a JTM45 for Clapton and Hendrix sounds.

    Then for the Jimmy Page "chainsaw sound" you don't really want, that was probably a Supro, not a Marshall, or a Tone Bender fuzz into a Marshall.

    Certainly worth making some switchable functions, but some will be hard to hear and too many will end up creating extra issues that slow the process of getting to actually play music.

    As afr as 6v6 or el34, I have tons of 15-20w amps and one 50w Marshall.
    I seldom play the 50w yet I really want a JMP50.
    Go figure?
     
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  13. Asmith

    Asmith Friend of Leo's

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    • JTM45 uses a GZ34 tube rectifier, early 70s plexi uses a diode bridge.
    Relatively simple since its a switch and 2 diodes.
    • Plexi splits the cathode of the first preamp tube stage, using different bias for each triode.
    You could do this but if its just changing the gain you could do the same with the gain knob if you just go with the highest gain bias.
    • Plexi uses EL34s instead of KT66s, and has higher value and higher wattage screen resistors (1k instead of 470r)
    Youd have to physically pull the valves and rebias the amp so nothing you couldnt already do with any standard plexi but impossible to practically put this on a switch.
    • Plexi has much higher filtering, using three 50+50ufs, instead of a 32+32uf and two 16 ufs in the JTM45
    This would be difficult, you'd need a hefty 3 pole switch that can handle over 400V and some filter caps/cap cans, I think youd struggle to make the space depending on if your using cap cans or axials and their location in the chassis.
    • Some difference in NFB which I haven't fully worked out yet (plexi has 100k coming from 4ohm tap, instead of 27k coming from 16ohm tap)
    Barely any difference really, youd want roughly 4x the value for the 4 ohm compared to 16 which is how it is. Its because the 4 ohm output will produce 4x the voltage to get thw same power as a 16, so a feedback resistor roughly 4x the value is required to feed back the same amplitude.
    • Lower PPI coupling caps in plexi, 22nf vs 100nf in the JTM45. This definitely would limit the bass and create a tighter, but less bassy sound in the plexi.
    Doable but you could just dial back the bass knob on the tone stack too which youll dial in my ear anyway.
    • Plexi has different output transformer, I think higher-rated for wattage, and also with a smaller primary, something like 4k instead of 8k, I think.
    This is pretty self explanitory
    • I think the power transformer is different too, but I don't know how exactly.
    As well as this
    • Some plexis also seem to have V2's cathode bypassed with a cap, probably to make it higher gain and brighter.
    This would be easy to implement and I used ampbooks tool to calculate gain of the V2a across the frequencies and the .68u will boost frequencies above 1kHz by 3db. It will make a for a brighter and tighter distortion but again you could probably again achieve a similar effect by tweaking the tone stack in the next stage.
     
    Last edited: Aug 19, 2019
  14. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    I called up Ken Fischer years ago with a question about the different OT in the '67 Plexi vs the '70 metal panel 100s.
    I had a '70 metal panel Super Bass at the time and wanted to know if it could be made to sound like a '67 Plexi 100 I used to own.
    Never mind being surprised that Ken actually answered, but he went on to describe part value by part value a long list of mods to convert the '70 metal panel into a '67 Plexi 100 circuit, or at least sound.
    He didn't build Marshalls in the Trainwreck amps line but had those schematics memorized it seems!
    He did not go into changing the extra power supply filtering, but he went on and on with component changes.
    Totally blew my mind.

    BTW, since the Plexi Marshall stopped being made in 1969, can we stop calling '70s Marshalls, "Plexi"?

    The plexi has plexi panels and was made from '66 to '68!

    From 1969 on, Marshalls had metal panels instead of plexi panels.
    From 1969 to 1973 Marshalls were hand wired holy grails, but they were not Plexi Marshalls!

    The Plexi is not the same as the early '70s metal panel JMP!

    Any more than BF Fenders or brown face Fenders are Tweed Fenders!
     
  15. itsGiusto

    itsGiusto Tele-Meister

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    Well, yes, you are technically correct.
    But if we're grouping amps by sound and function rather than front-plate material, the metal panels are so very similar to plexis. I've always heard them referred to as plexis, and referred to the sound they produce as plexi (I don't think I ever hear anyone mention "metal panel", except when they're trying to denigrate the sound of the amps compared to the late 60s plexis, when they're saying Marshall amps got too harsh-sounding).
    I've just always heard everything post JTM and pre JMP referred to as "plexi", and if I couldn't call them plexi, then I'd be at a lack of simple vocabulary to describe what I'm after, because I don't know of any other simple way to refer to everything within the set of amps post JTM and pre JMP.

    IDK, we probably just have different philosophy for how we describe things! If I built a 5E3, or a 5F1, or anything but put it in a dual rectifier cab, I think I'd probably still refer to it as a Tweed.
     
  16. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    I’ve built, modded, and owned 18watters of various types.

    A few tweaks to the PI can really bring it closer to Vox territory, but I never felt mine sounded Voxish as-is, per se.

    The speaker is the first place I’d go to tweak the nasal tone.



    Otherwise... I suggest a plexi.

    I’m so absolutely done with attenuators, though, so take my advice with a grain of salt.

    If you want a plexi sound, only a plexi will do.

    My plexi fix: a plexi clone build... with a PPIMV on the back panel (in place of a second speaker jack).

    The PPIMV allows me to attenuate the sound without extra gear. It sounds great in this circuit (and some other circuits).

    It still sounds best when a little loud, but it controls the volume without sounding like a bad attenuator or a cranked and fizzy pre on a typical MV control.

    It’s a few cheap parts (a dual pot and two resistors). If you know your way around an amp, it’s a quick mod that doesn’t leave any permanent marks. You could buy a reissue or build your own, and this mod will work either way.
     
  17. itsGiusto

    itsGiusto Tele-Meister

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    Idk, I have AB tested my Vox's PPIMV (switchable Trainwreck type 3) directly against using my Weber Mini Mass to get it to bedroom-playing range. While the PPIMV made it sound shrill and ice-picky, the Weber with the +3db treble switch was smooth and warm, but not dull. I definitely am on team attenuator, myself!

    And as for carrying extra gear, I think I'd first complain about my pedalboard. The Weber Minimass fits snugly into my amp's back cabinet.
     
    Last edited: Aug 20, 2019
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  18. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    The word on the street today about what happened back in the '60s is, uh, way off as it were.
    We live in the misinformation age, when repeating incorrect info wins out over reality in the end.

    First off, the metal panel Marshalls are really very different amps from the Plexi Marshalls that are actually different amps.
    As far as denigrating the metal panel amps for getting too harsh sounding, that is exactly what Marshall did to change the old softer less bright less punchy Plexi circuits to cope with stadium sound problems.
    They were also fixing other problems like hum and ghosting from the inadequate power supply filtering that gave the '65- '67 Marshalls the more liquid sustain and squishy feel.

    But while the '65, '66 and '67 100s were each a different circuit, sound and response; the amps actually changed the most drastically a year before the panels changed.
    So the last year '68 Plexi was basically the same as the subsequent metal panel Marshalls, yet still commands a premium price due to those panels. Smaller changes continued but when the filtering was increased to a spec that continued for decades, the Plexi sound was gone, replaced by a louder brighter more punchy less creamy amp ready for the '70s, a harder amp for harder music.

    In reality, the pre '68 Plexi 100 circuit wasn't even putting out 100 watts at full distorted volume.
    By '69 a full volume 100w Marshall was putting out measured wattage up around 150-160 watts, and stayed clean way past the full volume of the '66 and '67 100.

    Set up a '67 100 next to a '69 100 if you can find them in stock unmodded form.
    Plug in and play them for a while.

    You will no longer say they are basically the same amp, I can assure you!

    Right here right now you are trying to decide if you want a more modern metal panel circuit or an earlier more primitive Plexi circuit.
    So it's really worth revisiting the popular assumption that the Plexi and the metal panel Marshalls are basically the same amp.
    The PT is a big factor in actual tone, because you could put 400v 450v or 500v on the power tube plates and get drastic volume and tone change. A switchable tube/ SS rectifier can change the voltage maybe 40v and get an audible difference in tone, but you have to rebias when switching from tube to SS rectifier. So figure on another switch for the bias if you want switchable recs.
    You can also bias in between and call it good enough.
     
  19. itsGiusto

    itsGiusto Tele-Meister

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    Thanks, very informative breakdown of models!

    Definitely true. From all of our discussions I think I'm narrowing down towards primitive plexi, and not modern metal panel, though I want to be more sure!

    Offhand, I just stumbled upon this guy's channel. He plays a Brunetti Mercury 1, which is based on a 50 watter. Any clue which which specific model it sounds most like? Really a beautiful sound!

    What year do you think the Brunetti Amp might be based on? Does this amp have more filtering or less? Larger coupling caps or smaller?
    I think it sounds more liquidy and bassy, and therefore might be like a '67, before the metal-panel circuit change you described in '68, but that's just a guess as I haven't listened to different models back-to-back enough to know.
    Interested to hear your thoughts!

    I plan on having them both on the same switch, using a DPDT. It'll switch out the rectifier and switch to the other bias pot simultaneously.

    I don't want to just bias in between because then it wouldn't be an apples-to-apples comparison when I switch from one to the other. Part of what I want is to get some real listening experience, learning to understand by ear the different changes in sound something like switching out the rectifier makes, so the more apples-to-apples I can get the comparison, the better.
     
  20. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

    Age:
    59
    Posts:
    16,775
    Joined:
    Mar 2, 2010
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    Maine
    Brunetti doesn't say much about their tech specs, and with fuzz and delay I can't really hear what the amp sounds like.
     
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