18W Plexi Build suggestions?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Governator, Dec 2, 2019.

  1. Governator

    Governator Member

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    Hi, I've been contemplating an amp build for the past year of so and was originally looking at a 50W 1987 with a switchable circuit to a 2204 and PPIMV. However lately as I am getting more serious about ordering a kit, I'm now leaning towards an 18W Plexi voiced amp head (I also plan to purchase a speaker cab with 2 x G12H30's). Having never played an 18W I have lots of questions......

    I play mostly classic to hard rock (Guns N Roses, Zeppelin, ACDC, Hendrix) and some 80's and 90's metal. I currently just run a Vox AC10 on clean and use pedals for my desired sound, I don't gig at the moment, mostly play at home and jam with people, however since I've been getting back into playing, in the future some gigging could be on the cards and need something more powerful than the AC10. I'm thinking an 18W at 8-9 will sound sweeter than a 50w at 1-2? Will an 18W be enough for small gigs? What sort of headroom do you get with an 18W to be able to run clean sound at higher volume? Will an 18W be a good pedal platform for playing metal?

    What are some good kits? I've checked out the Valvestorm (they don't have cabs though by the looks), GDS Amplification (also don't have cabs by the looks), Mojotone and Trinity Amps), they all look pretty good.

    I'm in Australia so one of these kits + shipping + AUD conversion to USD is more expensive than buying an off the shelf Marshall 20W JCM800 Lead Series Studio Classic Head locally! Would I be better off just getting the 20W?

    I have an electrical trade and I have build over a dozen effects pedals, some fairly complex, so I'm pretty confident with my soldering and electronic abilities.

    Any help greatly appreciated, thanks.
     
  2. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    I would vote for just buying the 20W production amp.
     
  3. 00JETT

    00JETT Member

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    I think I agree with just buying the 20w production amp, especially the 2204 style one with the master volume, Or build a 20w 2204 style. There is a good chance the 18watt kit on 8-9 will still be louder than the 50w on 1-2 in your example. I think the 2204 production amp will get the gain levels you would need for the styles listed and the master volume will let you control it. Really any of your choices could be made to work though but as soon as I hear someone say they need more than just classic rock the 2204 circuits with its touch more gain than the plexi circuits come to mind.

    Building amps is addictive, I have had good luck with valvestorm and mojotone for parts, I don't use kits so I can't help you there.
     
  4. Vanyu

    Vanyu Member

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    If you do plan on playing out, then I’d suggest a 2204 over the 20w version. The MV’s in these amps are pretty good, it’ll sound good at practice volumes too. 2204’s are highly moddable too, and if you plan on going down the kit route then that would be a natural fit.

    I’ve gigged 18-20w amps in the past and it wasn’t too hard for my drummer to drown it out, I have a much easier time hearing myself with a 50-100w amp. I know that you’re currently not playing out, but a 2204 would certainly allow you to do so with ease if you choose to do so in the future.

    Don’t be put off by the wattage, 2204’s (and even 2203’s) do great at just about any volume, especially if you tweak that preamp to suit your playing.

    I agree, building amps is an addiction :)
     
  5. Dan40

    Dan40 Supporting Member

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  6. Governator

    Governator Member

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    Thanks, yes the 50w 1987 is what I was originally looking to build and then make it into a switchable 2204 along with mustard caps, CTS pots, etc. It’s a decent build for a first time build and decent $$, hopefully nothing goes wrong.

    The main thing i’m trying to figure out is what’s the difference between the 50w with some crunch using the PPIVM and the 18/36w when playing at home or jamming? As i’ve heard you loose a lot with the PPIMV/larmar which isn’t as effective as the real tube distortion?
     
  7. Vanyu

    Vanyu Member

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    PPIMV’s just don’t work that great at less than half volume or so. If you wanna give a PPIMV a good fighting chance at any setting, then I’d suggest removing your NFB loop and replacing your presence control with a Vox cut control. Doing that would address all of the problems inherent with a PPIMV, in particular at low volumes.

    This is because you lose NFB as you turn your MV down, because now less voltage is making it into the phase inverter. High amounts of NFB (like the 27K/16ohm setup) counter this to some degree, but Marshall’s just don’t take to PPIMV’s all that well (IMO) without other accompanying mods (like the one I mentioned above).

    As far as 18w vs. 50w, the difference isn’t as stark as you’d think, I’d suggest you play each one because they’re both gonna be pretty loud. FWIW I’ve heard that 18w don’t take that well to MV’s of any kind and are ‘meh’ with attenuators, but I don’t know that one from personal experience, never had one with a PPIMV and never had to use an attenuator with one.
     
  8. rumbletone

    rumbletone Silver Supporting Member

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    My Egnater seminar amp - SE20 -has been my main gigging amp for 5 years. It’s 2x6v6, 20 watts. The preamp is hotrodded Marshall - more gain than a JCM800 (but with a cathode bypass gain boost instead of cold clipper), but roll back the gain and turn off the gain boost and it will do ‘Malcolm’ levels of gain (and it is VERY touch sensitive, unlike many hotrodded Marshall designs).

    I’ve only had one gig where I ran out of headroom, and it was a 1000 seat venue and I wasn’t mic’d. In big rooms when I’m mic’d and in small rooms when I’m not mic’d, it always has more than enough power.

    And although it doesn’t move as much air as a 100w plexi, I’ve realized that a 20w in its sweet spot sounds better than a 100w with the volume on 1 ...
     
  9. joemonardo

    joemonardo Member

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    I just finished a JTM45 build, well not really, I'm currently troubleshooting. It's my first amp build and it's a headache, but it's fun and keeps you busy. I bought all my parts from valvestorm, the quality is top notch.
     
  10. VICOwner

    VICOwner Supporting Member

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    What is it that is giving you trouble?
     
  11. joemonardo

    joemonardo Member

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    Basically, I have completed the amp and have moved on to testing. Everything's great for about 5 min getting the right voltages where I need, but when I go to double check I will touch the pin with the voltmeter and the fuse sparks and blows.
     
  12. pdf64

    pdf64 Member

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    Repeatedly subjecting the amp to fault current is going to tell on something eventually. Build yourself a lightbulb limiter, it will protect the key parts from excessive current and may be used help to identify the cause of the issue.
     
    Don A likes this.
  13. PushedGlass

    PushedGlass Member

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    Something's wrong with your voltmeter - to potentially include (hey, see what I did there?) that it's not a voltmeter. A proper voltmeter would ideally have an impedance so high that it appears to the circuit under test to be no different than an air gap.
     
  14. VICOwner

    VICOwner Supporting Member

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    I have to guess the meter is also the problem. If you don’t use the meter, the amp won’t blow the fuse?
     
  15. joemonardo

    joemonardo Member

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    Yeah, I was up late last night working on it. Ended up re-soldering a few dodgy connections and it did the trick. I still have to grab some KT66s and a GZ34 and bias it today. Then it should be ready to rip!
     
  16. Don A

    Don A Silver Supporting Member

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    I have an Avatar 45 (JTM45 based) with a PPIV master and the master works really, really well. The master is and good as my THD Hot Plate attenuator at higher settings and is way better than the Hot Plate at lower volume levels.
    I can get the volume down to very comfortable (though still good and loud) volume levels. My Budda Twinmaster (18 watts, no master volume control) is way too loud in it's sweet spot compared to the Avatar 45, even when I take 4db off with the Hot plate.
     
  17. d95err

    d95err Member

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    I’d go for 6V6 rather than EL84s for a ”Plexi” type amp. I’ve experimented a bit with ”18W”-circuits and EL84, but never really got a tone I liked. Had much better luck with 6V6 or EL34.

    If you want to use the Plexi type preamp as a cleanish platform, it’s essential to make the bright cap switchable. With the standard 4n7 bright cap, the amp has to be turned up quite far to get a usable tone.
     
  18. big mike

    big mike Plexi Loving Admin Staff Member

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    1987 spec has a lot more crunch and gain due to the cathode follower arrangement
     
  19. aynirar27

    aynirar27 All You Need Is Rock and Roll Gold Supporting Member

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    I've built a few kits from Trinity. I bought their 18 watt kit, but only used it for the transformers and chassis, I kind of made up the circuit as I went along. They are a great company.
    But if you are concerned with "headroom", just go for the 50 watters. 18 watters still need to get pretty loud to hit the sweet spot and really don't stay clean for very long, so the lower wattage isn't going to necessarily keep things quieter
     
  20. Governator

    Governator Member

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    Thanks for all the comments, i’ve decided on the Mojotone 18w TMB and have one one the way, I think it will be ideal for me, it has their Jcm800 style pre amp with master volume on one of the channels which should get me the crunch and rock tones. Plus, this is a simpler build for my first build.

    The JJ EL 84 and 12AX7 tubes seem to be the go-to tubes for these 18 watters, anyone used any other brands? Are NOS tubes worth it?

    I’ll also be looking to purchase a 2 x 12” cab. What are your recommendations - closed or open back?

    - Speaker wise i’ve read the G12H30 are good, any other recommendation what to use?
     

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