6v6 Plexi build

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MartinBMartinB Frets: 19
edited July 9 in Making & Modding
I thought I'd make a thread for the amp I'm working on at the moment.  It's a little far into the process to do a full build diary, but then I've only recent joined this forum. 
Last year I built a 50-watt amp for a bandmate based on the Marshall 1987 circuit, with added reverb.  I like the sound of that amp but don't need that much power, so I've been working on a lower powered version using 6v6s.  I got the idea of building a 6v6 version from Mark Huss' website, and that with a single 12" should cover any of the sort of gig I do. 
I've used a set of 18-watter transformers from Valvepower, a JTM45 sized chassis from the same place, and a Hammond choke.  I already had an old high-impedance reverb pan sitting around, so after doing some digging around online I decided to try adding a cap-coupled reverb circuit using a JJ ECC832 (12DW7) with the ECC82 side for the driver and the ECC83 side for recovery.  I've used a single input going to both the normal and bright channels, and added a post phase-inverter master volume. 
I've just got the chassis up and running this weekend, and it's sounding good.  I'm still tweaking some things to get the tone and level of the reverb right, but otherwise I'm pleased that this one seems to have gone together without needing troubleshooting. 




Next up I need to make a nice looking front panel overlay and build a combo cab for it.  In theory I have access to a laser cutter for the front panel, but so far I haven't quite got around to figuring out exactly how to come up with a design in the right format for it.  I should probably try and find that out...
I'm going to put it in a 1x12" combo cabinet for it, with red basketweave tolex to match the knobs.  I've got a couple of different old speakers to try - a Fane Classic 12/70 from the early 90s and a Celestion G12/75 (not G12-75T) from 1978. I haven't tried them in a proper cab yet, but the Fane seems brighter and clearer and the Celestion perhaps a little dark (it's a 2" voice coil, apparently). The Fane has a patched-up cone, so I have no idea if it'll sound OK at volume or not. If neither of those is to my taste, I might look around for a new speaker.


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Comments

  • Winny_PoohWinny_Pooh Frets: 1643
    Nice work :)
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  • customkitscustomkits Frets: 325
    Nice work, I'd be interested to see the layout as I already have valve power transformers and chassis etc that were for an 18watt but I already have a lite one so something different would be good

    I really fancy trying the reverb
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  • ChalkyChalky Frets: 4169
    Very neat - schematic?
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  • MartinBMartinB Frets: 19
    Nice work, I'd be interested to see the layout as I already have valve power transformers and chassis etc that were for an 18watt but I already have a lite one so something different would be good

    I really fancy trying the reverb

    I don't have a neatly presented layout, just some scribbly pencil drawings as I figure things out.  But the layout of the main board, controls and sockets is just adapted slightly from a typical Marshall 1987 layout.  Surprisingly few changes are needed to run 6v6s - have a look at Mark Huss' page for some pointers;
    http://mhuss.com/Plexi6V6/

    I made up a little board for the filter caps and rectifier mostly because it was a little cheaper than using can caps.  I'm still playing around with the reverb (I think it's just about there now), but it's a similar idea to the "1 tube reverb" circuit discussed on the Hoffman Amps EL34 world forum, except that I'm using the 12DW7 driver, cap instead of transformer coupling and a high impedance 4FB type tank.  So far I've had the best results taking the signal for the reverb driver from just before the tone stack and mixing the return signal in after it.  It's sounding quite good run that way, if less wet than a blackface Fender type reverb. 
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  • MartinBMartinB Frets: 19
    Here's a bit more detail on what's going on with the reverb.  This a a roughly drawn schematic that I noted down so that I can remember what I did - it would probably be wise not to take it as a "how to" guide.  Having said that it does seem to be working well enough.  At first I tried it with the reverb driver taking its signal from after the tone stack, but I like it better this way.  I'll figure out whether I need to revisit it after I've played the amp some more. 


    I'm currently figuring out my cab dimensions and checking out the two speakers I have lying around.  No cab yet, but I've had a low-volume play with them sat in a cardboard box with a 12" circle cut out. 
    The Celestion is a G12/75, which I couldn't find much information about.  It has a G12H-sized magnet and Celestion tell me it has a 2" voice coil which makes it a bit different from most of the better documented models.  It has a silver/grey cloth dust cap, a 1978 date code and no stamp on the cone.  The Fane is from the early 90s and is from their general purpose Classic range rather than being a dedicated guitar driver, but apparently the paper-edged versions of the range like this one were recommended for guitar use.  With my other amp, a Princeton Reverb copy, I liked the Fane better and the Celestion sounded dull and lifeless.  With this amp, the Fane is OK clean but has a nasty edge with any dirt and the Celestion is much more balanced and doesn't seem dark at all.  I'll check them both out in a proper cab, but the Celestion is probably the winner with this amp.  I found it interesting how amp and speaker voicing interact like that. 


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  • MartinBMartinB Frets: 19
    edited July 31
    I keep ending up busy with playing and other things, but I've got the cab built in rough form and managed to get the chassis and speaker loaded up to take it down to a rehearsal this evening.  The cab is made from 18mm spruce shelf board, mostly because it's cheap and not too heavy, with the baffle and back panels made from 12mm birch ply I had left over from a bass cab project.  It still needs a bit of sanding down and some filler in spots before I do the tolex, grille cloth and piping.



    The sound seems quite promising - with my current band I like to run the amp close to clean with some hair coming in on harder played notes, and this gets there quite easily.  I might try removing the 0.68uF bypass cap I used on V2, just to make it a touch less gainy though.  The reverb has come out surprisingly well - the knob needs to be run quite high but it's a nice clear, open sounding reverb.  I have the impression that this Celestion isn't massively efficient, and it seems to be a relatively mellow, non-icepicky sort of speaker.  It might be interesting to try a different one some time though. 
    The cab is going to get red basketweave tolex, and I'm pondering whether to use up the black 70s/80s style grille cloth I have left over from another project or whether to pick up something else.  The large check Marshall style cloth might look good.  The front panel is going to be in laser engraved laminate, probably in white. 
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  • MartinBMartinB Frets: 19
    I've got my tolex and grille cloth done, and just have the front panel still to do. 




    My tolexing is not entirely flawless, but I'm happy enough with it.  I used the solvent-free version of Evo-Stick Impact, which takes some time to reach the tack point, but otherwise does the job well.  The piping is gold anodised aluminium florists' wire, as it's cheaper than anything sold as amp piping and doesn't have that odd greenish colour that some of the plastic piping does.  I had intended to use gold tadpole piping around the bottom and sides of the grille, but I hadn't reckoned for the thickness of this grille cloth and realised I didn't leave enough clearance for it.  Still, I think it looks OK without. 
    I've also treated myself to a new speaker as I wasn't quite happy with what I was getting from either of the ones I had lying around (neither was awful, but I felt like it could be better).  I noticed that Watford Valves have their Harma speakers on half price at the moment, and at something like £62 posted it seemed like a good deal for a nice UK-made speaker.  I'm fairly certain these are made by Tayden, and I went for the "Classic Ruby" G12H-alike, which from the specs looks like it is closest to the Tayden Great Brit.  It's got a fair bit more detail and articulation than the old Celestion and I don't have to push the treble and presence controls to get it, but it still manages not to be harsh with a bit of drive. 
    I've only done a couple of rehearsals with it so far, but I'm enjoying this amp.  It's taking a bit of time to get familiar with it after playing a Fender style amp for several years - the mid control has a noticable effect on the amount of crunch, and the presence control is something new to get used to.  With the band I tend to use it with the preamp volumes at 9-10 o'clock so that it's mostly clean, but it does do the classic Marshall-ish rock thing when they're pushed a bit further.  We're doing a bit of recording in the coming week, so I'll see if I can pull out some crunch for the couple of tunes that call for it. 


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