JCM800 6V6/6L6/EL34 in a Princeton Reverb Chassis

Discussion in 'Shock Brother's DIY Amps' started by robrob, May 1, 2019.

  1. omahaaudio

    omahaaudio Friend of Leo's

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    Isn't this a JCM800 version of the original Mesa Boogie?
     
  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    No, this is just a JCM800 circuit in a Princeton Reverb chassis. I believe the original Mesa Boogie was a hot rodded Princeton Reverb circuit.
     
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  3. 24 track

    24 track Doctor of Teleocity Silver Supporter

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    great post rob
     
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  4. EmWeAh

    EmWeAh TDPRI Member Ad Free Member

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    Hi all,
    new member, longtime reader of tdpri and of Robs fabulous site. Thank you very much Rob!
    I just finished up building the 6V6 Version of Robs RR2104, but with a few changes: I reverted back to the "standard" bias board, used a 5Y3 rectifier and a standby switch (yeah I know :)). Based on Robs layout I designed and cut my own board using the dimensions of Vishay PR resistors, Jupiter Vintage Tone caps and Hammond transformers. Bought a standard AA1164 chassis from TAD, did the cut out enlargements and had it powder coated.
    Everything fired up upon first start, but I think I need to think about changing pre-amp tubes or dive a bit more into amp voicing since this beast has a lot of gain. You can't turn up the pre-amp volume past 12 o'clock or it gets very harsh and muddy (sorry, I'm not very good in describing sounds :)). ATM I'm thinking of changing V1 for a 5751 and V3 for a 12AT7 as a starting point.
    Anyway, here are a few pics of the build (sorry for the lousy cell phone quality):

    Edit: of course I am willing to share PDFs and DIYs, as well as Inkscape SVGs of the panels upon request.
     

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  5. robrob

    robrob Poster Extraordinaire Ad Free Member

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    That's a fantastic looking build EmWeAh. Try a GZ34 in the amp and see if that helps clear up the mud. The amp was designed for a stiff solid state rectifier.
     
  6. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Looks great! No experience with that circuit, but I'll second what Rob said. The times I've subbed a GZ34 for a 5y3, it really transformed the amp. Adds headroom, punch, and definition in the Tweeds I've tried it in. Honestly, too much in my cases, but if the 5y3 is too muddy, I'd start there before modding the board as a first choice.

    Sent from my XT1585 using Tapatalk
     
  7. EmWeAh

    EmWeAh TDPRI Member Ad Free Member

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    Thanks for the kind words. I will try a GZ34, but unfortunately don't have one flying around here at the moment. Ordered already one from TAD and while browsing their shop I found a drop-in solid state rectifier replacement (this is plan B if things don't work out with the GZ34). Will try that before doing anything on the circuit.
     
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  8. Pierre67

    Pierre67 TDPRI Member

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    Great work !!! Really inspiring !
     
  9. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I believe the GZ34 or solid state drop-in will clean up the mud. High gain amps needs stiff power supplies and the 5Y3 can't supply enough current to prevent sag.
     
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  10. EmWeAh

    EmWeAh TDPRI Member Ad Free Member

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    it took me a while, but today I finally had some time to spend tweaking this little beast. As suggested by Rob and others the GZ34 was the key to stabilize the psu. I also plugged in the solid state rectifier, but couldn't hear much of a difference. I couldn't play it maxed out, so that's maybe the point where the sag of GZ34 could play a sonic role. For the moment I decided to give the GZ34 a go.
    Had the amp on for about half an hour and biased the 6V6 to 22.5mA. After talking final measurements I noticed that the GZ34 in combination with the Hammond 291CEX (330-330) brought up the B+ voltages significantly. 415V on B+3 seemed a bit high to me, so I decided to change the the first dropping resistor from 2.7K to 10K which brought everything downstream B+2 down almost exactly to Robs schematic values. The plates of the 6V6s are pushed a bit hard with 445 V, but it's still within spec (and JJ 6V6s are known to be sturdy little things :)).
    After playing the amp for a while I also changed the V1 bypass cap from the original .68uF to Robs suggested .47uF and the treble peak resistor back to the original 470k (for a little less gain). Was pretty satisfied then, but still started a bit pre-amp tube rolling :). Settled with a JJ 5751 in V1, a JJ 12AX7 in V2 and a EH 12AX7 in V3. This gives a nice Marshall crunch, but still a lot of gain when the pre-amp is cranked. Pretty happy right now, will try to record some sound clips, but this might take a while.
     
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  11. FenderLover

    FenderLover Friend of Leo's

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    It looks like you know how to adjust your voltages, but the schematic has some educated guesses that could be finessed a little. The 2K7 shows a 34V drop, which would indicate 12.6 mA supplied to 6 triode sections, which is unlikely.
     
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  12. robrob

    robrob Poster Extraordinaire Ad Free Member

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    @EmWeAh, I'm glad to hear you got the amp working and tweaked to your liking. What didn't you like about the preamp gain that you went back to the 470k treble peaker/attenuator resistor?

    The power transformer I spec is 310-0-310v versus the 330v you used so your voltages will be higher. I'll probably increase that first dropping resistor anyway though to get the preamp voltages down some.
     
    Last edited: May 19, 2019
  13. fastedtex

    fastedtex TDPRI Member

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    I built the Micro version of JCM800 from @robrob great design and finished the chassis about a week ago.
    Started on the head cabinet for it but have bogged down with no time to work on it cuz I'm too busy playing this thing. The more I do the more I like this amp!
    Just tried a NOS GE JAN 6829(12AT7) in V1 and liked it. Populated first with JJ's in all 4 spots, and they sound good also.
    LO had a different flavor with the lower mu AT7 with the gain(pre-amp Vol) up at 4 to 5 was more crunchy and less gainy but in the HI channel still plenty of Marshall tone there. And don't let the "micro" fool you, it is plenty loud for any home practice.. unless you live in the Grand Canyon.
    Considering adding an effects loop, have to stop by Rob's place and read up on that.
     
  14. EmWeAh

    EmWeAh TDPRI Member Ad Free Member

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    @robrob I just had the intention that it was a touch too much gain. Don't get me wrong it still has a lot of it ... I was really surprised how much. My 2203 reissue didn't had as much of it. For the moment I leave it be and wait 'til things have settled a bit and the speaker is broken in. I might consider tweaking it some more if there is any need for it then.
    Another project is rolling my way since a friend asked me to build him a AA1164 Princeton Reverb ... and then there is also your Blackvibe which is very intriguing :).
     
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  15. omahaaudio

    omahaaudio Friend of Leo's

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    So it's like a JCM800 version of the original Mesa Boogie.
     
  16. Bluey

    Bluey Tele-Meister

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    I'm not the sharpest tool in the shed when it comes to this sort of thing, but I'm sure they are different curcuits. If it's the wheel your referring to, yes they are all round.
     
  17. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The only thing the two amps have in common is the freakin' chassis :D
     
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  18. omahaaudio

    omahaaudio Friend of Leo's

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    And the installation of a high-gain amplifier in that chassis.
     
  19. omahaaudio

    omahaaudio Friend of Leo's

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    Each involve the installation of a high-gain "curcuit" in a Princeton cabinet.
    :)
     
  20. timewaster1700

    timewaster1700 Tele-Meister

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    Anyone have any sounds clips yet? I'm very interested in hearing this!
     
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