Two In One Princeton

Discussion in 'Shock Brother's DIY Amps' started by Nickfl, Nov 19, 2019.

  1. Nickfl

    Nickfl Tele-Afflicted

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    I built a modified AA1164 princeton about two years ago and decided recently to revisit it. The original build included some mods that I'd almost consider standard, like backup rectifier diodes and screen grid resistors as well as some based on @muchxs recommendations like a more substantial PT and slightly lower B+. I also removed the tremolo and added a couple of signal path mods including a three way stock/mid boost/raw switch on the tone stack, switchable NFB, pre PI master volume, and a "fat" control that blended in a spare triode in parallel with the second gain stage. My schematic as originally built can be seen here.

    It turned out to have a great clean tone, but the overdrive never really did much for me and the signal path mods were all kind of meh and were things that I wouldn't include on a build today. So, realizing that I hardly ever have any desire to play this amp I started thinking about what I could do to make it into something I would enjoy more. Eventually I settled on the idea of removing the fat triode, master volume, and tone stack switch and instead using all of them to set it up to switch between an AA1164 and something close to a 6G2 preamp.

    Here is the modified schematic:

    Princeton with 6g2 switch.png

    It switches out the 1164 tone controls and 2nd gain stage for the 6g2 controls and second gain stage and I split the cathode on the 6g2 2nd stage to reduce the signal so that after it is attenuated there and again at the reverb mixer it will feed the post reverb stage a signal small enough to more or less negate that extra gain stage and hit the PI with a similar signal to what you would see in a 6g2 without the extra triode post reverb. I also changed the NFB to switch between the stock BF princeton level and a slightly heavier level along the lines of the brown princeton, changed the rectifier to a 5y3 and adjusted the power supply dropping resistors to compensate. The 6v6es are seeing about 315v plate to cathode, but the preamp tubes are kind of splitting the difference between the plate voltages in the original amps.

    After making the modifications, I ended up adding large grid stoppers on the PI and 6g2 second gain stage as I seemed to be getting some blocking distortion when switched to 6g2 mode. I still need to play it more and with more guitars, but so far it is very promising with a telecaster at least.
     
    FRANKENJAG, Snfoilhat and King Fan like this.
  2. King Fan

    King Fan Friend of Leo's

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    What a cool project. I admire your circuit design skills.

    You're confirming my bias that:

    1) The PR is not an amp that really needs or wants all that many tone/circuit mods; I seldom use my raw knob or 3-level NFB switch.
    2) A 6G2 is what a 'raw' PR would sound like if it could.
    3) A PR is what a 6G2 would sound like if it finished high school and went to, well, Princeton.
     
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  3. Nickfl

    Nickfl Tele-Afflicted

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    Yeah, I'd pretty much agree with that assessment. I do actually like the "no NFB" option on the 1164 as it adds a little more bit to the overdrive character, but otherwise it isn't really improved by anything I've done to it. Multiple levels of NFB were definitely not needed, either some or none seems to be the ticket. Interestingly, with the 1164 channel I have no preference between the two levels of NFB I have available now, but I definitely prefer the heavier NFB with the 6g2 channel. I might tweak that a bit so that I have an option for even more NFB as I think that would go well with the 6g2 preamp.

    I used to put a raw switch on everything as it is such an easy thing to do, but I don't think I've ever really been inclined to use it once the novelty wore off and I've removed them from pretty much everything I've done it on now.
     
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