Princeton kit in SCXD chassis?

Discussion in 'Glowing Bottle Tube Amp Forum' started by swampyankee, May 23, 2019.

  1. swampyankee

    swampyankee Tele-Holic Gold Supporter

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    I have a spare Newell cabinet with 12" speaker I bought for a SCXD conversion. I think I may even end up with an extra SCXD chassis, so I'm wondering what I could use it for. Mojotone has a Princeton Reverb "small parts kit" which could make for a fun project. Has any one done a conversion like this, or heard folks that have? How difficult could it be? Although I'm not an electronics tech, I'm a decent electrician and solder dog, and I can follow instructions.
     
  2. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Seems like there're two questions here: Can you build an amp? And will the SCXD chassis house a PR?

    For the former, simple answer is yes. See many related 'can I' threads here for all sorts of safety, simplicity, and cost comments. The PR is a superb amp, but not (for sure) the simplest to build.

    For the latter, you want to count punch outs (tubes) and measure cutouts (esp. the PT). Even drilling, enlarging, and covering holes can be a chore, and for tubes, hole punches aren't cheap, even used. And nibblers? Well...

    I think you can hunt up a cheaper PR chassis than Mojo's (though they make an excellent one).

    http://rover.ebay.com/rover/1/711-5...0001&campid=5338148343&icep_item=223495587641
     
  3. swampyankee

    swampyankee Tele-Holic Gold Supporter

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    Actually the metal fab is the easy part since my "day job" is as an engineer at a tool and die shop, so I have pretty much any kind of fabrication resources I would need.
    I'm sure I could put the electronics together, even if I had to call on outside resources to help.
    My real question is, has this been accomplished? And what kind of finagling was required to get it done?
     
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  4. schmee

    schmee Poster Extraordinaire

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    I have considered this recently. The answer is "it depends" on how far you are willing to go. -Can you live with a SS rectifier? That would be easier to use the stock Power Trans. Or buy a small tranny for rectification to put in there.
    -I think there is plenty of space in there.
    -Will the PT power more tube filaments? I'm guessing probably.
    -If you want the Princeton reverb, buy a hole punch kit from Harbor Freight as you will need to add a couple tube socket holes. ($45)
    -Basically you are getting a chassis, faceplate and transformers. You have the cabinet and a good one from Newell. But if you have the chassis, not a bad way to go for minimal cost.
    -I would just order up components individually, you dont need to buy a "kit".
     
  5. swampyankee

    swampyankee Tele-Holic Gold Supporter

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    At $179, the kit didn't seem to be a bad value. Being a virgin amp builder, I'd be buying the how-to instructions and support that comes with a kit.

    Sent from my SAMSUNG-SM-G870A using Tapatalk
     
  6. King Fan

    King Fan Friend of Leo's Ad Free Member

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    A kit can save you shopping and shipping hassles. But Mojo and most other kits don’t include instructions or support worth mentioning.


    Sent from my iPad using Tapatalk
     
  7. schmee

    schmee Poster Extraordinaire

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    Yeah, I guess it depends on what it includes. You can buy a lot of resistors and caps for $180
    -Ceramic tube sockets?: I dont like them but they work fine.
    -You only want Switchcraft jacks.
    -CTS or Alpha Pots (You may consider if you want to buy pots that fit the faceplate holes in the SCXD.. they may be small diameter holes.)
    -Resistors: I always buy Metal Oxide as I find today's Carbon Comp to often be way off the mark tolerance.
    -Watch the wire, some of the cloth push back wire out there is terrible , getting fuzzy like a dust mop on the end after cutting! You want waxed cloth. marsh amps has great wire.
     
  8. archetype

    archetype Fiend of Leo's Silver Supporter

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    Some thoughts...

    The OEM SCXD transformers are a mystery. They are made in Taiwan and have a maker and part numbers, but, per the last time I looked, there are no info or specs available.

    I would assume the PT is quite inadequate to power an additional 2 tubes or 3 if you want a tube rectifier. As-is, the PT heater voltage is only borderline adequate just from substituting 6L6-GB or 5881 tubes that draw twice the heater current of the 6V6-GTs.

    The OT should be fine, as once you get past the SS preamp the SCXD is 99% Princeton. The OT might be a bit more robust than the PR OT, as the SCXD is less flabby in the bass when cranked.

    You could build a tube preamp (and reverb + tremolo if you wish) to hook to the existing Princeton driver, PI, and output circuit, but there's no room to do so. Everything is on the same PCB that spans the width of the chassis.

    Ulimately, the usable OEM parts would be the chassis, OT, and 3 tubes. There's honestly nothing else that's usable for a hand-wired PR.

    My advice:

    You're headed for frustration and possibly eventual failure trying to convert what you have just because you have it and may be trying to save money. It sounds like a fun adventure, but it absolutely will not be. You can convert a coffee table into a battleship, but you'd waste less time and money, and get an optimal result, by just building a battleship to start with.

    If I really wanted a PR, I'd sell the Newell cab. It shouldn't be difficult to sell, but selling it wouldn't recover the original cost. If the speaker would work well with a PR, I'd keep it. I'd then buy a PR kit and build it.

    If you really want to save bucks, if one of your SCXD chassis is operational then just build what you originally wanted to build. A PR has a few characteristics that an SCXD can't quite get to, but the SCXD gets pretty close to the way a PR sounds and responds to volume and picking. The Newell cab should help get it closer.
     
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  9. swampyankee

    swampyankee Tele-Holic Gold Supporter

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    Sound advice Archetype.

    I had a Warehouse G12C speaker in the Newell cab with the SCXD head. It sounded great at home but when pushed at practice, it got brash-sounding, for lack of a better term. Which is why I went back to the stock cab and 10".
    It would be fun to go back to the 12" and experiment a bit more. I have the G12C in my Concert Amp and I believe it's found a home there. But I have a few other speakers I could try for variety, and I could do some more research and find out what other's experiences are for other speaker swaps.
     
  10. DrPepper

    DrPepper Tele-Afflicted

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    If you have to add a preamp tube hole, consider locating it close to the input jack and make it V1... Then, you will need 3 and 3 near the back side.
     
  11. swampyankee

    swampyankee Tele-Holic Gold Supporter

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    So to update, I found a good deal on a SCX2 head, which I will install in my Newell 12" cabinet. That gives me one in the original 10" speaker configuration, and one with a 12" speaker. Even though it's not a Princeton, the OE version fills the need for a low watt, 10" speaker combo. Meanwhile, the 12" version will be a bit more of a hot rod.
    But in both cases, that disqualifies this thread from inclusion in the Glowing Bottle Tube Amp forum since SCXD's are only token tube amps...
     
  12. hamerfan

    hamerfan TDPRI Member

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    I always read this caveat with swapping power tubes from 6v6 to 6L6 /5881. But if you change the pilot light to an LED you've got enough headroom.
     
  13. archetype

    archetype Fiend of Leo's Silver Supporter

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    Thanks. I’ll look into that.
     
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