Princeton Reverb head build with Deluxe PT and OT...

Discussion in 'Amp Tech Center' started by Jerry Bee, Aug 13, 2017.

  1. Jerry Bee

    Jerry Bee TDPRI Member

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    Hey guys,
    I got a good deal at modulus amplification UK...a princeton reverb chassis with faceplates included:

    http://modulusamplification.com/Blackface-Princeton-Reverb-Chassis-Faceplates-P5266372.aspx

    But what I did not realised is that it has the mounting dimensions for the deluxe PT and OT, which is a good thing...I guess :)
    So I could build a beefed up PR now.

    Now my question is, should I build it stock or do you recommend any mods like Stokes, PaulC etc.?

    My plan is to build it with a switchable raw control, a pre pi mv and run it through a 12" speaker like a 12a150a...So I could use it for practicing, recording and gigging.

    Cheers
     
    Last edited: Aug 13, 2017
  2. Dacious

    Dacious Poster Extraordinaire

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    John Stokes doesn't recommend his mod with stock output transformer. Using the Deluxe transformers is a no-brainer in my opinion, or at least a higher rated power transformer as the Champ P/t gets pretty hot more so from the heater rating being marginal. You're not doing vintage so upgrades are in order. You do have to watch the power you run, I think.
     
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  3. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    The PaulC mod is flawed. It is based on incorrectly taken measurements. One can do a fixed bias phase inverter mod, but don't use the resistors that PaulC's mod recommends. If you want to do it, read more about the correct method here; tdpri. #post-7796029
     
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  4. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I'd throw a 470k grid stopper on the phase inverter grid. It won't affect the clean tone but will sweeten heavy overdrive.
     
  5. Jerry Bee

    Jerry Bee TDPRI Member

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    Hey Rob,

    I want to do your pre pi mastervolume with 1M pot anyway...
     
  6. danlad

    danlad Tele-Meister

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    I wholeheartedly second the grid stopper for that PI
     
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  7. Mart the Hat

    Mart the Hat Tele-Holic

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    You might already have considered this, but pre-PI master on a Princeton Reverb has limited usefulness compared to some other amps. I've tried it on mine, it doesn't really get any useful dirt and doesn't get you anything resembling the cranked amp sound, but for whisper-volume home use it still wakes up the preamp a little and gets the reverb working a touch better than if you have a single volume set somewhere below 1. If that's all you want from it it can still be worth having though.
     
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  8. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yea, with low-gain preamps you'll need a boost pedal to get some preamp distortion so you can turn down the master volume and still have dirt.
     
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  9. Jerry Bee

    Jerry Bee TDPRI Member

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    So a post pi mv would make more sense? Then the preamp would drive the pi more.
    I have good results with the post pi mv on my jtm-50 plexi Clone or does it depend on longtail vs cathodyn pi?

    I just want to lower the overall volume a tad to get a slightly overdriven sound for rythm work and then hit it with a booster or engage the raw control to let the beast roar and scream...

    An alternative would be to build a one channel deluxe reverb
     
    Last edited: Aug 13, 2017
  10. jhundt

    jhundt Doctor of Teleocity

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    no, the pre-amp will NOT drive the PI to any kind of useful distortion. The design of the cathodyne PI does not permit that. There is no gain added in the PI, and there is no advantage to using a post-PI master volume for that reason.

    I have a Princeton Reverb in which I have installed a Deluxe Reverb power transformer, and an upgraded output transformer that is even stronger than the Deluxe Reverb OT. It is a very fine-sounding amplifier, but it does not create a lot of overdrive distortion. And it is very, very loud.

    I played around with Stokes and PaulC mods, but I realized that the Fender PR is a very good amp just the way it was built - except for the weak transformers.

    I have considered the grid-stop resisitor or MV as described by Rob R, but because I rarely play with heavy overdrive or distortion I have not yet found it necessary. In fact I was happiest when I removed all my mods - except the upgraded transformers - and let the Fender sound like a Fender.

    I also have a PRII which I rewired as a single-channel Deluxe. It has a pre-PI master volume that works just fine. I tried a post-PI but did not feel that it added anything for my style of playing. (I do not play heavy blues-rock lead guitar)

    I would suggest that you make a very nice Princeton with upgraded power- and output transformers, and learn how to enjoy that amp. It may require pedals for low-volume distortion and overdrive - but that's an easy thing to do. Much easier than trying to make a well-designed amp do something other than what it was designed for.
     
  11. Jerry Bee

    Jerry Bee TDPRI Member

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    Thank you jhundt, that sounds very reasonable and I will do it that way.

    For dirty sounds i have my tweeds...
     
  12. jhundt

    jhundt Doctor of Teleocity

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    anyway, you can always do more mods later, after you have built and tested the amp. Please let us know how your build works out. There are lots of Princeton-lovers here!
     
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  13. Jerry Bee

    Jerry Bee TDPRI Member

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    Found a nice power transformer, originally for a 5E3 at the dutch tone factory, which is a bit over specs, see attached datasheet. But it will fit the mounting dimensions 100% and gives enough juice with 120 mA and 3A for the 5U4GB.
    I heard that Mabel Audio is the chinese tranny supplier for Weber and I only paid 69 Euro for it. The Hammond and TAD Deluxe pt are over 100 Euro. They have the 50 V bias supply, which I don't need anyway.

    First step for the build is to adjust the housing and I fished a mint and light birch ply cab in the bay.
    The height of the chassis cutout is 1/2" too tall, but the width will fit for the PR chassis as you see.

    5e3 PT-2.jpg

    DSC_0134.JPG
     
    Last edited: Aug 17, 2017
  14. Jerry Bee

    Jerry Bee TDPRI Member

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    Finally got it to work after i build it nearly stock , but still have some strange issues.
    After biasing the first time, there was no sound to hear, when i plugged in and playing some notes. I rechecked everything, all wirings and voltages...chopsticking etc. But all was fine. After a while i realised, that the reverb pot works as a master volume... even when the reverb is not engaged by the footswitch...still I don't find my mistake.
    Besides that the volume pot has no significant rise in volume till the half way up. Nearly cranked and the reverb at 12 oclock, the sound is fine, very blackface hifi. When i turn up the reverb pot even more, there are stroboscope like distortions on the lower notes, blocking distortion i guess. And there is 120Hz hum, could be a grounding problem.
    Some ideas?
     
  15. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Verify the value of the 1M grid leak resistor on the reverb driver 12AT7, the 100KL reverb pot, 470k attenuator attached to the reverb pot and the 3.3M reverb mixing resistor.

    If the 470k attenuator is too small all the dry signal will be bled to ground when you turn the reverb down.
    [​IMG]
     
  16. Jerry Bee

    Jerry Bee TDPRI Member

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    That was spot on...the 470k was a 470R!
    Now the volume pot works like it should, what a beautiful clean, very clear and punchy through the Jupiter 12sc, nice snappy response.
    But...the hum got worse, can't even say if it's 60Hz or 120Hz, maybe both. It's quite loud with the volume off, till 3 it gets less and then it increases again.
    Tomorrow i gonna recheck all resistor values and the grounding. My guess it's in the preamp stage. I should rewire the pin 1 to pin 6 connection of V2 with a longer cable, perhaps some interferences there.
     
  17. Jerry Bee

    Jerry Bee TDPRI Member

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    DSC_0161.JPG
     
    Last edited: Sep 23, 2017
  18. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    //Semi-hijack sorry!

    The jewel lamp holder looks kinda bent (?). Watch out if any of that touches the chasis. I glue a thin 1"x 1" strip of hardware store repair rubber under that lamp holder. Costs 10 cents and takes 3 minutes. Can save you debug headaches later if you do that.

    Has NOTHING to do with your hum prob though.

    //end semi-hijack

    Also: what are the switch and the pot on either side of the fuse holder doing?

    That pot for sure is not per AA1164-original. Is it a hum control mod or is it the mv you were mentioning? or?

    Is the switch for old school ground-switching? Are you sure you need that?
     
    Last edited: Sep 23, 2017
  19. Jerry Bee

    Jerry Bee TDPRI Member

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    I will check out your advice after work. I installed the lamp holder like that, because on the left it would collide with the mounting screw. It touches the chassis like that. But aren't the 6.3V connections isolated anyway?
    Yes, the pot on the back will be a MV.
    The toggle switch is power on-off.
    Thank you!
     
    Last edited: Sep 24, 2017
  20. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Nice neat wiring Jerry.

    On your output tubes, V5 and V6, can you confirm the filament/pin wiring is the way you intended? Squinting at the picture, they look reversed, maybe.

    I use different colored or sharpie-marked filament wires for this exact reason, to keep track of which wire is which through the twisted pair.
     
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