Thinking about a modified Princeton Reverb build

Discussion in 'Shock Brother's DIY Amps' started by Nickfl, Oct 11, 2017.

  1. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Offered for contrast :D
    Thanks very much for several recent threads that have got my mind going. I draw a lot of vaporware but this has a decent chance of getting built. Tweed Princeton+PR+PRII
    PR_PRII_20W_mashup_late_EQ_01.png
    Edit: likely needs the plate load resistor(s) on V3B split (82K/20K, 68K/47K, whatever) to attenuate some signal before the PI.
     
    Last edited: Oct 15, 2017
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  2. Nickfl

    Nickfl Tele-Afflicted

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    I see you have a SS rectifier on your design too, what sort of B+ are you thinking about? I also see you have a 300ohm sag resistor on the power supply, have you used that sort of setup before? I would be interested in hearing more about your thoughts on what you are looking to achieve with the power supply since I still mulling over that part of my design.

    I like the switchable negative feedback, that had crossed my mind for my build and may still get incorporated. I'm torn between wanting to keep things simple and thinking that would be a perfect use for the spot where the ground switch would otherwise go in the back of the chassis...
     
  3. Nickfl

    Nickfl Tele-Afflicted

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    Some more updates:

    Modded AA1164 schematic (1).png

    I added the passive fx loop, which I forgot in the first version. I also added switchable negative feedback with the stock resistor value, no NFB, and larger resistor bypassed by a 0.01uf cap on an on/off/on switch.
     
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  4. mgreene

    mgreene Tele-Holic

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    Hey Nick

    See the posts below for some nice info from others as well as my opinion.

    I will reiterate - the pull boost circuit in this amp does something WONDERFUL when sent to ground. I didnt study it to try to ascertain what that could actually be - I just accepted that when I completely removed it it hurt the sound of the amp (didnt measure either - probably something simple like changing the bias of one or more of the pre-tubes). It would be is easy to just clip these parts in the circuit to see if they do anything for you. After checking my files - it seems I just drew my actual mods over a printed schema in pencil. Can send if you are interested.

    http://www.tdpri.com/threads/princeton-reverb-mods-report-card.718675/#post-7588296

    http://www.tdpri.com/threads/prri-65-vs-pr-1980.717627/#post-7546100

    http://www.tdpri.com/threads/anybod...hieves-its-tremolo.709504/page-2#post-7487672

    http://www.tdpri.com/threads/anybod...hieves-its-tremolo.709504/page-2#post-7499293
     
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  5. mgreene

    mgreene Tele-Holic

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    Also - If you can go listen to Carr's Sportsman. IIRC it uses a SS rectifier and sounds great - clean. A very interesting development is that he uses Solen poly caps for PS filtering - usually employed only in HI-FI.
     
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  6. Phrygian77

    Phrygian77 Tele-Afflicted

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    Are you going to be using a PR chassis? I think the 550VAC secondary on the Classictone upgrade PTs is a bit low, even with a SS rectifier. I opted to go with a 5AR4 and the 630VAC secondary on my build because I wanted a fairly high B+ with minimal sag.
     
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  7. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Here are two of the many places I haven't taken the time to test by ear, but have just chosen a value and gone with it. 300R was a guess at emulating an old 5Y3, or a hedge against a Fender clone PT wound for too high a B+ (like a DR). My harmonic trem Princetonish has a 180R sag resistor for 15ishVDC drop, and I couldn't hear an obvious difference from the JJ 5Y3 it and the diodes replaced. Too long in between playing sessions, didn't make any recordings, too subtle for my ear. For B+ I aim for 360V because on paper the Fenders that ran higher B+, the families of amps from '63 - '65, seem to have been heavily redesigned toward more clean headroom at higher volumes, and I figure that's not what I want to achieve. And I figure I'm being more kind to tubes downstream in the supply, like the LFO or reverb driver, seeing voltages not so far out of spec for max plate voltage. But I haven't A/Bed 360V vs 420V, certainly could be missing out on something better.
     
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  8. Nickfl

    Nickfl Tele-Afflicted

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    Hmm, this has me reconsidering the tube ss rectifier switch option. Maybe I'll spec a PT to give me roughly 410vB+ through SS diodes and include a switchable tube rectifier to give me the option of dropping it down by around 30 volts, maybe a 5v4 rectifier? That would give me around 25v drop as compared to the SS rectifier and a touch of sag. If I build it this way I should be able to get a similar sound to the stock PR with the SS rectifier and then have the option to switch to the tube for earlier breakup and some sag, which should go well with the tone stack switch and NFB options...

    Yes, I am going to use one of the PRRI chassis sold by zachmdhunter on ebay. I've used them twice for builds before and they are great (and the price is really great!). I am probably going to go with an Edcor transformer. As you point out, CT doesn't have anything that is exactly what I want and Edcor seems to be a better choice when you want a non standard voltage. The Edcor xpwr014 should do nicely, 300-0-300 ht secondary with with suitable heater secondaries and it is a pretty comparable price to the classic tones. I think I will still go with a CT transformer for the OT since I want to stick a little closer to the original amp there.
     
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  9. Nickfl

    Nickfl Tele-Afflicted

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    Another update, this time I put in a tube/ss rectification switch as mentioned above. With the PT I plan on using I should get something pretty close to the stock aa1164 voltages with solid state rectification and drop 25v or so with a 5v4ga to give it a bit of sag and less headroom.

    I think this is pretty much where I want it, it preserves most of the original circuit and gives me some interesting mods to play with as well.

    Here's the schematic:

    Modded AA1164 schematic (2).png

    And a layout:

    Modded AA1164  Layout.png

    The layout is based on Hoffman's non tremolo princeton with my modifications. As far as I know it is correct, but I still need to go back and proof read it with fresh eyes.
     
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  10. Nickfl

    Nickfl Tele-Afflicted

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    I had a chance to proofread my layout against the schematic and found a couple of small errors, here is a corrected layout:

    Modded AA1164 Layout (corrected).png
     
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  11. Nickfl

    Nickfl Tele-Afflicted

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    Well, its four months later and I finally finished the build for this amp, so its time for an update! I finished everything in the chassis last night and got a chance to run through my startup procedure this evening. No shorts, no hum, and the reverb even worked perfectly on the first try! I got the bias dialed in at around 75% (with about 380v on the plates with a 5v4 rectifier) and everything was going well until I realized my tone controls didn't seem to be doing much and my raw switch was also making no difference when I switched it on and off. Eventually I realized that the DPDT switch for the raw control was bad, with the left half making no connection in any position, which left the amp permanently in "raw" mode. Fortunately I had another mini on/off/on switch on hand and swapped it in (with much swearing) and now everything seems to be working as intended.

    First impression is positive, but I didn't have much time to play after I got the tone controls sorted. I also think I may want to tone down the reverb intensity a bit before I put the chassis in the cab, it gets into outer space territory by about 4 on the dial, wich is a bit much for my taste.

    Here's a gut shot:

    IMG_20180215_223659.jpg

    And a shot showing the hairs breadth clearance between that big, beautiful Edcor PT and the coke bottle 5v4 rectifier tube:

    IMG_20180215_223751.jpg

    More pics to come when I get it in the cab, and I'll post some sound clips at some point when I get a chance to record.
     
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  12. Phrygian77

    Phrygian77 Tele-Afflicted

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    I liked my modded PR build so much that I decided that I want to try a non-reverb AA964 build. I'm still kicking myself for selling that amp.
     
  13. Nickfl

    Nickfl Tele-Afflicted

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    Today I tried a 12au7 in the reverb driver position instead of the 12at7 and that helped make it more usable, now at 4 on the dial it is in "surf" territory instead of "cartoon spring".

    I put it in the cab and played for a while this evening and it sounds real good. I think I am pretty happy with all the mods I made to the circuit, but especially the addition of a master volume. I always have a honeymoon period when I first finish a new amp, but I think I really like this one and it seems to be genuinely hard to dial in a bad sound with it.

    Here's what she looks like:

    IMG_20180216_205043.jpg

    Sound clips to come... eventually.
     
  14. schmee

    schmee Poster Extraordinaire Silver Supporter

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    I have built a few PR types. Mostly high power "turbo" ones. But also a couple stock PR's. I have tried MV on them and not liked it. The PR is a pretty nasty amp and MV makes it nastier, like it does even more powerful amps.
    I took my remaining Turbo PR and downsized it back to stock PR recently as I have too many loud amps. The one thing I left in the amp was a large power transformer. I even put the filter caps back to 20/20/20/20. But the large PT definitely cleans the dirt out of the amp some by itself... surprisingly. My main complaint with stock PR's is how nasty then can get even in smaller venues.
    BTW, I have a custom faceplate with Cut/Raw control, MV and Mid control I'm not using.
    [​IMG]
     
  15. schmee

    schmee Poster Extraordinaire Silver Supporter

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    Sorry, I just realized this thread is old!
     
  16. Nickfl

    Nickfl Tele-Afflicted

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    No need to apologize, it was an old thread, but I revived it to post about actually finishing the build, so consider it active again...
     
  17. Nickfl

    Nickfl Tele-Afflicted

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    I got an unexpected chance to record some samples today:

    First, clean with single coil pickups. The only adjustments in this track are switching between pickups, and then turning the volume up at about 2:50 and turning the reverb down.



    And cranked:



    And finally "raw". The first two are with everything set to "stock" aa 1164, with the master just turned down just below max. This track is with the tone stack lifted and the NFB lifted:

     
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