Finally -- Princeton Reverb

Discussion in 'Shock Brother's DIY Amps' started by theprofessor, Apr 9, 2020.

  1. theprofessor

    theprofessor Poster Extraordinaire

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    A great amp for strange times. Or for any time.

    A friend and I have a sort of collective going. He's a woodworker by trade and has helped me get off the ground in learning to make T-style guitars. Out of that collaboration have come my Cuban Mahogany T-style and my Walnut T-style.

    So now it's my turn to teach him how to make an amp. We've decided on a Princeton Reverb. And if I am going to help him build one, that means I should build my own, too, right?

    So that's what I'm doing. Ever since I built my first Princeton Reverb for someone else, I have wanted one for myself. And that one sounded so good, my plan has been to build one just like it -- a modern PR clone, using Hoffman's layout and a Deluxe output transformer.

    And when I say "teaching," I fully recognize that I'm no expert. I know enough to do what I need to do and to make a good amp. This will be my sixth amp build, but I have _loads_ yet to learn. Please feel free to pitch in and comment on anything as you see fit. In fact, I rely on you all to look over my shoulder to make sure I'm not doing something bone-headed and to explain stuff to me when I'm stumped (@FenderLover ..., @D'tar ..., others...).

    As usual, a lot of what I do in my threads is think out loud and document things for later builds or for others. I'll try to attend to several of the peculiarities of the build and its parts as I move along.

    The plan was essentially to make an exact copy of the last Princeton Reverb I built. That one used a ClassicTone 40-18027 power transformer, a ClassicTone 40-18002 output transformer (spec'd for a Deluxe), and an NSC reverb driver.

    The ClassicTone 40-18027 PT has been hard to find. Though Tube Depot stocks a good number of ClassicTone transformers, they carry neither the 40-18027 PT nor the 40-18002 OT. So I had to look elsewhere. The only place I found the 40-18027 PT was Triode USA, who does not seem to be open currently. I did find the 40-18002 OT from a seller on ebay, and I got a ClassicTone 40-18034 reverb driver from Tube Depot. So it was going to be all ClassicTone, but...

    Given that the 18027 is difficult to source even in the best of times, I decided to look elsewhere for a PT. The 18027 is spec'd at 315-0-315 for the high-voltage wind, with 100mA current capability, a center-tap on the heaters, and 5V/3A for the rectifier filaments. Speaking of rectifier, I've got an ANOS Mullard GZ-34 in my box that I'll be putting to use on this one.

    Classictone PT 40-18027.jpg

    So I started looking around for a comparably spec'd power transformer that won't give me crazy-high voltages. The Hammond 290AX, which is their Princeton Reverb transformer (among others), used to be spec'd for lower current, I believe, and with less amperage to the rectifier (EDIT: I just found the old specs: 325-0-325V@70mA, 5V@2A, 6.3VCT@2A). The new spec sheet lists the 290AX as 325-0-325V@100mA current rating, a heater center-tap, and now 3A to the rectifier filaments and 2.25A to the heaters, as @jsnwhite619 noted here about 15 months ago. In other words, it's identical to the ClassicTone 18027, save the 10V higher rating. We'll see if that is indeed the case when I get mine in the mail from Mouser in the next couple of days. If so, I think it will just barely meet the voltage maximum and current minimum recommended by @muchxs in this thread.

    Hammond 290AX (recent spec)
    Hammond 290AX.png
     
    Last edited: Apr 9, 2020
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  2. theprofessor

    theprofessor Poster Extraordinaire

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    On the recommendation of many here, I decided to get my chassis from the seller "zachmdhunter" on Ebay. I believe he may sell on Reverb as well. Since my buddy and I are both building one, I purchased two at a discount: $40 each, shipped. Man, that's a great price. Really well made, 16ga galvaneal, etc.

    Here it is with a bit of work done on it, sitting in my rough-and-ready "amp cradle" that is in no way fancy.

    As with my brown Princeton 6G2 build, I used Sandy Henry at Precision Design for my faceplate. I've got a Mojotone backplate on order. Ms. Henry's work is very nice, but it is expensive. I took my 6G2 design and name ("Lenox," after the Lenox House where a lot of my coursework was held in Princeton) and simply added "Reverb." To follow the Fender model, it would have said Reverb-Amp, but either we couldn't fit that on the design, or it didn't look right, so I just went with "Lenox Reverb."

    I'm not much a fan of the mid- to late-60's style skirted knobs on blackface amps, so I decided to keep the numbers around the dial. I'm still undecided on knobs at this point.

    IMG_2165.JPG

    I got the cap-can in and a little terminal block where the power cord will join up with the PT. The 3/8" grommets, and the 8-32 x 1/4" screws and keps nuts for the reverb driver.

    IMG_2167.JPG

    Speaking of the cap-can, I'm using a JJ 40-20-20-20 as I did on the last PR build.

    IMG_2173.JPG

    And if anyone needs a decoder-ring for the JJ labels X, Y, 0, and so forth in relation to the Princeton Reverb layout's abstract hieroglyphs, @King Fan has one somewhere on TDPRI. I can't seem to find it at the moment. At any rate, the "Y" on the JJ cap can is the 40μF node, and the rest are 20μF.

    IMG_2179.JPG

    Oh, and still speaking of cap-cans, @Phrygian77 notes in one of his PR builds, I think here, that the one-piece cap-can clamp that Hoffman sells does not line up with the mounting holes in the Mojotone chassis (or in the zachmdhunter copy, for that matter). As you can see in the pic above and in this one below that the one-piece cap-can clamp needs wider mounting holes. I drilled those with a 1/8" bit, just outside the ones already punched there. Apparently, the two-piece clamp that AES sells does fit the Mojotone mounting holes, but I have not tried that personally

    IMG_2171.JPG
     
  3. theprofessor

    theprofessor Poster Extraordinaire

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    Now for pots. I never have liked using the vinyl-shaft pots that Hoffman sells. I try to get everything I can from him, but I just can't deal with the vinyl shafts. I used to think that Mojotone was the only place for the solid-shaft pots for values like 3M-RA and the like, but the ones I bought from Tube Depot are in fact "Mojo"-branded. So this amp has a mixture of CTS pots from StewMac and CTS pots from Mojotone via Tube Depot.

    The StewMac ones have the solid silver-colored shafts, and the Mojotone ones have the solid brass shafts.

    IMG_2176.JPG

    The volume pot on the right is from StewMac. The treble pot on the left is from Tube Depot, and it is labeled "Mojo."

    IMG_2177.JPG

    A little backer board under the pilot lamp, held down with Liquid Nails.

    IMG_2168.JPG

    And one for the input jacks, too. I'll take those out and wire them on the front of the chassis before I install them permanently.

    IMG_2178.JPG

    Now to wire up the connections on the back of the turret board and to lace up the turrets with bus wire.
     
  4. D'tar

    D'tar Friend of Leo's

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    Sweet!

    But you are theprofessor!!!

    Subscribed:)
     
  5. E5RSY

    E5RSY Doctor of Teleocity

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    Awesome. Thanks for sharing this.
     
  6. BigDaddy23

    BigDaddy23 Tele-Holic

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    Awesome stuff, mate!

    Seeing as your made is the woodworker ,what are you planning on doing for the cab? Fancy wood or straight pine with tolex?
     
  7. theprofessor

    theprofessor Poster Extraordinaire

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    Well we decided to go the economical route on that and simply make cabs out of something he had lying in his shop -- some really nice 3/4" pine plywood and baffles out of Baltic Birch. I'm going to venture to do my first Tolexing job on mine. He's making the cab for me.

    While I'm going for a more traditional, refined aesthetic, he's going for something more industrial looking with his. This is not finished, but check out the steel corners he bent that serve both as corner protection and as the chassis straps. And the metal beehive-looking grill is fantastic, too. I think he's going to make his own faceplate.

    CAB_1.jpg CAB_2.jpg CAB_3.jpg
     
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  8. muchxs

    muchxs Doctor of Teleocity

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    I'll say right up front 'cuz I read your lengthy first post...

    Lotta folks don't remember that Bruce Collins designed a few transformers for Weber VST way back when. They're very nice! They're made with rice.

    Seriously, the W022772 is a Bruce Collins design. I'd say it's the more successful of the Collins designed transformers, no reflection on Bruce. It's one thing to get the design right, it's another matter to build it properly.

    Weber VST could easily discontinue their entire parts and kits line. Concentrate on speakers. Get 'em while you can!

    Considering how many I built... I don't have one of my own. I have big chunks of Princeton Reverbs, none complete.

    I don't get to keep the next one, either.

    Workin' on it...

    Good choices!

    Another way to go would be Allen Amps TP25 and TO26.

    Ultimately I'd like to have two Princeton Reverbs, one blowing through a MiniMass for my overdrive tone and another one for my clean tone. Clean tone version would be TP25, TO26 with real Tung-Sol 5881s.


    Couple notes:

    Why build the next one like the last one? Incremental change = evolution

    While I appreciate the work Doug Hoffman put into the Hoffman layout, I'm not feelin' it. I'm sure you've heard me mention feng shui in reference to layout. I'm more into the Tao of Leo. (Pronounced "Tee- oh of Lee- Oh)

    :D :D :D


    IMG_2015.JPG
     
    Last edited: Apr 10, 2020
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  9. tubegeek

    tubegeek Tele-Holic

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    Not "Tay-oh of Lee-oh"? I'm confused.*

    Beautiful boards!

    *my natural state.
     
  10. muchxs

    muchxs Doctor of Teleocity

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    Because "Dough of Low" doesn't look right, doesn't sound right, doesn't fit the meter and is even more inscrutable in context.

    :D :D :D
     
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  11. tubegeek

    tubegeek Tele-Holic

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    More inscrutable = more betterer

    I think?
     
  12. theprofessor

    theprofessor Poster Extraordinaire

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    That's really good to know about the Weber PT. Did you mean the WO22772? This one? I did consider the Allen transformers. I believe the PTs are made by Heyboer and the OTs are made by Marvel? Or the other way around? Anyway, the only reason I didn't go with those is because they're more expensive. And I know I can get the sound I want from these, since the sound I want is the sound I had last time. And I guess I don't need a clean machine with this one, either. Instead of vintage Tung-Sol 5881s (which I do have in my 5F4 clone), I'm going to be using a pair of '50's gray glass Sylvanias 6V6s.

    IMG_2182.JPG

    And that goes to why I'm doing things the way I am this time. I totally understand the idea of having these Princetons evolve. It sure seems like they've been good candidates for evolution for quite a long time now, from the 5F2 to the 6G2 to the blackface and silverface models to the Super Champ to the Mesa Boogie amps, etc. I have a boring reason or two why I'm not yet on the path to "perfecting" the evolution:

    (1) I know I liked the last one that I built this way, so I'm doing it again. I too prefer the Fender layouts in some ways, as I feel like I understand them better. I have a hunch, though, that Hoffman's layout led to a really quiet amp. So I've decided not to move away from what I found successful at this point.

    (2) I am still in the clone-making phase. It may be the only phase I'm ever in, but I have a ways to go yet to truly understand design well enough to modify and to know exactly why I'm modifying. So until then, I'll just be boring.
     
    Last edited: Apr 10, 2020
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  13. muchxs

    muchxs Doctor of Teleocity

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    We shouldn't hijack @theprofessor 's thread.

    :rolleyes: :rolleyes: :rolleyes:

    Hey, Professor...

    Where are the chassis mounting holes in relation to your burly corner protectors? I anticipate some heartfelt cussin'.

    :oops:
     
  14. theprofessor

    theprofessor Poster Extraordinaire

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    That's my buddy's cabinet he's making for his own PR. The chassis-mounting screws go through the top of the bent steel and into the chassis. That's how the chassis is mounted in the cab in the pics above. So, in essence, the chassis-mounting screws through the top of the amp and the steel connect to the chassis and make it a part of the corner joint, since the steel corner is also attached to the side of the cabinet. The corner is also the chassis-strap.
     
  15. muchxs

    muchxs Doctor of Teleocity

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    Aw, crapazoids. I usually try to tidy up my numerous typos. Went back and fixed it.

    Yeah, that one. If I had a five minute free shopping spree at Weber VST I'd grab a bunch of those. They were $44 a pop until recently.

    Allen transformers are typically Classic Tone PTs and Heyboer outputs.

    Respect to a dude who knows what he wants and who knows how to get it. The way he got what he wanted was to design his own line of transformers. They're a little more expensive but a lot more capable. They're usually twice as good as the expensive brand at half the price.


    My most memorable Princeton Reverb has an exactly precisely OMG! matched pair of those Sylvanias out of an old Hammond organ. Those are the "Easter Egg" in your build IMO, the really cool parts that indicate you're serious.

    :cool: :cool: :cool:


    No justification required. "Because I want to" is ample justification.

    :cool: :cool: :cool:

    Oh.

    (Wish I thought of that!!)

    :D
     
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  16. theprofessor

    theprofessor Poster Extraordinaire

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    Got a bit more work done today. My output transformer came in the mail, so I got that installed. I decided to put the keps nuts on the outside of the chassis, to make it easier if I were ever to need to replace the output transformer. It seems simpler to remove those than to try to dig them out from under the turret board, or worse, to have to take the whole board off to get to them. I used 3/8" 8-32 machine screws to mount these, since I have flat support washers under the tabs of the OT.

    IMG_2183.JPG

    Speaker came in the mail too. I'll be using an Eminence GA10-SC64. I've only owned the 12" big brother to this one, but I think it'll go well with the PR circuit.

    IMG_2184.JPG

    I also laced the turrets, using Hoffman's image as an exemplar. As he noted, this image is of his original design, which had two 100 ohm resistors on the far right of the board for an artificial center tap. He has since deleted those four turrets, so if you get one of his boards, you simply ignore the last two columns on the far right of that image.

    IMG_2185.JPG

    IMG_2186.JPG

    I'll double-check all these tomorrow and tack-solder them. Then on to the back-of-the-board connections. And never let it be said that I enjoy lacing turrets.
     
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  17. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    Noticed the braided blu/brn/red OT primary leads.

    This is a test from the professor. Isn't it?:D

    Let's not lose your highs. Un-braid them.:cool:
     
    Last edited: Apr 10, 2020
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  18. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Good luck with it! As far as the transformers, the 290AX and Hammond 1750E OT through the Cannabis Rex was more amp than I ever needed gigging. It was stout! I used the 325v taps and 5u4 rectifier. I think I ended up around 410-415v plate voltage, but it's been a little while. Still the best amp I've built.

    I think that the only mods I did was 3 way NFB switch, and I changed a resistor that either decreased the amount of reverb send or increased the dry signal...either way, the intent was to decrease the reverb so that it was useful past 2 on the knob. And I used 4x 22uf F&T caps on a board instead of a cap can.
     
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  19. tubegeek

    tubegeek Tele-Holic

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    100 or 200pf? Against 8 ohms? Really?

    I'm looking at 1uF being what you'd need to get a 20Hz cutoff here. I don't think my 100pF estimate is off by THAT much?
     
    Last edited: Apr 10, 2020
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  20. tubegeek

    tubegeek Tele-Holic

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    Oh wait...

    PRIMARY leads....

    15Kohm against 500pF for 20KHz, -3dB cutoff. I guess just maybe, you COULD hear this... my bad...
     
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