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Harvard in a 5F2A Chassis Build

Discussion in 'Shock Brother's DIY Amps' started by James Knox, Nov 22, 2020.

  1. James Knox

    James Knox Tele-Holic Silver Supporter

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    So when I learned that Classic Tone was folding I though I would buy some Iron for a build. I decided to build a Harvard 5F10 to be able to see/hear/experience the differences of the 5E3 Deluxe. After reading through a couple Harvard build threads, I decided to 1) use a Princeton Chassis and drill xtra holes, 2) use a 12A*7 for V1 rather than a 6AT6, and to 3) try a Hoffman board for the first time.

    I bought a 40-18021 for the PT and a 40-18023 for the OT.

    87991AA2-8AAC-4BB8-865F-BB8F75C14EF8.jpeg

    When the Chassis and Trannies arrived, I realized the PT was bigger than the hole in the Chassis. So I “modified” the chassis for the PT and drilled the xtra socket holes.

    FA7BC12D-5728-470C-80E3-BB8551604F18.jpeg 7FEA6601-9993-4975-B252-BE83C2AD0F68.jpeg

    This will be my first “Fixed Bias” build. Since the PT does not have a 50v Bias Tap, I will need to figure out how to tap into the 5V Rectifier pin. Also need to figure out what B+ voltages to shoot for.

    Open to advice and suggestions here!

    Also, if you have built a Harvard, what do you wish you had done different?
     
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  2. James Knox

    James Knox Tele-Holic Silver Supporter

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    How do you like to mount the Hoffman boards? I bought some 1/2 Standoffs. Where is the best place to drill the board so the mounting screws are not in the way of components?

    C7EA5D39-A8B7-42B6-90D9-C29F1D115143.jpeg

    I bought 4 standoffs. Do I put them at the 4 corners, or just use 2 like old school fiber boards? (Yes, I’m still putting More eyelets in...)
     
  3. Viejo

    Viejo Tele-Afflicted Silver Supporter

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    I used a Classictone Transformer when I built a Harvard in a 5F2 chassis but it fit the chassis with no mod. I don't remember the PT number but it had dual secondary voltages and that's why i chose it. I believe low B+ voltage is the key to what makes a Harvard sound like it should
     
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  4. Cali Dude

    Cali Dude Tele-Holic

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    I have an old tweed Harvard. I always wondered how it might sound with a 12ax7 in place of the oddball 6at6.
     
  5. Mongo Park

    Mongo Park Tele-Holic

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    I built a 6g2 and uses Hammond 290CAX and it worked fine, kinda the same ballpark as your build
     
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  6. Nickfl

    Nickfl Friend of Leo's

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    That power transformer is probably going to be a good bit too hot for a harvard. I'm guessing a little over 400 volts B+, which is about 100 volts higher than you want.
     
  7. screefer

    screefer Tele-Holic Silver Supporter

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    Great thread.....subscribed

    What 5f2a chassis are you using?
    Aluminum or steel?
    How did you punch the holes?
     
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  8. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    That 1/8" FR4 stuff is extremely stiff. You can probably get away with just 2 standoffs if they're 8-32, or 4 for 4-40. My first experimental build was built on an AA764 Champ board on just 6-32 standoffs. That board wasn't going anywhere.
     
  9. jsnwhite619

    jsnwhite619 Friend of Leo's

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    The 325v I use biases around 375v-380v B+, so, yeah... That's gonna be a hot one.

    Check out the Princeton Reverb bias setup, and copy it. I imagine you'll be at 400v or more with that PT.
     
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  10. King Fan

    King Fan Poster Extraordinaire

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    Great project. The Harvard is a wonderful amp. If I were gonna build another amp, it's the one!!!

    For sure, the guys are telling it like it is. Edit: Whoa, I see the Harvard wants B+ 305? FWIW, I barely got my 6G2 down to 325 (target is 315) with a 40-18027 on the 275-0-275 taps, and your 40-18021 is 355-0-355?

    The right PT is the most important ingredient on old Fenders, especially re B+ -- with current and sometimes rectifier specs getting attention too. Seems like we've seen two dozen "wrong transformer" builds here -- just in the last couple weeks! :D

    The "Princeton Reverb setup" mentioned by @jsnwhite619 is nicely detailed by Doug Hoffman here. The 10KL pot works perfectly, but sometimes you need to adjust the tail resistor (the 27K often works well on a PR). And of course you don't have to solder that resistor to the pot -- it's easier and smarter to ground it to a small bolted solder lug. FWIW, Jason is somebody who really knows the Harvard. I don't, but I seem to recall he's aware of a lot of tribal wisdom out there about voltages, the input array, OTs, etc.

    As for standoffs, *in tweeds* go as short as possible. The ¼" work just as well as ½" and give the board at least a chance to fit under input jacks and stuff. Also, your eyelets will beat turrets for these simpler boards where altitude conflicts are a big deal.
     
    Last edited: Nov 22, 2020
  11. schmee

    schmee Doctor of Teleocity Silver Supporter

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    I usually use nylon standoffs from the hardware store rather than anything metal. I just tap the metal chassis for a machine screw. But those you have will work, I have some here.
    If the voltage is too high on your power transformer your Harvard may not sound as it should.
    Can you just take the bias tap off the rectifier like the BF Princeton does?
     
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  12. rjtwangs

    rjtwangs Tele-Afflicted Silver Supporter

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    20190317_001902(2).jpg
    My Harvard, built by Michael Clark. When I ordered my amp I told Michael that all I wanted was a stock 1960 Harvard, I wanted it built as though I got into a time machine and went back to 1960 and bought a brand new Harvard. I love it, it's the amp I gig with. It has a 1961 Jensen P10Q in it. Miced up it sounds HUGE!! It's a great amp you've chosen, good luck with your build, I'll be watching as well!!


    RJ
     

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  13. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The free range electrons cause too many problems so I try to herd them by keeping the wires close to the chassis. For an electron the 1/2 inch standoff is a verrry lonnng distance.

    DSCN1451[1].JPG

    The board isn't going anywhere so three bolts should be plenty. Just a bolt with some kind of spacer is adequate for a standoff.
    I used some scrap from the board cut-off to make these standoffs. With this design the board will not rock on the standoff points. Only two bolts hold the board securely.

    DSCN1441[1].JPG

    Since you are using eyelets, some fish paper under the board is the usual insulation.
     
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  14. James Knox

    James Knox Tele-Holic Silver Supporter

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    I agree I like the lower voltages.
     
  15. James Knox

    James Knox Tele-Holic Silver Supporter

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    Good point. I think I need a different PT. What do you think about this one?
     

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  16. James Knox

    James Knox Tele-Holic Silver Supporter

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    Steel...
     

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  17. James Knox

    James Knox Tele-Holic Silver Supporter

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    Will do Jason! What PT do you recommend? I’ve read through All your Builds and learned a lot. My last build was your 5F2A 5881. Currently my favorite amp.
     
  18. James Knox

    James Knox Tele-Holic Silver Supporter

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    Definitely going to source a new PT. Probably Hammond since probably impossible to find a Classic Tone now.

    Thanks for the link to Hoffman’s Bias info. I did get a 10k bias pot! I agree, @jsnwhite has shared some amazingly stuff on his build journeys.

    Yeah, I’m liking eyelet over turret these days. A 5E3 build cured me of turrets, lol.
     
  19. James Knox

    James Knox Tele-Holic Silver Supporter

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    The standoffs did seem to long to me. I like the idea of short nylon bushings. I have some of those. I plan now to take the bias tap off the rectifier.
     
  20. James Knox

    James Knox Tele-Holic Silver Supporter

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    Yes, I’ve seen your amp on the Harvard threads. Super nice man. I’m a big Clark Fan myself. Don’t have any now but have owned A couple Beauforts and gigged a Tyger in the 90s.
     
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