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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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    I am learning that Solving Problems is a big part of building Amps. Now, to solve the problem of “cutting a Big hole in the chassis” for the Previous wrong Power Transformer.

    I had some washers laying about and figured out that I could put a washer on either side of the chassis to bite down and hold the PT in place. Like a bracket. I tightened everything down and that Tranny ain’t goin’ nowhere!

    3D092593-6BE3-4590-9A29-EB303B835A4B.jpeg CDEBB909-C79B-4421-8D35-7E2208C30A47.jpeg
     
    Last edited: Nov 23, 2020
  2. BigDaddy23

    BigDaddy23 Tele-Holic

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    Excellent! I’ll be watching this build!

    BTW - my bias resistor is 100K with a 10k bias pot with that tranny. I reckon I have a 27K resistor to ground off the pot.
     
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  3. James Knox

    James Knox Tele-Holic Silver Supporter

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    Today I got the Diodes, Grid Stoppers and Screen Resistors on the Octals and mounted them. V1 and V2 sockets in ready for filaments tomorrow. Loose fitted the Bias Pot, 1/10 Power Mod Resistors and made sure the Board cleared everything. My first Hoffman Board. Man, that thing is nice!

    934AD01C-AA1E-4C1E-AF95-8C861E22AA77.jpeg
     
  4. James Knox

    James Knox Tele-Holic Silver Supporter

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    yeah, I have a 10k pot there. The fixed bias part Of this build is the “new territory“ for me. I’m going to study up on it tonight. It’s still a little confusing to me.
     
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  5. James Knox

    James Knox Tele-Holic Silver Supporter

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    Regarding the 1/10 Power Switch Mod, I put the Fuse on the bottom of the Chassis which freed up the hole on the top for the SPDT ON/ON Switch.

    Does it look like I have this wiring scheme right?

    19199243-DB22-40C8-AACE-7E85DC1F69B0.jpeg 9A8590CA-A458-4496-80EE-137EEDD31A2A.png
     
  6. James Knox

    James Knox Tele-Holic Silver Supporter

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    Today I wired up the 1/10 Power Switch, Rectifier Tube Socket, Lamp Assembly and Heater Filaments ran...


    755E7931-504E-42F2-8F2A-83F6B5DA6EED.jpeg 0E7D1DB1-2526-4C63-B80F-D686937F242A.jpeg
     
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  7. BigDaddy23

    BigDaddy23 Tele-Holic

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    Looking great, mate!

    Have you got heat transfer paste under those big resistors? It's a good idea to ensure they operate to their rating.
     
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  8. James Knox

    James Knox Tele-Holic Silver Supporter

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    No, but they are bolted down tight. I don’t have any. How important do you think it is?
     
  9. BigDaddy23

    BigDaddy23 Tele-Holic

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    I think it is required for correct rating as it takes up any air gap. It’s the white paste that you see between big transistors and their heat sinks in SS equipment. I used it in my attenuator builds where I had the big 25 and 50w resistors.

    It would be easier to chuck some in now before that chassis fills up more!
     
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  10. James Knox

    James Knox Tele-Holic Silver Supporter

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    Good point. I’ll get some! Thank you!
     
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  11. BigDaddy23

    BigDaddy23 Tele-Holic

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    It’s handy gear to have - you only need a small amount......a little goes a long way! Just wipe the excess ooze out
     
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  12. David Barnett

    David Barnett Doctor of Teleocity

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    And by small amount, that means the thinnest film possible. The least amount that will spread out to fill the area when you screw them down.
     
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  13. Kevin Wolfe

    Kevin Wolfe Tele-Holic

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    @James Knox, something you may be interested in trying on your builds.
    The terminals on the topside of the tube sockets, you can bend them downward at something like a 45 degree angle and get yourself more room to manipulate all that wire to decrease the chance of shorting something out.
    Can’t remember where I learned that, but it helps me anyway.

    Edit: Oh, and by the way, nice build.
     
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  14. James Knox

    James Knox Tele-Holic Silver Supporter

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    Ive bent them here and there when there is a lot going on. I’ve never bent them all at a 45 degree angle. It’s actually a cool idea. One could do it prior to mounting in the chassis.

    I saw this diagram from a Fender Service Manual and wondered if they were bending them....

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  15. James Knox

    James Knox Tele-Holic Silver Supporter

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    When wiring up V1, since I am using a 12AY7, do I just leave the second triode empty (no wires)? I seem to have seen something about “parallel” wiring both V1 triodes.
     
  16. Kevin Wolfe

    Kevin Wolfe Tele-Holic

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    They probably won’t fit through the holes if you pre-bend them.
    Some circuits do parallel the triodes, but others do not. I’d follow the schematic. Someone with more experience than I should be able to answer that. I myself, have never substituted a dual for a single triode.
     
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  17. gabasa

    gabasa Tele-Meister

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    I'm just collecting parts for a Harvard build myself. I didn't even know that this part of the forum existed until now, so I'll also be starting up a build thread soon.
    Subscribed and watching!
     
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  18. Emiel

    Emiel Tele-Meister

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    Nice build! Subscribed!

    I'm using the very same transformers in a DIY 5F10 Harvard. The lower 275V tap gets you right into 10% of the original 305V measurement. After reading jsnwhite619's comments I'd like to try the hotter 325V tap, to put it more into 5F11 territority. Johan Segeborn did a Youtube video on an original 5F11 and it's rocking! Mine doesn't do that, it's a bit more compressed and 'congested'.
     
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  19. James Knox

    James Knox Tele-Holic Silver Supporter

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    Since I’m only doing two instruments inputs (Rather than the Harvard three) do I just wire them like a two-input 5F1 Champ? I would like to attach the resistors directly on the tube socket and use shielded cable.
     
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  20. Kevin Wolfe

    Kevin Wolfe Tele-Holic

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    A 5e3 has two sets of 2 inputs. That would work. If the Champ inputs use 2 68k resistors, that would work as well.
     
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