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New (and first) build: 5f2a Tweed Princeton

Discussion in 'Shock Brother's DIY Amps' started by theprofessor, Nov 2, 2017.

  1. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    So use flat washers over the tabs of the OT?

    I have been looking at this, and I can't figure out any place where one of the tabs of the OT wouldn't be under the eyelet board, unless I moved the whole thing way over by the PT. Any suggestions?
     
    Outlaws likes this.

  2. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    I reviewed my 5F2A builds as well. I can’t see how to get the one OT mounting hardware out from under the board either.

    The chassis is long enough that the board could maybe be moved toward the far end of V1 I suppose. Then maybe the OT fasteners may be fully exposed. I’m not sure I’d go through the trouble. If by chance, some day you want to play with different OTs you would just have to partially lift the board.
     

  3. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    That seems right to me, Keith. I think I'm just going to mount it where it would normally go. I don't really see an alternative, and the likelihood of me changing the OT in the future (other than for failure, for some reason) is very small.
     

  4. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    What did Mojotone send you for a Power Transformer? I thought I read somewhere that Mojo transformers that are made in the USA are built by Heyboer.
     

  5. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Hi Keith - I think I've heard that, too. The PT is here (MOJO 760): http://www.mojotone.com/amp-parts/T...d-Blackface-Princeton-Champ-Power-Transformer
    And here is the wiring diagram for the PT:
    MOJO-760_Power_trans_wiring_diag.jpg

    As I said in one of the posts above, the page for the 5f2a Mojotone kit states that it comes with a Blackface output transformer, but that is not the case. It comes with the Blackface Champ transformer (Mojo 771). http://www.mojotone.com/amp-parts/Transformers_1/Blackface-Champ-Output-Transformer
     
    Last edited: Nov 3, 2017

  6. screamin eagle

    screamin eagle Friend of Leo's

    Oct 9, 2008
    S. CA
    I've got a 5c2 clone (2 channel however) and it's a great amp. Good luck and enjoy the amp.
     

  7. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ


    Well it appears not to be a goer in this case. I'd just make the hole where the mounting nut won't affect the board mojo too much.
     

  8. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    That looks to me like (1) 1-Meg resistor (connecting sleeve and tip of input 1), (2) 68k resistors (connecting tip of both inputs to the board), and (1) 1.5k resistor that is connected to the 25μF one on the far right side of the board. Right?
    Here is the Weber 5f2a layout. Weber_5f2a_layout.jpg
     

  9. jtcnj

    jtcnj Tele-Meister

    Age:
    53
    355
    Feb 2, 2015
    Brick, N.J. USA
    King Fan likes this.

  10. Outlaws

    Outlaws Tele-Meister

    156
    Jan 16, 2007
    None
    Yup. Even just a hair more surface area goes a long ways.
     

  11. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    The optimum situations for metal film resistors (for a quieter noise floor) are the resistors in the signal path. (This includes plate resistors, grid leak resistors, tail resistors in LTPs, and cathode load resistors in cathodynes, any resistances in the tone stack, grid stoppers and input resistors)
     

  12. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thank you!

    I went ahead with mounting the output transformer where it seems it's "supposed" to go. I almost got myself an extra hole in the chassis by getting mixed up by existing holes when I turned the chassis over. Thankfully, I was cautious enough and did not finish the hole before I realized what I was doing. And as it turns out this aborted hole is now under the OT. So it's invisible to the naked eye. The new hole is the one on the right. It's out of focus, but the machine screw does not come up even to the surface of the keps nut. So the under-the-board space is only taken up by the nut height.
    IMG_3994.JPG
     

  13. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    I also reinforced the tabs of the OT with a couple of fender washers, as @Outlaws suggested.
    IMG_3998.JPG

    I also decided to add some toothed lock washers for parts that didn't come with them, like the Switchcraft 12A shorting jacks.
    IMG_3996.JPG
    You can see the toothed washer in the picture under the speaker jack. You can also see that I wasn't going to let @King Fan be the only one who got in on the braiding fun!
    IMG_3995.JPG
    For all those building a 5f2a kit from Mojotone, you'll see that they have the chassis drilled for a female RCA jack, such as those used on the 5f1 Champs. However, you don't use those holes for the 5f2 1/4" speaker jack.
    IMG_4001.JPG
    So I've got most of the chassis together now. I'll begin working on the board this evening and will post pics of that soon. Here is the chassis. I don't love it that my Alpha 24mm 1Meg pot with SPST switch (for the tone) does not have as long a shaft as the Mojo/CTS one that comes with the kit (compare the volume pot). The shaft and body is not as wide, either. But I can make it work. I'd really like to have the feature where I can defeat the tone pot or use it if I want.
    IMG_3999.JPG
     

  14. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thank you, and thank you very much for pointing me to the Aiken resource on resistor types. Those white pages are magnificent (even if I'm only understanding parts of it)!
     

  15. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    This is your first build right? Get ready to systematically remove, what seems like everything that you installed in the chassis. One piece at at time for some reason or other, as you advance along the build. LOL. I'm kidding. Not really. I swear at some point or another it felt like everything was removed again somewhere down the line. Oh, and all that nice fancy writing on the F&T caps? Gone... You seasoned builders, you know what I am talking about and you're laughing.

    I am enjoying the build Professor. Good work. Keep the updates coming. For an amp junkie. the only "hit" better than building your own amp is watching someone here go through the process!
     
    King Fan and FenderLover like this.

  16. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks very much, @tubeswell . At this point, I think I'll just replace a few at the beginning of the signal path and then keep the carbon comps from there. I'm tempted to put the metal film everywhere you mentioned, but then I'd have a bunch of 1/2 watt carbon composition resistors that I probably would never use. So I'll simply say that I believe in the "magic" and "harmonic distortion" of the carbon comps, even if I don't know if I do. But just because it's what I have!

    Here is my populated eyelet board. You'll see that the 1.5kOhm resistor is hanging on for dear life. He's just there to show you where he's gonna be soldered on (roughly). I'm gonna replace the (2) 68k carbon comp Xicons with metal film ones that are on their way to me. And the schematic calls for 8 μF filter capacitors in the B+3 and B+4 positions, but since I wanted to use F&T caps, I went with 10 μF (they don't have 8 μF).

    Three questions:

    (1) Any issues here? It sounds like the "indented" (positive) end of the filter capacitors go downward in the chassis of a 5f2a, where the "regular" (negative) side of them point upward in the chassis, toward ground (not the ground, but electrical ground!). Also, the foil-ends of those Mallory 150 coupling caps are pointed downward. Is that right?

    (2) @robrob : In your "How to Build a Tube Amp" document (for all you readers who would like to reference it, it's here), you suggest the possibility of using some light glue under the filter caps or possible even a zip-tie around the board and the caps to keep the caps from hanging from their leads. Can you recommend a certain type of light adhesive for this? Maybe a dot of silicone under each one?

    (3) If all looks ok with the board, how does one treat the leads on the resistors and capacitors when preparing to solder them into the eyelets? Do you clip them at about the level of the eyelet, or just below, and then treat the eyelet as if it is a flat surface to which one solders the end of the lead? That is, how much lead is left on the back-side of the eyelet board? None? A little?

    Thanks!
    IMG_4412.JPG
     
    Last edited: Nov 3, 2017

  17. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Oh, and I meant to say: As I put the resistors on the board, I took a pencil and wrote in their actual measurements in pencil on the Mojotone 5f2a layout.
    IMG_4413.JPG
     

  18. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    Yes, Positive poles of polarised caps (electrolytic caps) go towards the 'more-postive voltage' parts of the circuit. (Note that in amps where a negative voltage is required for a bias voltage source, the +ve pole of polarised caps needs to go to ground, which is, relatively, at a more positive potential than the negative voltage output of the bias supply. But you don't need to worry about that in this type of amp, because all the tube stages in the 5F2A are cathode biased). Talking about polarised caps in this way, is different from the discussion about the orientation of the outer foil on (non-polar) film caps.

    Film caps are made by rolling two foil sheets together* with an insulating film between them. One lead connects to one foil sheet. The other lead connects to the other foil sheet. As these two foil sheets are rolled up into a cylindrical shape, part of one of the foil sheets is left forming the outer layer. The lead connected to outer foil of film caps should go to the low-impedance side of the circuit, for better signal shielding. For a coupling cap, the low-impedance side is the dc/plate side. For a plate bypass cap, the outer foil goes to the (pure DC) B+ side of the cap (i.e. the opposite of the plate side) - don't worry, there are no plate bypass caps in a stock 5F2A circuit. If using a (non-polarised) film cap as a cathode bypass cap, the outer foil goes to the ground side of the cathode resistor - however, (due to the demands of physical space saving, and the desire to have as-large-a-capacitance as possible for cathode bypassing in a 5F2A) all the cathode bypass caps in a stock 5F2A are the polarised electrolytic type, where the +ve pole of the cap goes to the more-positive voltage side of the cathode resistor (which is the cathode side).

    *electrolytic polarised caps are constructed the same way, but the dielectric barrier between the capacitor electrodes is maintained by a one-way polarised charge, which quickly breaks down if the charge is (absolutely) reversed, which is why polarised caps self-destruct if you run a reverse voltage across them
     
    Last edited: Nov 3, 2017

  19. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Just wanted to point out that it's a better practice to use a schematic to determine polarities for components than a layout diagram. Layouts show you where components go and the general routing of wiring, but for actual connection detail, use a schematic. I would also very much recommend that when wiring up an amp or any device, use a highlighter to mark the schematic as you make/solder connections between points... that way, you know when you're done. It's the electrical equivalent of checking off items on a checklist.
     

  20. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks, RLee77. I'm looking at a 5f1 schematic and layout right now (admittedly not the same as a 5f2a). I've gone to read @robrob 's "How Tube Amps Work" document again to see if I can get a handle on reading a schematic and comparing Rob's annotated schematic of the 5f1 with the annotated layout of a 5f1. But I admit that I can barely swim, and I've decided to cross the Channel.

    I'm trying to make sense of @tubeswell's comments above, especially vis-a-vis the low-impedance side and the high-impedance side. The coupling, or "blocking" caps (in my case, the Mallory 150s) are supposed to have their foil-end toward the low-impedance side. Since a low-impedance circuit is a heavy load that allows AC current to flow, and since the coupling, or "blocking" caps block DC voltage, wouldn't it be the case that in the 5f2a, the foil-end is downward? The signal comes off the plate of V1A and straight into the "bottom" of the first coupling cap (C1) and then later comes off the plate of V1B and straight into the "bottom" of the second coupling cap (C2). So I have them oriented correctly, do I not?

    I feel a bit like I just learned to dog-paddle and folks are telling me how to do the butterfly... But I'll get it eventually (at least the crawl, maybe not the butterfly).
     
    tubeswell likes this.

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