1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Critique My 5F2A Build

Discussion in 'Amp Tech Center' started by Sylas Wojo, Oct 14, 2020.

  1. BigDaddy23

    BigDaddy23 Tele-Holic

    Age:
    46
    Posts:
    639
    Joined:
    Jan 4, 2019
    Location:
    Australia
    Looking good, mate. Just be sure to check the fit in your chassis - you don't want that bar to foul the input jack tips. By the looks, you shoild be fine but it is always good to check.

    I reckon one of those solder tags will suffice for support. Pop it in the middle of the long span. You have used good, stout wire for the turret connections so it will be nice and solid.

    Regarding the preamp DC supply, under the board is neater IMHO and should not pose any issues if you bring the wire up through the turret and hook it over the top (like an upside-down J). This way, you can change components on the board without fear of the wire droping out.
     
  2. Sylas Wojo

    Sylas Wojo TDPRI Member

    Age:
    17
    Posts:
    76
    Joined:
    Aug 24, 2020
    Location:
    United States
    I've been doing a bunch of research over the past few days to find out how I would implement these mods (I'll focuse on switchable NFB first) into my amp's circuit board. But I just can't figure it out... From my understanding it sounds like I'm going to need a on-off-on 3 way switch, and a 56k resistor. But I'm not sure where all of those components hook up to.

    Another thing that complicates things a bit is that I would like to also add a switch to change between OT impedances; 4 - 8 - 16 ohms to the speaker. But I guess that part is a little less important... It would be nice to have, but I'm not sure how often I would actually use a different speaker other than the stock 8 ohm that I'll install into the cabinet.
     
  3. Nickfl

    Nickfl Friend of Leo's

    Posts:
    2,274
    Joined:
    May 24, 2016
    Location:
    Florida
    I don't think those are necessary.
     
  4. Sylas Wojo

    Sylas Wojo TDPRI Member

    Age:
    17
    Posts:
    76
    Joined:
    Aug 24, 2020
    Location:
    United States
    They probably aren't, but I don't see a down side to them. If one gets in the way I can always unsolder it.
     
  5. jmp81sc

    jmp81sc Tele-Meister

    Posts:
    314
    Joined:
    Feb 6, 2015
    Location:
    Orange County
    Sylas Wojo likes this.
  6. Sylas Wojo

    Sylas Wojo TDPRI Member

    Age:
    17
    Posts:
    76
    Joined:
    Aug 24, 2020
    Location:
    United States
    Heres an update for the build:

    20201023_212246.jpg 20201023_212332.jpg

    Holes were drilled/cut for PT an OT. In the first picture you can see how I routed the OT wires, later I move the holes for the wires more to the left to clear the circuit board.


    20201028_210636.jpg

    20201031_160407.jpg

    And I wired up most of the stuff that does not directly connect to the circuit board. If anyone see's anything wrong with any of these pictures so far.. feel free to critique me.

    I have an idea for the switchable V1a bypass cap. I'm thinking of mounting 2 bolts on the v1a cap end of the board with holes drilled through them. I'll poke the 'legs' of the cap through the holes in the bolt, and tighten it down with a nut... Hopefully that made sense. Does that sound like a good idea? Is there anything that could go wrong with that?
     
  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    2,258
    Joined:
    Dec 30, 2019
    Location:
    california
    That would make a good mechanical connection. Corrosion or the bolt loosening could be a factor down the line. It would be best to follow up the mechanical connection with solder.
     
  8. Sylas Wojo

    Sylas Wojo TDPRI Member

    Age:
    17
    Posts:
    76
    Joined:
    Aug 24, 2020
    Location:
    United States
    Hmm, then maybe I should solder it to its own turrets. That would make it a bit more permanent so that's not ideal. Down the line I'll unsolder it and test out a few different cap values, if I like the sound of multiple, then I'll think about putting them on a switch.
     
    Lowerleftcoast likes this.
  9. Sylas Wojo

    Sylas Wojo TDPRI Member

    Age:
    17
    Posts:
    76
    Joined:
    Aug 24, 2020
    Location:
    United States
    So my understanding about switchable negative feedback is that to build this mod I will have to switch out the existing 22k ohm resistor with a ~10k ohm (so its not possible to 'bottom out'), and a 500k ohm linear taper pot wired as a variable resistor between that 10k resistor and the speaker jack.


    5f2a with mods.png

    Does this look correct?
     
  10. Sylas Wojo

    Sylas Wojo TDPRI Member

    Age:
    17
    Posts:
    76
    Joined:
    Aug 24, 2020
    Location:
    United States
    Pardon my terrible Microsoft paint skills :)
     
  11. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    2,258
    Joined:
    Dec 30, 2019
    Location:
    california
    That is correct.

    The pot doesn't need to be over 100K.

    The unused lug can be connected with the center lug.
     
    D'tar likes this.
  12. Sylas Wojo

    Sylas Wojo TDPRI Member

    Age:
    17
    Posts:
    76
    Joined:
    Aug 24, 2020
    Location:
    United States
    Thanks for the reply.
    *fixed*

    5f2a with mods.png
     
    King Fan and Lowerleftcoast like this.
  13. Sylas Wojo

    Sylas Wojo TDPRI Member

    Age:
    17
    Posts:
    76
    Joined:
    Aug 24, 2020
    Location:
    United States
    I measured the unloaded PT voltages at:

    Heater to heater - 7.5 vac
    HT(Red) to HT CT - 370 vac
    Yellow to yellow - 5.9 vac

    They seem a bit high, maybe too high for comfort? Anything alarming here?
     
  14. King Fan

    King Fan Poster Extraordinaire

    Posts:
    5,710
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Unloaded voltages will be high; that range is not unexpected.
     
    Sylas Wojo likes this.
  15. Sylas Wojo

    Sylas Wojo TDPRI Member

    Age:
    17
    Posts:
    76
    Joined:
    Aug 24, 2020
    Location:
    United States
    With the 5y3 installed (and filter caps not wired) I get a B+ voltage of 380vdc, does this sound like its in ballpark?
    (I tried reading the Harvard schematic, but was not sure what voltage I should be looking at)

    Also my V1 grid stoppers read an ohm-age of 33.5k ohms, but the color code says they are 68k resistors. Is this one of the situations where reading the resistance is impossible once in place?
     
  16. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    2,258
    Joined:
    Dec 30, 2019
    Location:
    california
    Yes your B+ is in the ballpark.

    They are not impossible to read. Plug a cord into the input jack and measure resistance from tip to sleeve with your meter. The *low* jack should read ~68k ohms and the *high* jack should read ~34k ohms.

    EDIT: I wasn't figuring in the 1M to ground. Never mind.:oops:
     
    Last edited: Nov 13, 2020
  17. Sylas Wojo

    Sylas Wojo TDPRI Member

    Age:
    17
    Posts:
    76
    Joined:
    Aug 24, 2020
    Location:
    United States
    Hmm very strange. I'm getting 134k ohms for the low, and 0.981M ohms for high. Would a picture be helpful?
     
    Lowerleftcoast likes this.
  18. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    2,258
    Joined:
    Dec 30, 2019
    Location:
    california
    Sorry, I wasn't thinking of the 1M to ground.

    Your readings are correct.

    Editing my former post.
     
  19. Sylas Wojo

    Sylas Wojo TDPRI Member

    Age:
    17
    Posts:
    76
    Joined:
    Aug 24, 2020
    Location:
    United States
    Ah ha that's right! Pfew.
     
    Lowerleftcoast likes this.
  20. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    2,258
    Joined:
    Dec 30, 2019
    Location:
    california
    So, if you measure from V1 pin2 to the low jack tip with the jack switch opened and closed you can get the ~68k and ~34k readings.

    Hope I have it right this time. lol.;)
     
    Sylas Wojo likes this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.