Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

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
    Thanks, guys! I'll see if I can cancel the order, since it hasn't yet been processed. Thanks for being a voice of sanity...
     

  2. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    Or just keep them for your next build...
     

  3. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    I'm going to have lots of parts left over! At least 2x 16μF filter caps, 2x 8μF filter caps, 2x Mojotone (CTS) 1 Meg audio pots, 1x Switchcraft 12A mono jack, 1x amp fuse holder, 1x Carling SPST switch, 2x 6V6 Mojotone tube sockets, 1x 12a*7 tube socket, 1x Belton 12ax7 tube socket, 2x Orange Drop .002μF coupling caps, and several different values of 1/2 watt carbon comp resistors, in addition to all the 1w and 2w Allen-Bradley carbon comps I already have. A bunch of cloth hookup wire, etc., etc. It's almost like I have to build another amp!
     

  4. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    gets under your skin pretty quick
     

  5. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    I'm waiting on some new cloth hookup wire to come this week so I can finish up my board. I'm being quite liberal with the lengths I'm cutting, and I'm sticking with the color scheme Mojotone uses.

    So while I was watching (American) football this afternoon, I started working on annotating the Fender 5f2a schematic. I compared it with the layout, and I think I'm beginning to understand the signal flow through the amp a lot better. I feel confident now that I can wire up all the pots, Jacks, and tube sockets properly. I still have some more learning to do to understand how to wire up the 5Y3 rectifier and the PT and OT. I attempted to number the capacitors and resistors, but I must confess that I'm not always certain as to how they are ordered (sequence-wise). I was working here largely from @robrob's 5f1 schematic, which doesn't have quite the same number of parts as the 5f2a.
    Princeton_5f2a_schematic_annotated_2_SCJ.png
     

  6. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    Yes, basically. On the 12AX7 socket, you tie pins 4 and 5 together when wiring the 12AX7 heater for 6.3VAC. Pins 4-5 goes to one side of the heater winding, Pin 9 goes to the other side.

    B+1 and B+2 are in parallel (so they act like one joined up 32uF cap, or 40uF - if you are using 20uF caps etc)

    The one (sensible) 'mod' you could make to the 5Y3GT socket, is to add 1 x 1000V 1A protection diode in series between each HT winding end and the respective 5Y3 plate pin (Pins 4 and 6). The banded end of each diode points towards the 5Y3 plate pin. You can use 1N4007 diodes. These will protect the power transformer and filter caps from a potential short of the 5Y3GT, but they won't affect the normal functioning of the 5Y3GT forward current supply of forward voltage drop.

    See the attached article by R.G. Keen
     

    Attached Files:

    RLee77 likes this.

  7. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks, tubeswell. I like the idea of added protection, should the 5Y3 short out. I'll read Keen's article on the subject, though I imagine I'll still need more explanation. The idea of B+1 and B+2 acting in parallel makes sense, in light of the fact that the Hoffman 5f2a kit uses one big 33μF filter capacitor in the first stage.
    Hoffman_5F2A_Page_1.png
     

  8. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    When mounting the protection diodes, you can use the un-used pin lugs on the 5Y3GT socket. Run each of the PT's High-Tension ('HT') winding ends to (say) pins 3 and 5 respectively, and then run each diode between Pin 3 and 4, and Pin 5 and 6 respectively (with each diode's banded end pointing towards Pins 4 and 6) - instead of attaching the HT winding ends directly to Pins 4 and 6.

    Edit: When mounting these diodes on the 5Y3 socket, the job is easier if you mount the diodes on the socket with the socket out of the chassis. (Same thing with making a jumper wire between pins 4 and 4 of the 12AX7 socket). Its a good idea to do those jobs before you get to wiring up the amp.

    (Fender never did this back in the day, because SS diodes were bigger and more expensive)
     
    Last edited: Nov 5, 2017

  9. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    This really does create a very reliable circuit. Tubes are exceedingly resilient to momentary overvoltage conditions, but susceptible to mechanical shock and aging failure. Conversely, silicon diodes are extremely resilient to mechanical shock/vibration, and are virtually unaffected by age, but will fail immediately if their max ratings are exceeded for even an instant. So the two together is an ideal combination, each covering for the other's weaknesses.
     

  10. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    The diodes are good stuff, and Tubeswell and RLee77 give you all the expert information. I always refer to Rob's nice diagram to install them:

    diodes.png

    Or if it helps, here's a photo from a recent build:

    IMG_0063 (1).jpg
     
    Last edited: Nov 5, 2017
    jtcnj and tubeswell like this.

  11. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks very much, @tubeswell , @RLee77 , and @King Fan ! Great information. I'll circle back around to this when I wire up the rectifier. In the mean-time, I'm ordering some of these 1A 1000V rectifier diodes (1N4007) from Tube Depot. Thanks for the pics and diagram, too. Very helpful!
     

  12. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States

  13. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks very much, Rob! I've got some of these 1N4007's on the way to me now.
     

  14. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    I should have some parts here tomorrow so that I can starting working on the board again. In the mean-time, I have two questions:

    (1) How do y'all go about mounting the fiberboard in the chassis? The kit I have comes with a backing board, which I assume goes in first, and then you put the wired-up eyelet board on top of that. What are some options here? The backing board and the eyelet board are going to need to be up off the chassis just a bit, since my OT attaches to the chassis with a keps nut in the middle of the chassis, where the fiberboard will need to be placed over it.

    (2) The Mojotone kit comes with 22 ga cloth hookup wire. I'll have plenty of that to work with. However, I'll also have some 20 ga and 18 ga wire, if I need some. Is there any reason to use such thicker-gauge wire in some spots?
     
    Last edited: Nov 6, 2017

  15. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    Vintage Fender eyelet/backing boards just got screwed straight up against the chassis with self-tapping metal screws.

    The only reason you would use thicker hookup wire in an amp is for a ground buss bar, and is higher-powered amps, the heater hookup wire for the big bottles, but in a 5F2A, the heater current is pretty minimal (750mA for the tubes and 150mA for the pilot lamp) so 22ga is fine for that.
     
    King Fan likes this.

  16. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Excellent, tubeswell! Thanks very much for this information. Just what I was looking for.
     

  17. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Not only is Tubeswell right (of course he is -- I only mention it cuz conventional wisdom is often to build heaters with 18ga) but you'll save yourself a lot of grief by using 22 ga there. 18ga solid-core is hard to shape and solder through the holes in the socket lugs -- more than one builder has broken a lug while working with 18ga.
     

  18. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Professor, the only comment I'd make about your annotated schematic is the B+1 and B+2. They're both B+1 because they are at the same voltage so they are the same power node. So B+3 is really B+2 and B+4 is B+3.
     
    sds1 likes this.

  19. sds1

    sds1 Tele-Meister

    Age:
    41
    370
    May 4, 2017
    Orlando, FL, USA
    The 18ga wire is tough to work with, I would love to work with smaller heater wire when possible! What chart or guidelines do you guys go by on wire gauge and current rating? So far I've looked at 3 charts and they are all different.
     

  20. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks very much, Rob! That makes sense. I appreciate the comment and the guidance.
     

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