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Building a 5F2A. Here Goes.

Discussion in 'Shock Brother's DIY Amps' started by Tapatio, Apr 16, 2018.

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  1. Tapatio

    Tapatio TDPRI Member

    81
    Jun 13, 2016
    New Jersey, USA
    I was inspired by a recent thread by the theprofessor who recently built a 5F2-A Princeton. See:

    http://www.tdpri.com/threads/new-and-first-build-5f2a-tweed-princeton.779349/

    The best thread I’ve seen in a long time. He had no shame in asking all the questions the rest of us were thinking about anyway, all answered by the more experienced folks in the community. It made a fantastic resource for new builders.

    I’ve committed to build one too as a challenge so I can refine the skills necessary for more ambitious projects later on. These are my initial thoughts:

    -Switchable to exclude the tone control so it can be a Champ circuit as well.

    -Switchable negative feedback –on/off, as well as choice of none, 10uF or 25uF cathode bypass caps on the 2nd stage 12AX7. Or maybe a choice of different power tube cathode resistors (which is best? worth it? Silly idea?). What would be better, varying the distortion amount or the bass response?

    -10” Speaker. Have 2 on hand: New Weber alnico Sig 10 & a Jensen C10Q well broken in.

    -Old pine for the cab. Tweed covering. Custom cast bronze emblem (more on this idea later).

    -My own turret board, to be laid out being mindful of changing out parts if necessary down the line.

    -Hoping to get B+ right without the need to tweak with zeners, etc. but willing to mod if necessary. PT choice is a Classic Tone 40-18109 for flexibility: US 100, 110, 120V plus Euro primaries; Secondaries of 660, 630, 550V. OT will be Classic Tone 40-18031 with choice of 5 or 8K primary and 4, 8 & 16 ohms secondary.

    -F&T filter caps and fancy Sozo blue coupling caps. Disclaimer: I don’t buy into boutique parts hype because I can’t normally hear much difference. I’m simply going for pretty here… I like the F&T’s look, price and temp rating in comparison to the blue Spragues. The Sozos’ values are bang-on every time. (I did draw the line, however, at the new $8.00 Sozo silver mica tone cap vs. the regular $0.70 one).

    -I’m using metal film resistors throughout. No carbon comps, I can’t really hear much there.

    Any thoughts/comments are greatly appreciated!

    All the parts are in, I just need to decide on the switchable mods. And please don’t look at the horrible speaker cabinet with the Ikea fabric. That’s old…


    Parts_5F2A.jpg
     
    dan40 likes this.

  2. totterer

    totterer TDPRI Member

    Age:
    41
    20
    Mar 19, 2018
    Nebraska
    Howdy! Im on the same road, I have a thread over in the glowing bottle forum. http://www.tdpri.com/threads/5c3-questions-1st-build.814356/ I've also read through the professor's build about 5 times. Im sure ill be referencing it throughout. I just finished upholstering my cabinet with tweed. I used hide glue and highly recommend it. It seems people are scared of it, but it is completely forgiving. It goes a bit slower than using contact adhesive but its cool working with traditional materials.
    Im a bit held up at the moment. I received a small parts kit from Mojotone, and just realized its missing 4 parts! I didn't receive a response today and am getting impatient. I figured Id have to order some more parts anyway, but still annoyed by this setback... I see you have a Classictone 40-18031, I have the same so I can run bigger tubes. Im also changing the cathode bias resistor from 5W to a 10W, to handle more current. What power transformer did you get? Good luck with your build, it sounds like you have a good handle on it!
     

  3. kleydejong

    kleydejong Tele-Holic

    587
    Aug 30, 2010
    Orange City, IA
    Good luck! I have a soft spot for the 5F2A as it was my first dive down the rabbit hole as well.

    Easy. As I understand it, the tone control on a 5F2A is a combination of a high pass filter and a variable bright cap. The signal enters the tone pot on the wiper. Then it either bleeds high frequencies to ground through the .005 uf cap connected to ground - or it allows those same frequencies to have a straight shot past the volume control with no attenuation.

    You could use an SPDT switch where the signal leaves the .022uf coupling cap before it hits the wiper. Send it to the middle terminal on the switch. In the up position it is routed on as normal. In the down position you leave it blank so it gets lifted.


    Also very easy. Put a SPDT switch in line with the 22k nfb resistor from the speaker output to the V1B cathode.

    I'd also perhaps recommend Rob's 5E3 NFB switch mod. It uses an ON-OFF-ON SPDT switch instead. It adds in a cathode bypass cap to the equation for even more range from clean to dirty.

    I've tried a lot of mods on a lot of amps. This is perhaps the most universally applicable mod that can offer a very different tone and feel that is almost always usable while also giving you the option to have the purity of the original design.

    I don't know if you'll experience a meaningful difference between 10uf and 25uf. In my experience 5uf and higher provide a full frequency boost for electric guitar that is all pretty much the same. 1uf is tighter. Marhsall's .068uf is also a popular choice. So I'd consider dropping the 10uf to something more in that range.

    I wouldn't tweak the power tube cathode resistor unless you plan on doing something like a 6V6 / 6L6 switch.

    I wonder if you meant 'preamp' tube cathode resistors. If so, I spent some time with my 5F2A on the bench a while back using alligator clips to audition a range of cathode bias resistors from 820 ohm to 10k. This begins to touch on stuff I don't fully understand, but I believe the value of the resistor sets the bias of the tube. The bias of the tube can certainly affect the tone and feel of the amp. Especially when it comes to how that tube clips as it gets pushed into distortion. My testing was limited though and I had a kind of hard time really hearing the differences. Especially on a clean tone. I would like to revisit this.

    Ultimately I personally would first choose the bypass cap mod first because controlling low frequencies can be very important to me.

    My last bit of advice - when I built my 5F2A I finished it as bone stock to the original. Then once I got through initial startup and confirmed the amp was a success I dove into mods. I found this to be very helpful. Startup can be a challenge and you may need to debug. Easier to do so without mods. It also gives you a good feel for your baseline starting point. You may discover some mods are more or less desirable once you hear the amp as it is.
     
    JuneauMike and Tapatio like this.

  4. jimgchord

    jimgchord Tele-Meister

    Age:
    44
    128
    Apr 3, 2018
    victor,ny
    The Weber should work out good for you. I have the more expensive 10a125 and a sig series, I prefer the cheaper one. Good choice on the ot, I've used several of that particular one.
     

  5. SSL9000J

    SSL9000J TDPRI Member

    Age:
    46
    8
    Dec 18, 2016
    North of Atlanta, GA
    My first build started as a good ol' 5F1 Champ that morphed into something more like a Princeton when a tone control was added. In retrospect, it was a mess, noisy as hell, but the fact that it worked at all had me excited enough to keep going.
    Recently I built a Vibro-Champ with a similar circuit design to a Princeton. (12AX7 / 6V6GT) Tried two different Jensens, C10R (ceramic) & P10R (alnico) and went with the P10R. Though you can't really go wrong with either, both sounded good.
    Be careful and good luck!
     

  6. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    @Tapatio and @totterer - I'm very, very happy that my (lengthy!) thread on the 5f2a has been helpful to you. That's exactly why I did it, and it gives me a lot of pleasure that others can learn from it. I'm excited to watch these builds. The 5f2a is a fantastic amp! Cheers!
     

  7. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    I prefer a pot in series in the NFB loop because I like finer adjustment than on/off allows. Ommv.
    Ymmv on this too, but an eyelet board might be easier to deal with if you want to replace components??? Maybe I feel this way because I am more familiar with eyelets, and/or most of the turrets I have dealt with are more problematic to undo because of the wraps on the turrets.
     
    kleydejong and King Fan like this.

  8. totterer

    totterer TDPRI Member

    Age:
    41
    20
    Mar 19, 2018
    Nebraska
    Ok I see now you are using the 40-18109. Ive got the 40-18019. Thanks for the detailed parts rundown. Its making me rethink my own choices!
     

  9. jimgchord

    jimgchord Tele-Meister

    Age:
    44
    128
    Apr 3, 2018
    victor,ny
    That's why you put the components n the holes
     
    Wally likes this.

  10. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    I understand that...but that is not always how those are done. Many times build techniques, even invery well built old school handwired circuit, are not conducive to repairs, ime.
     
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  11. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    ^^this. A full NFB cut is a fairly big step on a 5f2a, so a pot is cool. Admittedly many people end up preferring just one or two places on the pot, but at that point it's easy enough to replace the pot with a switched NFB circuit.

    Great choice to build a 5f2a. Agree with Kley about not modding the little guy too much, but even more I always say build stock first. Sure, plan ahead (eyelets, turrets, chassis holes). But if the amp doesn't work perfectly on fire-up, mods make trouble-shooting painful. If it works, you'll be all set to know what mods you actually want or need. Mine ended up with a rotary off switch on the tone pot and a (subtle) single-option NFB cap/resistor switch. Perfect.
     
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  12. Tapatio

    Tapatio TDPRI Member

    81
    Jun 13, 2016
    New Jersey, USA
    Thanks for all the great responses! I think I will take this wise advice and keep things simple. I've got it down to the switch for the tone control, and I really, really like the NFB pot idea.
    I'm working on the final layout right now before I start drilling the board. I hope to get something up tonight for your valuable ideas. BTW, one day I'll try eyelets but I still have about 8 feet of brown garolite board I bought many years ago. I also have a sackful of hide glue crystals for the tweed when the time comes. I've been sizing canvases with the stuff for years as well as using it as glue & never thought of it for this use.

    Now, before putting the finishing touches on the layout sketch, I've gotta go finish the taxes which are due today. Don't forget!!
     

  13. Tapatio

    Tapatio TDPRI Member

    81
    Jun 13, 2016
    New Jersey, USA
    OK, while I procrastinate with the taxes here....
    A quick Q.
    On the pot in series with the NFB resistor, what would be a good value for the resistor? Is the pot ok at around 50K? I've seen many variations out there.

    I figure I would set it up like this:
    View attachment 507189
     

    Attached Files:


  14. jimgchord

    jimgchord Tele-Meister

    Age:
    44
    128
    Apr 3, 2018
    victor,ny
    I use 2.7 k and a 50k pot. That way you get a wide range of values
     

  15. Tapatio

    Tapatio TDPRI Member

    81
    Jun 13, 2016
    New Jersey, USA
    Thanks jimgchord, I'll set it up that way. BTW, the woodworking and design is impressive on your projects.
     

  16. jimgchord

    jimgchord Tele-Meister

    Age:
    44
    128
    Apr 3, 2018
    victor,ny
    Thanks very much, enjoy your build!
     

  17. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    The 5F2A calls for a 22K top resistor tied into a 1500 ohm tail resistor. If you want to increase the effect of the negative feedback loop at the extreme of the range of adjustment, then I would suggest that that ‘top’ resistor’s value is important. It might be ineteresting to first play the amp stock. Then reduce that 22k resistance to maybe half....to 12.5K and judge that change....it will get cleaner...one might like it. I doubt that going much smaller than the stock 22K there would be what I would want...ommv. For my purposes, I might leave the stock circuit in place and use something between 25K and 50K for the potentiometer to adjust that top value.
    Aiken has a good tutorial if one wants to study designing NFB loops. I am sure the Valve Wizard does, as well. Here is a ratio between those two resistances...top and tail.... that determines the strength of the signal being injected as negative feedback.
     
    King Fan likes this.

  18. Guitarteach

    Guitarteach Poster Extraordinaire

    Aug 6, 2014
    UK
    +1 on variable NFB, it’s a full on character changer and lets you dial on the sweet spot for you, cab and speaker.

    The mod that is way above all others though was adding a VVR. I made on with a small clutch of components and a mosfet on piece of tagstrip.

    IMG_1189.JPG

    I can dial in all sorts of gnarly, bright marshall like dirt at really useful volumes with it. Makes for an amazing amp now. Have added a couple of cathode boosts either side of v1 but I’d have to open it to recall the values I settled on. One was full range and one highs. Can use them both for glass melting fuzz.

    A 12” Celestion Alnico Blue in a large light pine cabinet really works well with it. It now has an extra switch and a tone control as it started as a champ.

    IMG_1123.JPG
     

  19. wmprivett

    wmprivett Tele-Meister

    243
    Sep 22, 2009
    Brick, NJ
    Uncle Doug on YouTube talks about, and shows how to install an adjustable NFB in his how to build a Champ series. It works great. I use it all the time now.
     

  20. Tapatio

    Tapatio TDPRI Member

    81
    Jun 13, 2016
    New Jersey, USA
    @Guitarteach dang, that's a cool idea. I can see where this is going to lead eventually! But first I need to conquer the original circuit. After that

    I will revisit the Uncle Doug video on the NFB.
     

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