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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. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    No to this part. Separate means no sharing with the power grounds / circuit grounds / PT grounds / coffee grounds. :D

    Don't overthink this. If I'm right, the ground off the filter caps should just go to the same point as the center tap off your PT (have you ID'd that little sucker yet) -- and many many of us just use a PT bolt for that.

    As we discussed, other than the safety ground, don't sweat the small stuff *too much* -- as long as your (current, modern) *circuit* grounds go somewhere, the amp will work, and a few simple power wiring steps will make it work safely.

    The 100R artificial CT resistors anchor to the chassis, not the lamp. Rob has a tidy way to put 'em on a socket, but if there's any space under the lamp, I just run 'em to a solder lug or *another* PT bolt. If it isn't clear, *this* 'CT' is for the heaters; the one we talk about above is for the B+ / HT winding. Here's the best pic I could hunt up.

    100R.jpg

    Let me take your mind off grounding for a minute :) : Have you built your input jacks yet? Simple way is to do it on the outside of the chassis so the spacing is right (L and R are maintained just as you look at 'em -- no mirror image -- don't get confused). I like the pic and notes Doug Hoffman has for these, except 1) the lugs will line up if you use the chassis holes for a jig, and it's easier to do it outside than inside and 2) he puts the 68ks on the input array. This has theoretic advantages, but it's a lot trickier to assemble and solder both upstream and downstream, and for a small tweed amp I would definitely leave the 68ks on the board where you have 'em.

    Inputs.png
     

  2. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks, King Fan. That's really helpful. Yes, that makes sense. So "separate" does not only mean separate from the preamp section, but separate from the other grounds in the power section. The earth ground of the power cable is grounded completely separately, then the B+ and center tap off the PT are grounded separately, and then (in my scheme), the preamp is grounded separately, all to input 2. That makes three grounding points in total, if I'm understanding correctly. Of course the chassis is the "super-ground" to which all these other things are grounded, but you know what I mean.
     

  3. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Three total ground anchors, right. Again, from my insight altitude of 2" off the ground... :D
     

  4. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Nice pun! Yes, if I just look back at Rob's 5f1 layout, it all makes sense. The only thing I'm doing differently is that I'm connecting the C6/R8/R9 area to the preamp ground bus, rather than daisy-chaining it over to the power amp ground bus and then to the power amp ground. At any rate, his layout has (1) an earth ground from the power cord to the chassis, (2) a power amp ground to the chassis, and (3) a preamp ground bus to the ground lug of Input 2, which is, of course, connected to... the chassis, through contact via the toothed lock washer, etc.

    5F1_Layout_Robrob.png
     

  5. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Alright, the board's in. I'm going to start wiring it all up tonight.

    IMG_9489.JPG
     
    King Fan likes this.

  6. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    I'm gonna let someone smarter than me decide if that's right in theory, but it's what I did in practice and it works fine. :)
     

  7. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Oh, and here's the other diagram you might like on 12A jacks...

    TS jack lugs.png
     

  8. LudwigvonBirk

    LudwigvonBirk Tele-Meister

    Age:
    115
    139
    Aug 26, 2017
    Madison
    King fan said: “ run the ground off the first filter caps to the same point you run the PT center tap -- often a PT bolt with the serrated washer, crimp ring, keps nut, loctite security from yesterday.

    Then the other board ground and the two pot grounds drop to the 'preamp bus' which is anchored once and only once to the chassis at an input jack.

    Does that make sense...?”

    It does to me. Because, I have had very good results doing that. Safety checks out (first order of business), and the amps I have built/fixed have been very quiet when they are supposed to be.

    There are alternatives that also will work (and some that won’t ).
     

  9. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    Or look at the attachment on one of my posts on about Page 6 of this thread
     

  10. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks, King Fan! I found a graphic amongst Rob's stuff that got me where I needed to go, so I wired the input jacks a few days back.
    Switched_Jack.jpg
     

  11. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    OK, everyone. I spent most of the day wiring the chassis. I've included a bunch of pictures here and in the next posts. Tomorrow I'm going to go step-by-step through the layouts and schematics and mark things off as I see them done correctly.

    I wonder about my wiring of the tone pot, which is a SPST switch.

    Please feel free to comment and especially to point out any errors you see!

    IMG_9522.JPG IMG_9523.JPG IMG_9524.JPG IMG_9525.JPG
     

  12. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Last two:
    IMG_9535.JPG
     

    Attached Files:


  13. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    looks very tidy for a 1st build. It would pay to check all your wiring with an ohmmeter to make sure you have DC continuity/resistance where you should have, and not where you shouldn't have.

    Use a yellow highlighter and a copy of the schematic to work your way systematically through the amp from input jack to speaker jack and from B+ to ground.

    Then test it (with a speaker or dummy load) and a light bulb dimmer, before putting the rectifier tube in and taking the DC voltages and VAC heater voltages etc.

    Plugging in the 6V6 and 12AX7 comes at the end.

    Then comes the live check about whether you have the NFB loop around the correct way
     

  14. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thank you, sir! I went back and looked at your comments on the solid state diodes when I was wiring up the rectifier. Very helpful. And I also went back to your comments on the B+ power supply coming from the OT transformer to the positive node of the first two (parallel) filter caps and the B+ send to the rectifier from the opposite (negative) node of those same two capacitors. Your narratives were very helpful as I was trying to interpret layouts and schematics as best I could. Thanks!
     

  15. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thank you! Will do!
     

  16. tweedman2001

    tweedman2001 Tele-Holic

    825
    Aug 3, 2014
    North of There, NY
    Hey Prof. Is there some reason you have the tone pot with a switch? Normally one would use either a toggle power switch or a switched volume pot. I haven't been studying your build so maybe you are using it to switch the tone in or out?
     

  17. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Hey tweedman2001 - Thanks. Yes, I'm using a switch to switch the tone in or out. In = Princeton; out = Champ. That's the idea.
     
    keithb7 likes this.

  18. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    When the Professor switches out the 5F2A tone pot, it then becomes the venerable 5F1 circuit! Smart man The Professor, watch him. He's crafty. Unlike Gilligan.
     

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