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. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    No. A heavy load presents a high AC (signal) impedance. For film coupling caps the outer foil should go to the opposite side, i.e. the plate side. The plate is the signal source. The circuit that follows the coupling cap after the plate is the AC load. (Note that the plate resistor itself also forms part of the AC load, but the plate per-se, in a common cathode inverting gain stage, is the signal source)
     

  2. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks, @tubeswell . I'm using @robrob's "Tube Amp Dictionary" here, but perhaps imprecisely. Here is why I'm a bit confused and why I worded things as I did. I quote: "A high impedance circuit is a low load circuit that restricts AC current flow. A low impedance circuit is a heavy load circuit that allows AC current to flow easily."

    Aren't both of my coupling caps facing the plate side? The foil of C1 faces the plate of V1A (pin 1), and the foil of C2 faces the plate of V1B (pin 6), right?
    Fender 5F2a schematic.jpg
     
    Last edited: Nov 3, 2017

  3. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    The guys are right, Prof, but you're right too. It wasn't until my third amp that the schematic really made sense to me, and I found that I could a) build from the schematic *and* the layout and b) find more useful checks and guides to what I'd done by looking at the schematic. Look up the pinouts of your tubes, mark the pin numbers on the *schematic*, and just go back and forth as much as you can.

    Yes, the outer foil end should go down on your coupling caps -- as you say, toward the plates. Remember how the cap was quieter with the outer foil end connected to ground on the input jack? Well, ground there was the low impedance connection, and the plates are that on coupling caps.

    Oh, one more thing when wiring from layouts. Mojo, Weber, and many other modern layouts can be useful, but they get weird, each in different areas. As much as possible go Fender on wiring the board and leads, and go with Rob on the power and heater wiring. Figure out your PT and OT and household wires using your good eye and the wiring diagrams, but then check all choices with the gang here. Transformer wire colors ain't rocket science -- rocket science makes sense.
     

  4. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    Yes, both coupling caps in the stock 5F2A circuit connect between the (source) plate and the (higher impedance) AC load, so the outer foil lead points to the plate.

    (I should add clarification in relation to one of my earlier comments, that the reason that the outer foil lead on a plate bypass cap points to the B+, is that to AC, the B+ and the ground 'look' one and the same, because the filter caps act to shunt any AC that would otherwise appear in the power rail, to ground. However, a superior form of plate bypass cap would actually go from the plate directly to ground, because electrolytic caps aren't perfect 'shunters' of AC to ground, and any AC that appears in the power rail can be AC-coupled back into the signal at the plate if plate bypass caps are connected between plate and B+)

    Rob's grammar is a bit confusing in that sentence. A 'high AC load' is presented where there is low AC-resistance* between signal source and Ground/B+. An 'AC load' is 'high' where 'AC-resistance' is 'low', because current (which is present in a dynamic state within the signal 'AC') can get from the source to the ground return (and B+) more easily. Likewise, if you have parallel resistances between the signal/AC source and the ground, this increases the AC load - and the signal strength at the source gets 'sapped' more easily (by the cumulatively lowered AC-resistance). An analogy is a spider's web, where the centre of the web can oscillate the greatest distance because that is where there is the lowest impedance to oscillation. But as you get closer to the edges of the web (were the web is anchored to something more solid), this oscillation becomes increasingly constricted. The more strands that the spider can add from the centre of the web to solid anchor points at the edge of the web, the more tension there is in the centre of the web, and the harder it is for the centre to oscillate. The centre of the web is like the source part of the signal circuit, and the edges where the web is attached to solid objects, and like the B+/ground return. The lengths of each web strands presents the amount of resistance. Shorter strands represent lower resistance (shorter pathways for the energy to oscillate along), and vice versa. The 'web-anchor' points in the 5F2A schematic are the bits where all the ground return triangles are, and the pathways to those triangles present the 'strands' of AC signal impedance. (How's that for a natural model?)

    *'AC-resistance' is the same thing as 'impedance'. But AC resistance also includes those parts of the circuit that present DC resistance to the signal source. Hence a plate resistor, that presents a DC-resistance pathway between the signal source and the B+, also is part of the AC-resistance/load for that signal source, and any AC coupled resistances on the 'other-side' of the coupling cap will be in parallel with the plate resistor, when it comes to calculating AC load, because to AC, the B+ and the ground 'look' one and the same (because of the AC-shunting function of the filter caps).
     
    Last edited: Nov 4, 2017

  5. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    When I mentioned using a schematic, I was referring to schematics that are fully filled in with component numbers (R1, R2, C1, etc), and these would also be matched on the layout drawing, with pin numbers on the sockets, colors on the transformer leads, etc. Since it looks like the schematic you're using is unmarked, I can certainly see why you're using the layout as a reference. So in that case just use the layout and then verify connections on the schematic if the layout is unclear.
     

  6. Axis29

    Axis29 Poster Extraordinaire Ad Free Member

    Jan 2, 2007
    Virginia, USA
    I can only answer for me.... What I do is get the leads in there nice, tight and neat from the top, tugging them and bending them back to hold the component tight. Then, I solder from the top as well. When all is said and done, I turn the eyelet board over and snip off the leads tight to the eyelet.

    That's usually when I realize I had one or two component soldered into the wrong hole... I wait until, I've screwed the board into the chassis and connected all of the tube and pot wires before I realize I forgot two under board wires. :)
     

  7. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    That's really helpful, Axis29! Thanks a lot. I'm going to start soldering some today.
     

  8. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    When applying solder to the eyelets, I have my lead wires in place as well. A heat sink is nice for soldering close to components. You can even use a small alligator clip. Just something to absorb heat. I personally have not used glue or silicone on caps to hold them down. Its not a bad idea. I have serviced many vintage amps with original caps where nothing was used either. Everything seemed to be fine. In addition, today's equivalent caps are generally smaller in size.
     
    Axis29 likes this.

  9. Outlaws

    Outlaws Tele-Meister

    157
    Jan 16, 2007
    None
    I leave about 3/16” - 1/4” of lead into the hole if it goes inside the turret. Remember, you might be pulling these out at some point. You don’t want it solder and lead all the way to the bottom, plus you might have jumpers down there. If you have to desolder you will end up having to heat the turret enough to loosen the bottom and then that jumper might fall out when the lead pulls out. Which means then you don’t do that because it’s gonna 30x the work to yank the board and have to just clip the lead near the comment instead and then solder two leads together to swap a new component...that’s not bad, but it looks sloppy imo.
    There just is Zero good reason I can think of to have excess lead length in the turret, and plenty of negative against doing it.
     

  10. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks very much, @keithb7 ! Two questions:

    (1) Your comment leads me to ask a question about leads, which I would have asked eventually anyway. The fellow on the Tube Depot video who builds a 5f1 says he makes his leads about 3" long. How long should they be? Is that about right?
    (2) As for the heat sink, does one simply connect an alligator clip to one of the leads of the component and then the other end of the alligator clip to a big piece of metal (like the chassis)?
     

  11. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks, @Outlaws ! That makes sense, especially when thinking of the possibility of removing components later.
     

  12. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    Put the alligator clip on to the component lead. It will get hot while you solder, as it transfers heat away from the lead and absorbs into the clip. No need to try and hook anything up to the clip. Just clip it to the component lead. I generally leave about 6" of lead wire. Maybe more or less as required. I will place the board inside the chassis once a few times to check my wire length.
     

  13. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Excellent! Thanks!
     

  14. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    A couple of things: I don't like the fuse holder that comes with the Mojotone kit. I'm sure it would work just fine, but it looks cheap, and the top looks like it's going to pop off at any moment (in which case, it actually wouldn't work). So I went around looking for a new fuse holder.

    In the process, I came across the Hoffman parts list, which I found extraordinarily helpful. I also saw there some washers that step up the smaller Alpha pot shaft to the slightly larger chassis hole. That will help solve some of my annoyance with that. And I got the Hoffman fuse holder.
     
    Last edited: Nov 4, 2017

  15. Outlaws

    Outlaws Tele-Meister

    157
    Jan 16, 2007
    None
    If you are not versed in soldering to the back of pots with heating times of 2-3 seconds, I would practice that before doing your board as it will make you better at it. If your soldering iron doesn’t have a fat chisel tip with enough heat it will feel like a struggle rather than being enjoyable.
    I won’t even touch the component leads until the turrets are hot enough to flow the solder, because then the component lead is either already hot on its own from the transfer, or a split second is all it needs at the end to allow all the solder around it to then join with it. And flux.
     
    Last edited: Nov 4, 2017

  16. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Great pace despite deep research, Prof, very scholarly indeed, no wonder the committee responded so quickly on your tenure action. You have a big committee of top faculty, too!

    IME the Alpha short-shaft will hold a knob just as well as the others, and some experts really love Alpha pots. I agree the look is odd -- but only until you put the knobs on.

    One option to consider: I totally agree soldering to the back of pots is hard. OTOH, in an amp it's not actually needed. Marshall did, so many do, but as Merlin notes it is neither necessary nor even best practice -- you can just run a separate ground bus and drop ground wires from pots to it. IIRC Rob's layout shows a separate bus? See builds by Keith and many many others here.
     

  17. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Thanks, @Outlaws . I *think* I'm good with soldering to the backs of pots. I've done some of that on my guitars. And I'm working with a variable-power Weller 40-watt soldering station, a chisel tip, Alpha-Fry 60-40 rosin core solder, and Rosin paste flux. I'm not saying I'm some kind of expert, but I *think* I can do well enough... I also love the Hakko solder tip cleaning wire and holder that I use. I found @Rob DiStefano 's YouTube video on basic soldering practice to be very, very helpful.

    Thanks, @King Fan ! I have noticed a ground bus in some layouts, including @robrob 's 5f1 layout. If I were to go in that direction, what gauge bus would be best for this application. I noticed this morning that Hoffman's 5f2a parts kit includes 4x 24ga bus wire and 4x 20ga bus wire.

    As for my "deep research," I suppose that's just my way. It doesn't mean I won't make mistakes or ask dumb stuff (asking lots of questions is part of the research!), but I try to be methodical. That is kind of my real-life job. I'm also using the forum here as a way to think out loud, get feedback, and record everything, with as many pictures as possible, for all those who, like myself until a couple of weeks ago, have never thought of building an amp and don't even know which way is up. My hope is that the data will be a teaching tool for others.
     

  18. tweedman2001

    tweedman2001 Tele-Holic

    827
    Aug 3, 2014
    North of There, NY
    Note there is some differences when soldering turret vs eyelet boards. Looks like you have eyelets. Turrets can accept longer leads while the eyelets will be much shorter. Also the eyelets will be easier to heat. But both need to be heated enough for the proper flow of solder. :)
     
    Outlaws likes this.

  19. Outlaws

    Outlaws Tele-Meister

    157
    Jan 16, 2007
    None
    Oh you have eyelets. Forget everything I said about turrets lol.
     

  20. keithb7

    keithb7 Friend of Leo's

    Age:
    46
    Jan 9, 2010
    Western Canada
    I agree on the cheap fuse holders. I often upgrade them.

    Re ground scheme. See pic. Ignore the ground off the second input jack. I had to remove it. Was a rookie error that created a ground loop. You can see that the ground of each pot is via a wire to the ground bus.

    [​IMG]
     

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