5f4 / 5e7 build

Discussion in 'Shock Brother's DIY Amps' started by theprofessor, Feb 2, 2019.

  1. theprofessor

    theprofessor Friend of Leo's

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    After some planning and selling a lot of stuff to raise money, I've started in on my 5f4 build. I decided to use (at least for now) the larger value for the local NFB resistor on the preamp (10M versus the 5f4's 4.7M) to make it exactly like a 5e7, but with two speakers. I can always change that later by adding another 10M, therefore halving the resistance and increasing the NFB to 5f4 levels.

    I'm still waiting on some parts. The Mojotone chassis came to me with some chrome flaking, and they quickly put a new one in the post. So I've got to wait on that. Also waiting on my Hammond choke that's sufficient to filter the plates.

    I've populated the board, and I now have a question regarding adding an adjustable bias pot:

    In the 5f4/5e7 circuit, the bias section is as follows (pics in next post): the junction of the 220k resistors in-between the two .1uF coupling caps goes to the negative side of the 100uF bias electrolytic and then up to the positive side of the diode. I'd like to put a 10kl pot in there, and I'm not exactly sure how to wire it up. I had hoped to use one of those cute little plastic bias pots that I used on a Hoffman-designed build, but I got the turret spacing wrong. It doesn't fit. So now I'm going to use a more traditional Fender-style 10kl pot off the board, but I need to understand better how to wire it.
     
    Last edited: Feb 2, 2019
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  2. theprofessor

    theprofessor Friend of Leo's

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    Here is my failed design:
    IMG_0997.JPG
    And here is the idea I'm going with for now (below), with a stand-in 10kl Alpha pot. What I don't understand is what attaches to the wiper and what attaches to the right lug of the pot. And is the right lug connected to the wiper?

    The Fender Bandmaster reissue schematic shows what I'm hoping to do, but I don't understand the notations for how the pot should be wired up. What does an arrow (from the 220k junction) going straight into the resistor/pot mean? And which lug is connected to the positive side of the diode? Fender 57 Bandmaster reissue_BIAS_Page_1.jpg IMG_1001.JPG
     
  3. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    The arrow is the wiper. That should make things clear.

    Check your 100uF though. It should be after the diode.

    Edit: which outside lug is connected to the diode is not that important. It only determines what rotation direction causes the amp to be biased hotter. It is indicated though, by the (other) arrow and the letters c(lock) w(ise).
     
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  4. theprofessor

    theprofessor Friend of Leo's

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    Thank you very much. The arrow is the wiper. Very helpful! That would seem to mean that I have it correct in my picture above.

    As far as the 100uF electrolytic: See the little wire sticking up out of the turret on the positive side of the diode? That connects to the negative side of the 100uF electrolytic, and then on to the 220k junction.
     
  5. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    The wiper with the yellow wire should go to the 220k junction. The diode/capacitor should not go to the 220k junction.
     
  6. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Got my popcorn.

    No coffee yet, tho, prof, so check me on this. Pots in this implementation - voltage divider - work in either direction as noted (CW / CCW) but have a wire on each lug. You said the 220 junction should go to the wiper = central lug? I’m not sure I see either feature in your photo.

    BTW nice work finding a reissue scheme. Those TP voltages are super useful when it comes to testing (let’s not say troubleshooting, no, not happening).

    Edit: I see Tom said it better and quicker.
     
  7. theprofessor

    theprofessor Friend of Leo's

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    OK, thank you. I followed the Weber layout when putting in back-of-the-board wires. Here's what it looks like:
    5e7_layout Weber.jpg
    In this scheme, without a pot, the 220k junction goes to the bias electrolytic and then on to the positive side of the diode. Are you guys saying that instead of running the 220k junction first to the electrolytic, that I need to move that wire to the wiper of the pot, and then attach the right lug of the pot back to the positive end of the diode, which then goes to the 100uF cap?

    Like so:
    5e7_layout Weber_BIAS.jpg
     
  8. theprofessor

    theprofessor Friend of Leo's

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    And if what I've drawn out above is correct (maybe it isn't), I still don't understand Hoffman's diagram for adding a bias pot to a fixed bias amp (Princeton Reverb, in this case) in relation to this amp. Should the right lug of the pot be attached to the negative side (cathode stripe) of the diode, instead of the positive side, as in my drawing above?

    biascircuit4.gif
     
  9. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    I think you've got your positive/negative/stripe mixed up:
    6683456305_0ace237008.jpg

    Bias circuit are always a bit weird to think about as they have a negative voltage. The capacitor is drawn empty from the negative side as opposed to being filled from the positive side.
     
  10. theprofessor

    theprofessor Friend of Leo's

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    Thanks, Tom.

    First, does my drawing on the 5e7 layout look correct to you?

    Secondly, yes, the cathode stripe thing along with pos/neg current thing, is confusing. The negative (cathode) is marked with the stripe. But you'll see on Hoffman's layout above that he has a positive symbol on the non-striped side of the diode. Is he simply marking negative current, rather than labeling the sides of the diode?

    The + and - signs on Hoffman's layout are confusing, since I understand the diode markings as you posted above. Like this:
    diode-current.jpg
     
  11. Uncle Daddy

    Uncle Daddy Tele-Holic

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    There's plenty of space in the chassis for something a bit more man sized. This be my one.

    P1120555.JPG
     
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  12. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    Drawing looks ok (after removing the original bias wire and a resistor).

    Hopefully I find some time later to clarify it a bit more. Family rush hour is starting...
     
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  13. theprofessor

    theprofessor Friend of Leo's

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    OK, so here's what I've got. I think this is correct. 220k junction to wiper of pot. Left lug of pot to resistor to ground. Right lug of pot to positive side (anode) of diode through turrets laced together. Then from the negative (cathode) side of diode down to the negative side of the 100uF cap (under the board; the yellow wire there is simply to show the connection).
    IMG_1002.JPG
     
  14. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    Short answer: Looks good.

    Long answer:
    The diode is made of a semiconductor (e.g. silicon or germanium). The anode is made positive by doping it with e.g. boron atoms that result in an excess of 'holes' (a deficit of electrons). The cathode is made negative by doping it with e.g. phosphorus atoms that result in an excess of electrons.
    This is what is depicted by the + and - in your image in post #10.

    For a diode's functioning in a circuit, you can forget all of the above. Just think of the diode as a one way valve. The bias circuit is arranged in such a way that the voltages are as depicted in the Hoffman bias image in post #8. <poetic license>The diode keeps the negative voltage in the capacitor and it allows the transformer pull the positive out. </poetic license>
     
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  15. theprofessor

    theprofessor Friend of Leo's

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    Thanks so much, Tom. I really appreciate your taking the time to reply. I'm going to see if I can wrap my head around the technical details as you explained them in your post. In the mean-time, I'm glad to know I'm on the right track now with the fixed adjustable bias.
     
  16. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Taken with the generalized Hoffman diagram, (not the PR version) and ignoring for a moment several variants (like recto-pin4 bias borrow for amps without a bias tap, and 50K pots, and the option to wire the pot in 'either direction' relative to lugs 1 and 3, and the ambiguity of how pots are drawn (usually shaft down, here shaft up)) I do like Doug's explanation on that page:

    "Here we have a typical Fender bias circuit. The AC voltage comes from the Red/Blue wire on the power transformer. The center tap, Red/Yellow wire, is the other end of the transformer winding. The Red/Yellow wire is grounded to the chassis to complete the bias circuit. The AC voltage from the power transformer Red/Blue wire then goes to the 470 ohm bias range resistor which knocks the AC voltage down a bit. The AC voltage then enters the bias diode and gets rectified into a fluctuating negative DC voltage. The negative DC voltage is then smoothed out by the 47uf at 100 volt bias capacitor. The smoothed negative DC bias voltage then enters the bias pot. The other end of the bias pot has a 27K 1/2 watt resistor that is soldered to ground. The total resistance for this circuit to ground is 10K + 27K or 37K. The bias pot and 27K resistor form a voltage divider. The middle wiper of the bias pot sweeps across the 10K resistance of the bias pot and selects the voltage that gets sent to the power tubes. The negative voltage then leaves the middle wiper of the bias pot and goes to a junction where two 220K 1/2 watt resistors are soldered together at one end. The bias voltage goes through each of the 220K resistors and appears at the two junctions. Each of these two junctions have a wire that leads to each power tube input grid, pin 5. Usually the bias voltage goes through a 1.5K grid stopper resistor first and then on to pin 5 of the power tube. That is the complete bias Circuit complete from transformer to power tube."
     
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  17. FenderLover

    FenderLover Friend of Leo's

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    One problem with that bias circuit is that if the wiper ever fails or gets dirty and looses contact, there is no bias and you get red plates. Simple fix for bullet-proofing: add a 1M from the wiper to the outside terminal connected to the C21 (-) on post #2. If the pot/wiper ever fails, the bias will be held negative and save your outputs.
     
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  18. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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  19. theprofessor

    theprofessor Friend of Leo's

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    Thanks, @FenderLover ! Good to see you around again! So a 1M resistor where I've drawn it in between the wiper and R lug that is then connected to the junction of the diode and the (-) side of the 100uF cap? It's in aqua blue.
    IMG_1002_bias 1M wiper to R lug.jpg
     
  20. FenderLover

    FenderLover Friend of Leo's

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    I've got my eye on you, Prof ;)
    You got the location correct. Anything greater than 100K will work fine.
     
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