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Harvard in a 5F2A Chassis Build

Discussion in 'Shock Brother's DIY Amps' started by James Knox, Nov 22, 2020.

  1. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    BTW back at bending socket lugs.

    Fender only drew 'em that way for diagram clarity.

    You don’t need or want to bend 'em out much if you’re wiring diodes, resistors, or jumpers on 'em. Especially if you then need to pass the lugs through the holes for outside mount. That’s especially true for nonals, which ironically is where a little separation can be nice. Ask me how etc. :)

    The nice Belton octals are best not bent, and if you must bend 'em, use a good needlenose and a minimal bend. From Doug Hoffman...

    C21C0281-720E-4F7B-9EE9-B95828143386.jpeg
     
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  2. Kevin Wolfe

    Kevin Wolfe Tele-Holic

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    I’m pretty sure I learned the hard way that they won’t go if you pre-bend’em. Never had a problem with fitting resistors and diodes though. In fact it seems the extra room makes it even easier.
     
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  3. James Knox

    James Knox Tele-Holic Silver Supporter

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    Waiting for Thermal Greece and Diodes (how did I run out of those?) so I populated and soldered the board, attaching leads, etc today. I’m trying to really understand the bias section of the board. While double checking my “underside” jumpers I came across this...

    263D4DD5-88BB-4B30-B58A-F20EA74490B5.jpeg

    I ran the underside jumper that is highlighted in yellow. Then while checking other layouts I this alternate version...
    71C5651B-A99D-4C73-8EAF-7724E90D8FEA.jpeg

    Then when comparing the MoJo 5F11 Layout, thinking all along that is the way to wire up the bias pot I notice that there is NO JUMPER from the bottom of the 22k’s to the Bias circuit.

    AD10568E-FE2F-40DA-8B7E-21BB0FF8AE2C.jpeg

    Since I’m waiting for N4007s I have to pause anyway but I’m stumped on laying out the Bias Control. And why no jumper for the 5F11?

    I was planning on tapping B+ off of the Rectifier Tube to a resistor to a diode to the 10k pot like on the Mojo 5F1 Layout. Is this correct?
     
    Last edited: Nov 25, 2020
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  4. gabasa

    gabasa Tele-Meister

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    That's because in the 5F11, the connection to the 220k resistors is via the the trem depth control.
    The Mojo layout is correct, a wire on the top of the board straight from the pot, so no connection is necessary underneath.
    Check out the schems for both amps, you'll see the difference easier there.
     
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  5. Kevin Wolfe

    Kevin Wolfe Tele-Holic

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    Yep, the 5f11 uses a bias wiggle tremolo. And Me, I’m done with wire under the board. Had one come apart when I was re-building the 5e3 and had to unsolder a bunch of stuff to lift the board and fix it. Every thing will be up top from now on. Just sayin’.
     
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  6. printer2

    printer2 Poster Extraordinaire

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    With the closeups I don't really get a sense of scale, I wondered how big the chassis was compared to the one I am building (eventually, got an acoustic guitar going for me today. I went and measured it, looked up the 5F2 and though, mine is 1/4" smaller. I do have ears (Tweed chassis) but 8 pin socket for the output tubes. So yeah, a little tight but doable. It is looking good.
     
  7. James Knox

    James Knox Tele-Holic Silver Supporter

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    Ok, I see that now - Thank you!
     
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  8. James Knox

    James Knox Tele-Holic Silver Supporter

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    I have thought about that myself!
     
  9. James Knox

    James Knox Tele-Holic Silver Supporter

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    Here is an early pic from farther back...

    58747D9E-47E0-4A01-8FB6-350E8E3FA4B9.jpeg
     
  10. James Knox

    James Knox Tele-Holic Silver Supporter

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    Here Is the populated board with the bias section unfinished. I want to make sure I have it correct before I solder the bias circuit in.

    A714E5D9-8F76-4EA0-871B-049E8B839CE2.jpeg

    Unfinished Bias Section...

    C7978B8E-F2F7-4DA8-BBCD-584AA2E135A8.jpeg

    Is this correct? 27k or 33k?

    73EDE639-6D89-4A09-8070-534F422D31E3.jpeg
     
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  11. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I'm not sure about bias off the B+. Do you have a pic of the scheme you've adopted? Anyhow, I think Mojo's a great outfit, but I don't follow their wiring layouts.

    Meanwhile a minor point of practicality; I wouldn't solder the tail resistor to the pot. It's difficult and unneeded (just take it to the power amp ground). But most of all it makes swapping resistors (often needed) to get the right value wayyy tedious.
     
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  12. BigDaddy23

    BigDaddy23 Tele-Holic

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    Re the 27 vs 33k, you won't really know until you fire it up and look at the negative bias range you get out of the circuit. Just make sure you have access into that area for the soldering iron and a pair of needle nose pliers!

    The other thing to consider is putting the PT CT right on the negative for the first filter cap.
     
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  13. BigDaddy23

    BigDaddy23 Tele-Holic

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    Re the bias power source, refer back to my post #34.

    You want to take a feed from ONE of the PT secondaries where it lands at the rectifier pin. That would then connect to your 100K bias resistor to provide the appropriate voltage to your bias circuit.
     
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  14. mjcyates

    mjcyates Tele-Meister

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    I am pretty sure this is how I did mine:

    5F10 Bias.png
     
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  15. James Knox

    James Knox Tele-Holic Silver Supporter

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    Hey, thanks for that! Is the 10k resistor because of the 100/100 Cap? Most others I see used a 100k, but that was with a 50/50 cap.
     
  16. mjcyates

    mjcyates Tele-Meister

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    Looks like I got that 10k value from the Mojotone layout for the Tweed Vibrolux.

    G1TVK044_04.gif
     
  17. James Knox

    James Knox Tele-Holic Silver Supporter

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    Ok, cool! Looks like ima have to experiment a little.
     
  18. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    You shouldn't have to experiment much, though the 'tail' resistor value may need a bit of trial and error which is why I suggest making it easy to solder / unsolder.

    But the first resistor? The small Mojo 10K resistor is **if you have a bias tap**, as they show in their layout.

    If you don't have a bias tap, the standard way is to tap the HT off the PT, as @BigDaddy23 says. That's gonna bring ~300VAC to the party instead of 50 -- you need a *much* bigger first resistor.

    Here's my drawing of Doug Hoffman's 'PR adjust bias method' @jsnwhite619 mentioned. You can move the parts of course if you don't have a bias board.

    upload_2020-11-27_11-11-44.jpeg

    Doug explains this on his bias page (link in my post #10).

    Basically, that HT voltage is much higher than a dedicated bias tap so must be brought down a lot. Here it goes to the 100K bias range resistor, which knocks it down to an acceptable level before it enters the diode to be rectified.

    He also notes the 27K resistor in the PR may be a different value on other amps, but you should not make this resistor less than about 10K or you'll have too large a current drain to ground. If you need a smaller value than 10K to get the amp biased right, you should adjust the 100K bias range resistor as a coarse, initial adjustment. (In practice on these Fender amps, you don't usually need to go as low as 10K.)

    Details:
    • As some smart person already pointed out, if you run rectifier protection diodes, take the HT tap off the pin *before* the diode (eg, pin 3 instead of pin 4).
    • The first, 'range' resistor handles high voltage, so 2-3W metal oxide is useful.
    • Bias filter size isn't critical, but values like 47-100uF are often used in modern versions.
    Final note: As you'll know, James, there are two ways wire a pot, as a variable resistor or a voltage divider. Merlin (and Doug) like the 3-terminal voltage divider method for bias, though you will see some smart folks argue for the 2-terminal variable resistor method that Mojo shows.
     
    Last edited: Nov 27, 2020
  19. mjcyates

    mjcyates Tele-Meister

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    King Fan's answer is much better than mine:(
     
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  20. gabasa

    gabasa Tele-Meister

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    Here's a photo of Victoria's Ivy League I found online.
    If you zoom in, this is what they do:

    Pin 4 -> 100k resistor -> diode -> 25k pot -> 15k resistor

    Very close to what you drew in your plan, but a little extra range on the extreme ends of the trim pot.

    The pot-resistor you drew gives you 27k-37k range between the diode and chassis ground, but easy to dial in with a little screwdriver.
    What Victoria seems to do gives a 15k-40k range, but perhaps needs a little more precision with the screwdriver.
     
    Last edited: Nov 27, 2020
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