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? about Blackface pF capacitors & pots for scratch-ish build

Discussion in 'Shock Brother's DIY Amps' started by Huddy, Apr 21, 2021.

  1. Huddy

    Huddy Tele-Holic

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    I've got something in the works that I've been collecting parts for until I had time... and all the parts. Hope to get started May 1st.

    • For an AB763 amp, am I correct to assume that anything in the pic(a/o)? fared range should be a mica capacitor?
    • The first reverb recovery triode after the the tank couple cap - .003 µF - is that value crucial? .0022 or .0047 µF okay?
    • I've heard that J Taper pots are pretty crucial as well for the Volume and treble positions? Linear more ideal for the mids? What about bass pot?
    I think that's it for now... Thanks in advance!
     
  2. David Barnett

    David Barnett Doctor of Teleocity

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    BF amps used ceramic disc capacitors in the pF range.
     
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  3. Bob Arbogast

    Bob Arbogast Tele-Afflicted Silver Supporter

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    I just built a black-panel style amp with J tapers for Volume and Treble. I quickly replaced them with 10% audio taper pots. For me, the J pots were too much too soon.

    On the plus side, now I get why people think a Deluxe Reverb at "4" is loud. Other than this recent experiment, all my builds have used 10% taper pots (except for the linears, of course).

    But it doesn't matter whether the pot is J taper or 10%. The amp will sound and perform the same. It's just that for a particular result, the number on the knob will be different.
     
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  4. Huddy

    Huddy Tele-Holic

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    Yeah this is what I was concerned about... especially on a very loud amp with no master... but then again... I am choosing to build it that way so... :lol:

    I have a tweed deluxe I built with linears and I was barely taking it off 3 until I realized 8 wasn't really all that much louder but it did have a bit more crunch/grit to it.
     
  5. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    I've never used J taper pots on any of my builds. Although I never measured the pots in them, the controls on the old blackface amps I've played and worked on behave just like the clones I've built using CTS audio taper pots.

    For a black/silverface style circuit, a J taper isn't a bad idea for the treble control. Not such a good idea for the bass or volume controls though.
     
    Last edited: Apr 22, 2021
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  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Ime Silver Mica or Polystyrene are a better choice for signal applications. Make sure to put a heat sink on them when soldering.
    .0033uF are not too hard to find. You can parallel two caps. .001uF paralleled with .0022uF makes .0032uF.

    The .003uF in the circuit forms a High Pass RC filter with the reverb pot. It rolls off low frequencies at 6 dB per octave below the cutoff frequency.
    With a .003uF the cutoff frequency knee is around 530Hz.
    With a .0022uF the cutoff frequency knee is around 723Hz.
    With a .0047uF the cutoff frequency knee is around 338Hz.

    The .0022uF and .0047uF caps will start to roll off the lows approximately one half octave above or below where the original .003uF would roll off. I am not sure how noticeable it would be. Ime, it is possible the .0047uF could create oscillation trouble with a temperamental amp.
     
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  7. Huddy

    Huddy Tele-Holic

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    Very good info. Thanks a bunch! ;-)

    Regarding the reverb cap, I'm just trying use as much of what I have on hand as possible - which is a lot and you'd think wouldn't be an issue - but I'm kinda OCD about things matching unless there's a good purpose. So it's gonna be a nail biter deciding whether to parallel vs different brand. Yikes!

    Just need a handful of resistor values some silver mica caps and I'd like to try some PVC insulated wire this go 'round. I like cloth fine but... ya know... just wanna trying something different and see how it goes.
     
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  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    OCD... Make it match a Dumble. Spread silicon goo all over. It all matches. hah.
     
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  9. dan40

    dan40 Friend of Leo's

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    Do your self a favor and try some pretinned Topcoat wire. So nice to work with and holds it shape almost as well as solid core. I normally buy my 22awg Topcoat from Valvestorm but most tube amp suppliers also carry it.
     
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  10. Huddy

    Huddy Tele-Holic

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    That's what I've got coming from AES. I think the brand is Weico.
     
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  11. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    The reverb recovery has a fairly heavy load on the output, around 87k.

    That's close, but you've got to add the 470k and 220k in parallel with the 100k pot, so:
    2200pF = 828Hz
    3000pF = 607Hz
    3300pF = 552Hz
    4700pF = 388Hz
     
    Last edited: Apr 22, 2021
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  12. sds1

    sds1 Tele-Afflicted

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    The Weico topcoat 22AWG is good stuff, it's all I use anymore.
     
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  13. mabinogeon

    mabinogeon Tele-Meister Silver Supporter

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  14. NTC

    NTC Tele-Holic

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    I didn't know those existed - I need to try some.
     
  15. mabinogeon

    mabinogeon Tele-Meister Silver Supporter

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    Just to warn you, the leads are all of 3/16" long, whether you need it all or not... I just bend a little loop and solder some wire to it. The FKP2 has longer leads, but it's metallized polypropylene, not film/foil, if that sort of thing matters to you.

    (The MKP375 is also metallized polypropylene. It's rated to 1,000VDC though, which is fun. It also has copper leads whereas the WIMA's leads are tinned steel.)
     
    Last edited: Apr 23, 2021
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  16. mabinogeon

    mabinogeon Tele-Meister Silver Supporter

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  17. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    There is nothing wrong with silver mica. Get CDE 500V 5% or better from Mouser or Digikey. The ROHS versions are a tannish color, and look right at home where a ceramic would be used. I use them for treble caps, bright caps, the PI input on blackface circuits, and I have even used 3300pF ones for the reverb recovery coupling caps.
     
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  18. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Thanks for putting in the extra time to make it a little more accurate. Granted, I didn't put in the extra time. That is why I wrote around. Accurate? Nope. "Close" was enough to get the OP's question answered ime.
     
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