Filter caps & bias/output

Discussion in 'Shock Brother's DIY Amps' started by jsnwhite619, Feb 9, 2020.

  1. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I started a thread about a month ago when I recapped and tweaked my 5f11. I thought I'd start a new thread in case some interesting info came of it.

    I found my original build thread from it - lucky for TDPRI because I'd never keep up with notes this long - and compared the original Mojo 3x16uf filter caps to the measurements with the current 3x22uf F&T caps. Wall voltage tonight was 121v which is approx. what I usually measured at my old house when I built it, but I don't know what it was when I measured it then. I adjusted the bias to get to the same plate to cathode voltage and measured everything else from there.

    From my old thread: https://www.tdpri.com/threads/starting-a-5f11-vibrolux-build-bias-question.691516/page-4

    • -32.94v bias
    • 373v plate to cathode
    • 24.7mA at 1-ohm resistor.
    • Voltage on the 5Y3 side of the 100K resistor is 338v. Diode side of the 100K is 193v
    • B+ is
    • 377v
    • 374v
    • 289v
    • Rob's Bias Calculator says I'm at 65.8% dissipation
    Same tubes and tonight's measurements:
    • -32.99v bias
    • 373v plate to cathode
    • 21.8mA at 1-ohm resistor
    • Voltage on the 5Y3 side of the 100K resistor is 311v. Diode side of the 100K is 129v
    • B+ is
    • 378v
    • 375v
    • 290v
    • Rob's Bias Calculator says I'm at 58.1% dissipation

    So, the only real differences are voltage from the rectifier to the bias circuit (-8% before the diode and -33% after the diode), and the bias dropped nearly 8% cooler.

    I just finished up the new cabinet, and it's been a long time since I've played it, but I felt like it was cleaner & punchier than it was before. I had it set around 55% and it stayed clean with a Tele all the way up the dial.

    I guess my question for discussion is "Can heavier filtering affect tube bias?" We see posts all the time about upgrading filter caps and getting better bass, output, clarity, etc. But, is the bias related, and if so, is a cooler bias at the same voltage level the reason for the other better known attributes?
     
  2. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Btw, here is a shot if the new cabinet. Still got the Cannabis Rex in there, but I have to say that the Legend 1258 has kinda won me over to that Fender sound.
    [​IMG][​IMG]

    Sent from my moto g(6) using Tapatalk
     
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  3. Wally

    Wally Telefied Ad Free Member

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    Jason, where is this 100k resistor? The original scheme calls for a 10K, doesn’t it?
    As for the differences, I don’t see how a change of filter caps could affect those voltages and current draw there that have obviously changed. Odd, it seems to me. Is the vibrato circuit shorted out of the circuit for these measurements? Same tubes?
     
  4. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Sorry. My PT doesn't have a bias tap, so this is the method I used, except a pot vs. fixed resistor.

    [​IMG]
     
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  5. D'tar

    D'tar Friend of Leo's

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    Temperature perhaps?
     
  6. Wally

    Wally Telefied Ad Free Member

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    Odd problem, ime, or perhaps variation would be a better word?
     
  7. schmee

    schmee Poster Extraordinaire Silver Supporter

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    I have found that , at least in some amps, filter cap changes don't make a huge difference. I'm not a super loud player though.
    For instance, In a single channel hot rod Princeton Reverb (6L6/Super Reverb trannys) I built, I tried 44/44/44/20, 33/33/33/20, and 20/20/20/20. The difference was very minimal... not sure I could tell any difference actually at reasonable volumes.
     
  8. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I think it helped deepen the tremolo on this one, but I don't have any evidence other than my ears and memory.



    Sent from my moto g(6) using Tapatalk
     
  9. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    Are we making assumptions about a new electronic part that the manufacturer states has a +/- 20% tolerance? (The datasheet indicates up to 6uA leakage).
     
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