5f1 6v6 bias

Discussion in 'Shock Brother's DIY Amps' started by jlang, Apr 6, 2019.

  1. jlang

    jlang TDPRI Member

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    I'm getting the following readings in my latest 5f1 build. I'm I wrong in my math that my bias is currently 14.4? This seems really high for a 6v6 and I can't get a good reading from pin 5.


    Wall Voltage: 123v

    Power Transformer Output: 358v

    Heater Voltage: 6.3

    Rectifier Heater Voltage: 5

    B+1 Measured at the first filter cap: 398

    B+2 Measured at the second filter cap: 336

    B+3 Measured at the third filter cap: 279


    The 6V6:

    Pin 3: 386

    Pin 4: 337

    Pin 5: 0

    Pin 8 DC: 21


    The 12AX7:

    Pin 1 DC: 136

    Pin 3, Cathode DC: 2

    Pin 6 DC: 131

    Pin 8: 2
     
    Last edited: Apr 6, 2019
  2. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    Have you got the heater supplies mixed up?, it should be signal valve heaters at 6.3 and rectifier at 5, or is it a typo.
     
  3. jlang

    jlang TDPRI Member

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    Sorry about that. That was a typo. I'll edit that now
     
  4. jlang

    jlang TDPRI Member

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    Also, I forgot to add that I have a EH 6V6 and TAD 12AY7 installed. NOS 5y3 Rectifier.
     
  5. dan40

    dan40 Tele-Afflicted

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    Pin 5 on the 6v6 should be 0 because the 5f1 is cathode biased and does not need a negative bias voltage on the grid.
     
  6. jlang

    jlang TDPRI Member

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    Ok, thanks @dan40
     
  7. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Plate-to-cathode voltage, Vpk = 386 - 21 = 365 V

    You should have a measurement of you cathode resistor. Let's assume for now it is exactly 470 ohms.

    Voltage drop across the cathode resistor Vk = 21 V.

    Find cathode current, Ik, using Ohm's law. Ik = Vk/470 = 0.045 A or 45 mA

    Plate current, Ip, is estimated from cathode current. In a 6V6GT it is like 90-95% of the cathode current.

    .94 * 0.045A = 0.042 A

    Plate dissipation is plate current multiplied by plate-to-cathode voltage, (Ip)(Vpk) = 15.5 W

    15.5 W / 14 W = 110%
    15.5 W / 12 W = 130%

    https://robrobinette.com/Tube_Bias_Calculator.htm Thanks, Rob!
     
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  8. jlang

    jlang TDPRI Member

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    Wow that claculater does make things easier. Bad news though, it's worse than I thought. I should probably change out that resistor now. Thanks all for the help. I really appreciate it.
     
  9. jlang

    jlang TDPRI Member

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    @Snfoilhat So, If I use a 500ohm I bring it down to 14.5, which is still over. It looks like the next resistor value is 680ohm, bringing it down to 10.7watts, which seems really cold in a cathode biased amp. Does a value between those two exist?
     
  10. Uncle Daddy

    Uncle Daddy Tele-Holic

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    Throw in a JJ 6v6 and forget all about it!
     
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  11. peteb

    peteb Friend of Leo's

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    It’s likely biased exactly where it should be. Is there a problem with the amp? I wouldn’t think there would be.






    Are you concerned about the amp or the tube?



    The tube can take it. There are people who expect a single ended 6V6 amp to dissipate at least 16 W in order to sound the best. Older style 6V6s, not even JJs, are known to be able to take 20 W. I’ve seen it myself.
     
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  12. jlang

    jlang TDPRI Member

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    @peteb Well, everything is running ok and it sounds really good playing through it and I haven’t noticed any red plating or anything. I just don’t want to cook the tube or cause other issues because the bias is too hot. The specs for the EH that I have in there shows 14watts max. So that’s really what’s worrying me.

    @Uncle Daddy I have a JJ sitting around so that is an option.
     
  13. peteb

    peteb Friend of Leo's

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    I just lost an EHX in an early end, I just bought a new EHX to replace it because EHX have worked fine in the past and it was only $15.






    check it in the dark. my champs have a little faint (white stripe) running down the center of the plate.
     
  14. dan40

    dan40 Tele-Afflicted

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    These little amps have been running hot like that for years and some still have their original power tube. I would button it up and enjoy it right where you have it now.
     
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  15. jlang

    jlang TDPRI Member

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    @dan40 I’ve ordered a few resistors just in case I need them but I think that’s what I’m going to do. Close it up and just have fun with it. At this point it sounds so good I really hate to change anything.
     
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  16. John How

    John How Tele-Meister

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    You could also use a combination of resistors if you really wanna tweak it to a certain dissipation or just put in a robust tube and use it like has been mentioned...
     
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