5e7/5f4 adjustable bias

Discussion in 'Shock Brother's DIY Amps' started by mgrossm, Sep 17, 2021.

  1. mgrossm

    mgrossm Tele-Meister

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    Anyone have experience using one of the mini 50k trimpots to do this? Just used one on an 5e8a a d it worked out nicely...I think I had bias tap>4.7k>diode>50kpot>10k>ground. Went something like this picture but with 4.7k instead of 2.2k:



    (but any thoughts on what value resistors to use or experience with this would really help. Seems after reading through all the threads people have tackled it all different ways)

     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Bias taps from the transformer can be anywhere from ~50VAC to over 350VAC. Depending on where you start, the value of the resistors will vary. You can try to crunch the numbers to arrive near the values or have a handful of resistors to sample in the circuit. One could also use a pot to dial in the resistance, but remember you may have 300V on the pot so using that method will require careful safety measures.

    Ime it is fastest to audition several different resistances by just clipping them in. When a resistance is chosen solder it in. OMMV.
     
  3. mgrossm

    mgrossm Tele-Meister

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    Its 50vac
     
  4. 2L man

    2L man Tele-Holic

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    When using lower bias AC voltage, I think there should be rectifier diode first and filter capacitor should be just after. And then follow the voltage divider circuit. Then 50VAC turns to about -70VDC which is more stable bias PS voltage and bias circuit design comes easier.

    50k potentiometer make bias adjustment coarse!

    On your drawing I think voltage does not get low enough? Potentiometer can be used as "voltage divider" and "adjustable resistor" which your draving does and then because there is 10k resistor to Ground it effects so much that I think Grid voltage never get low enough for tubes to conduct bias current.

    I can't say what would the input voltage will come but lets assume it comes -60VDC. If pot is turned to 0 ohms there are 2,2k and 10k resistors doing voltage dividing and math comes: -60V : (2,2k + 0k + 10k) x (0k + 10k) = -49V which seem too high.

    If you change it taking jumper away and connect bias current leaving from "slider", the pot turns to a voltage divider it works better, but still might be coarse to adjust. Then high end comes 60V : 62,2k x 60k = ~58V and low end 60V : 62,2k x 10k = ~10V. And here the biggest variable is what the bias PS voltage will be?
     
    Last edited: Sep 18, 2021
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