5E7 Biasing issues

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jscballer, Dec 17, 2009.

  1. jscballer

    jscballer Member

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    So i built a 5E7 using a similar setup as the weber layout, and from pin 3 of my power tube to the center tap of the ot it reads 12 volts, which I think is way to much, it read earlier when i was using the a 6.8k resistor that the schematic says for fixed bias, but then I swapped it for a 4.7k and added a 50k pot instead of the 56k resistor and it still ran hot so I kicked the 4.7k to a 3.3k and Ive gone from 8 to 12volt and the fins in the tubes start to glow red.

    Thanks,

    John
     
  2. mark norwine

    mark norwine Member

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    Vdrop over the OT pri......by itself......tells you nothing.

    You think 12V is way too much? Based on what?

    What you want to know "current through"....not "voltage across"

    Take that voltage (12V) and divide it by the cold [i.e. amp off & drained] resistance from the same points. The qoutient of that will be the current through the plate.

    It is that current....and only that current.....with which you should be concerned.
     
    Last edited: Dec 17, 2009
  3. jscballer

    jscballer Member

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    i did that, and I know that value, but isnt current a function of voltage and resistance, so if the voltage changes and the resistance stays the same then the current changes
     
  4. jscballer

    jscballer Member

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    the reading I get from pin 3 to the center of the ot is 66 ohms the data sheet for it says 4000 so could it be a bad transformer?
     
  5. mark norwine

    mark norwine Member

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    Correct.

    I'm only as good as the info presented. Your original post said nothing about cold resistance, and didn't indicate in any fashion that you had done any math or knew your quiescent current. Thus, I offered what I did.

    Let's start over: you (say) that you know your resistance; you know your voltage, therefore you know [or can calculate] your current. Can you provide these numbers?

    Were it mine to do, I'd multiply the current I want (say .035A) by the resistance [presently unknown to me] to determine an approximately-desired Vdrop.

    Then I'd fire it up and tweak the neg supply (you said you put in a pot, right?) until I read the Vdrop desired. Having pre-calculated what that voltage means (in terms of the corresponding current), I’d set it & forget it.
     
  6. mark norwine

    mark norwine Member

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    I wrote my response (above) while you were posting....sorry

    First of all, do NOT confuse impedance with resistance! They are not the same thing! For this calculation, you want to use the DC resistance.

    66 ohms * 0.035 amps = a Vdrop of about 2.3 volts.

    You said you measured 12 volts of drop? That was 181mA through the tube. That's bad. You may have cooked yoru tubes already.
     
  7. hasserl

    hasserl Member

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    Doh! That sucks!

    I'd suggest you take the output tubes out of the amp before you keep going. In case you haven't already damaged them this will keep them from cooking while you get this figured out. Then measure the voltage you're getting at pin 5 of the power tubes, you need to have around -50vdc there. Once you get that established, then put the tubes back in and try it again.
     
  8. jscballer

    jscballer Member

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    so i tried another set I have and it still read 8v and it didn't change when I turned the bias pot so what else would I have to change to lower this value.

    Thanks,

    John
     
  9. mark norwine

    mark norwine Member

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    your bias supply is either wired wrong or not going "negative enough".

    rebuild your bias supply,
     
  10. hasserl

    hasserl Member

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    See the post above yours. You need a negative bias voltage to pin 5. Get that established first.
     
  11. Trout

    Trout Member

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    Exactly,

    the 6.8K resistor should not have been changed, Get the voltage reading between the center lug of the bias pot and chassis/ground.

    Post the voltage reading measured at the center lug of the bias pot, unloaded should be XX volts, and loaded with tubes should be XX volts

    Note diode polarity orientation.

    Look at this layout and confirm you are wired correctly.

    Or look at this schematic if you prefer,
    5E7 Schematic

    I have used both versions of this particular bias circuit, the Ceriatone works great and has a finer adjustment,
    The Killback version works well also. I like the Killback version simply because it is just a bit more aggressive when turning the pot.
     
  12. jscballer

    jscballer Member

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    so I rewired it with a 6.8 k resistor and a 50k pot to ground, but it still reads 8volts between pin 3 and the center tap of the ot, I will try and get readings from the bias pot to ground and get them posted.

    Thank you,

    John
     
  13. mark norwine

    mark norwine Member

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    Your bias supply is wrong. Do not tweak values; scrap it & rebuild it, from the ground up. I'm 99% certain that you have a construction problem somewhere.....not a "component value choice" issue.

    Also....it is NOT the "voltage across" that means anything.....it's the "current through" that you should be caring about. Take the "pin 3 to CT" voltage & divide it by the cold resistance of the OT's winding to determine the quiescent current. You need to be "thinking current". Get your mind around the difference; doing so will aid you in the concept of this.
     
  14. Trout

    Trout Member

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    Is the bias cap backwards? The positive goes to ground, that is a common mistake guys make.


    +1, its pretty clear it is wired incorrectly.

    OP,, Post some pictures of the bias wiring where the pot, diode and cap are mounted.
     

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