New 5F6-A: what's up with these measurements

Discussion in 'Shock Brother's DIY Amps' started by TelZilla, May 13, 2019.

  1. TelZilla

    TelZilla Friend of Leo's

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    Details:
    5F6-A running a JJ5U4GB, two JJ 6L6GCs, and all three preamp tubes. All done with build and double checking, setting the bias, and I'm getting some really puzzling voltage drop measurements from pin 8 of recto tube to pin 3 of 6L6. Basically, I'm getting milivolt values.

    Measurements:
    • Resistance (Pin 3 of 6L6 to Pin 8 of 5U4): 59.4 ohms-outer, 54.5 ohms-inner
    • Voltage Drop (Pin 3 of 6L6 to Pin 8 of 5U4): -0.324 volts-outer, -0.327 volts-inner
    • Plate Voltage (Pin 3 of 6L6 to ground): 421 volts-outer,422 volts-inner
    • B+ (first node): 429 V
    • For what it's worth, it sounds great- Loud, dynamic. Ya know, like a Bassman.
    So if I move the decimal point of the voltage drop one decimal place to the right, I get a reasonable if a little too hot bias of 23 and 25 watts. Of course, I don't think there's any reason to do that. But math can be fun.

    Edited to add: I've measured with 3 different multimeters (including a fancy-pants Fluke) and keep getting these weird mV readings for the voltage drop.
     
    Last edited: May 13, 2019
  2. charlie chitlin

    charlie chitlin Doctor of Teleocity Silver Supporter

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    Whatever it is, by the time you get it sorted, you'll probably feel the need to send @Wally or @robrob a fruit basket. ;)
     
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  3. jhundt

    jhundt Doctor of Teleocity

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    what voltage drop do you think you should be seeing between those points?

    If the voltage at B+ first node is 429, and the voltage at the plate of your outboard tube is 421, I would say that the voltage drop across the output transformer is 8 volts.

    with a resistance of 59.4 over the output transformer, I=V/R would return a current of 0.135 amps. that sounds like a pretty high figure.

    perhaps I am missing something in your description. I never measure these kind of things this way.
     
    Last edited: May 13, 2019
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  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Are you trying to take a transformer shunt bias reading? I'm not really sure what's happening here.
     
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  5. TelZilla

    TelZilla Friend of Leo's

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    Something measurable in volts not Milivolts, right?

    For instance, something around 3 volts. I'm getting around 0.3 volts.

    Let's take the Outer 6L6 as an example: Ohms law says I=V/R, So my plate current would be 0.3241volts/59.4 ohms, which is .00545 amps or 5.45 miliamps. I would expect a value for the plate current 10 X that.

    If I then calculate Plate dissipation (Plate voltage * plate current or .00545 amps *421 volts), I get 2.29 Watts. Again, if I multiply this by ten, I get 22.9 Watts, which makes sense, even though I'd probably shoot for more like 21.
     
  6. TelZilla

    TelZilla Friend of Leo's

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    Just doing what my Uncle Doug taught me to do- Setting bais by calculating plate current (voltage drop/resistance from The plate of the power tube to the filament of the Recto tube), measuring plate voltage, and multiplying the two to get plate dissipation in watts. I've used this process before with no problem.

     
  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Well, measure the voltage at pin 8 of the rectifier and pin 3 of the tube (with respect to ground) and then see what numbers you get? If that's the same as your first B= node, looks like you have 7-8V, which would be your drop.

    Why you're getting that reading, I can't say.
     
  8. Wally

    Wally Telefied Ad Free Member

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    I have never taken such a measurement. The pin 8 on the rectifier is the source for the B+. That first filter node feeds the OT center tap. The plates...pins3...of the power tubes get their voltage from the two ends of that OT primary winding. Pin 5 is your bias voltage, which sets the bias.
    Other than to check the B+ voltage, the only time I contact that B+ is to perform a current draw measurement via the transformer shunt method.
    You have a drop between that pin 8 on the rectifier and pin 3 on the power tubes of approx. 8 volts...your numbers from above. 429vdc at the rectifier pin 8, 421vdc at the power tube plate. Your screen grids should be just a bit lower, I would ht8nk....or about the same.
    What is your current draw with the plates at 421vdc? My numbers sow me that at 421vdc, a current draw of 50ma gives you a plate dissipation right at 70%.
     
  9. Wally

    Wally Telefied Ad Free Member

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    okay.....I see what you are trying to do. Robrob has an excellent tutorial on this method. I don’t use this method.
     
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  10. jhundt

    jhundt Doctor of Teleocity

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    I used that method once under abnormal circumstances, far from home in a foreign land. I can't remember why, but it seemed to be an answer to whatever shortage of equipment I had. I had never used this method before, and probably will not again, since there are other methods that are (in my opinion) easier and more useful. This post reminded me of now much I forgot about that method. I think I better go read Rob's tutorial again, just to keep my chops up!
     
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  11. TelZilla

    TelZilla Friend of Leo's

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    So what method do you use instead? Where can i read about it?
     
  12. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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  13. robrob

    robrob Poster Extraordinaire Ad Free Member

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    On this webpage I detail how to measure bias three ways: Transformer resistance, Cathode resistor voltage drop & Transformer shunt. I recommend the first method because it's safer than #3 and it's universal. #2 only works with amps that have cathode resistors.

    https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm
     
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  14. TelZilla

    TelZilla Friend of Leo's

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    Interestingly, that page describes the method I'm using, among a couple of others (I bolded the part that matches what I'm doing):
    • The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side.
    Looks like I need to just do (B+)- Plate voltage at the 6V6s to get my voltage drop. As has been mentioned, that number seems to be somewhere around 8V. Once I'm able to verify those measurements, I may just have to leave the mystery of the milivolts where it is.
     
  15. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Right - you're not doing anything wrong, but I have no explanation where the mV are coming from.
     
  16. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Right - you're not doing anything wrong, but I have no explanation where the mV are coming from.
     
  17. Wally

    Wally Telefied Ad Free Member

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    I may have to try this method out sometime just out of curiosity. I doubt that it will change the way I do things. I read a comparison of the methods last week......there was so little difference that it was inconsequential. The shunt method is so quick that I am hesitant to use any other method. All of these methods demand taking measurements in a live circuit, right?
     
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  18. peteb

    peteb Friend of Leo's

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    pin 3 6L6 to pin 8 5U4 should be same as the difference between the plate voltage and the B1+ voltage. (they are both the drop across that OT primary)



    what does the voltage drop across the OT measure? 429-421=8V? or 0.32V?


    it cant be both at the same time
     
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  19. jhundt

    jhundt Doctor of Teleocity

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    maybe you have to stop and check what scale you are using when you are reading those questionable measurements. The fact that you are reading them the same on 3 different meters is difficult to explain. You should be reading them on the VDC (DC voltage) scale.

    Are we all using the same definition for "voltage drop"?

    you are not switching to the DC-current scale are you?
     
    Last edited: May 13, 2019
  20. TelZilla

    TelZilla Friend of Leo's

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    You have effectively restated my question and identified the source of my confusion
     
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