Alternate Individual Bias Adjustment Modification

Discussion in 'Fisher' started by dcgillespie, Jun 30, 2014.

  1. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

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    I just got one installed in my Fisher 400, but I had an issue with a mix up with my trimmers, being that 2 were only 1 k pots.....
    Followed some diagrams and looked at lots of pics and it is actually quite straight forward. After doing one board, it's a piece of cake
     
  2. RS Steve

    RS Steve Tube Junkie Subscriber

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    Assembled my first individual bias board, still have to solder it up and connect the leads, then 5 more to go. :beerchug:

    20161218_165728.jpg
     
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  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Well that oughta keep you busy. Looks good!

    Dave
     
  4. RS Steve

    RS Steve Tube Junkie Subscriber

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    So this was the easy part for me, trying to solder everything up correctly is going to be my biggest challenge. I'm just not grasping the schematic enough to make it transfer to the layout. I get brain freeze to easy I guess. :confused:

    20161221_150142.jpg
     
  5. triode17

    triode17 Active Member

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    Dave, I built a stereo, PP Pentode amp. a while back which included a 10 ohm resistor in each cathode leg to individually adjust each tube for approx. 70% plate dissipation. A common bias control is used for each pair and a dc balance control is installed as well. Fine, but, I now realized that it may be more important to set the dc offset from plate to plate to zero volts than to match the output currents. Of course, due to the fact that most output tranny's use unequal length windings on each side of the primary (because one coil is usually wound on top of the other, making one length of wire longer, thus having more resistance).
    This led to me realizing that each tube in the pair will be running at a different power dissipation, like 65% and 78%. Wouldn't one tube would then wear out faster than it's brother?
    So, ultimately, which is more important to set, equal currents or zero dc offset?
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Equal quiescent current. In the bigger scheme of things, the slightly different DCR between the halves of the primary winding is insignificant.

    Assuming your question is based on Class AB operation (I take it it is, based on your target for plate dissipation), in an ideal PP output stage (perfectly balanced output stage drive, output tubes, and primary winding except for DCR), it is the proper amount of balanced quiescent current between the two output tubes that sets up a smooth and seamless hand-off between the two tubes when Class AB commences. It takes both of these elements for a seamless event to occur. If the tubes are unbalanced under quiescent current, or the quiescent current is not of the correct amount, then distortion increases due to an imperfect hand-off.

    This is not unlike trapeze artists performing at a circuit event. When the timing is perfect, the two trapeze both come together at a precise time and distance, such that the artists can transfer from one trapeze to the other in a perfectly seamless event. But.....

    1. What if one trapeze is suspended lower or higher than the other? This is the equivalent of the two tubes being out of balance. The two trapeze may still both come together at exactly the same time, but if one is significantly higher or lower than the other, the transfer cannot be made because of the gap created due to the unequal height of the two trapeze. Or.......

    2. What if based on the point at which the two trapeze are suspended, the two trapeze are of equal length (balanced), but they are both too long or too short relative to their points of suspension? The two trapeze with either collide in mid air, or again, never come close enough for the transfer to be made -- even if their timing is perfect.

    The analogy is not perfect, but it serves to make the point. It is the balanced current draw through the tubes that is the important point of consideration -- not an exactly equal DC voltage drop across both halves of the primary winding -- which would imply that the quiescent current through the two tubes is in fact not equal. This is because it is the tubes that control the current flow through the winding -- not the winding that controls the current flow through the tubes.

    Now, an exactly equal AC (dynamic) balance is critical across both halves of the winding, which is a product of both the construction of the transformer itself, and in the real world, the tweaking the AC drive to the output tubes so that an equal drive is in fact presented to the primary winding by the output tubes. Of course, in our perfect output stage, no tweaking of our perfectly balanced drive to our perfectly balanced output tubes would be required. A slight difference in DCR between the two halves plays very little if at all into AC voltage developed across each half of the winding.

    Finally, if the output stage is designed correctly, then at the point of correct, balanced, quiescent current draw (the operating point), the plate dissipation of the tubes will be at a safe level, as well as throughout the entire dynamic operating point (the load line) of the tubes as well, such that an appropriate amount of tube life will be achieved.

    I hope this helps!

    Dave
     
  7. triode17

    triode17 Active Member

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    I hope some one hears this. I know it's an old thread. But I have this problem right now. In my Regent P-P amp (my design), if you adjust the tubes for Zero Vdc offset, the idle currents will be different. On the other hand, if you set the idle currents the same, then the Plate to Plate offsets are different. This is all due to the output trans. primary having different resistances from CT to either side (plate). So which do you choose???
     
  8. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Set the Idle Current's! Read Dave's post above your's very closely. I believe this will answer your questions.
     
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  9. triode17

    triode17 Active Member

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    Dave- I did an experiment, (using my True RTA, 24th octave spectrum analyzer) in which I set the tubes for equal bias current, to produce 70% Plate dissipation, then to zero plate to plate voltage to compare any differences in harmonic distortion. The former test ( @1w RMS, 1kHz) produced only second and third harmonics. But the latter test produced only the third harmonic, at the same level as before. So, with the transformer 'balanced', the even harmonics truly do cancel, as the theory predicts.
    I performed another experiment and unbalanced the current through the two tubes until the 2nd harmonic rose to the same level as the 3rd harm, then measured the DC offset across the trans. primary. The offset had gone to a 1.4Vdc difference before it really may a difference. With the tube currents set to be equal, there was only about 0.3vDC difference. So your point is well taken that the DC offset is insignificant once the tubes are balanced.
     

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