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    Splitting cathode resistors in Dynaco tube amps



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    Splitting cathode resistors in Dynaco tube amps

    Post by GP49 on Tue Jun 15, 2010 1:01 am

    In conjunction with modification to the bias circuit to enable individual DC bias adjustment on the output tubes, owners generally will split the cathode resistance in the output stage of the Dynaco power amplifiers to facilitate measurement of the current flowing between cathode and ground.

    I've noted that in a lot of these home-brew modifications, as well as on some published modifications, the only thing that is done is to replace the single, shared cathode resistor with one cathode resistor per tube, of double the resistance. But in doing this, isn't something being left out?

    In the original ultralinear output stage, the two cathodes were tied together and grounded. When adjusting bias, the cathode current was measured by inserting an ammeter in the cathode circuit of each tube; the current was measured directly. David Hafler's Biaset circuit lifted the cathodes above ground by the value of the common cathode resistor but the cathodes were still connected together. That common connection of the cathode is broken when the modifications using two separate cathode resistors are done.

    That's of importance, actually. When current flow increases in the output tube, as in when a signal is being amplified, the voltage at the cathode rises. This increases the difference between the (negative) DC voltage on the control grid and the cathode, thus tending to make the tube conduct less. The effect is negative feedback generated by the signal current within the stage, causing nonlinearity, and I don't think this is being accounted for when this kind of circuit modification is done. Common-connecting the two cathodes tends to negate this effect because the output stage is push-pull, so when more current flows in one output tube, less current flows in the other. The change in the cathode voltage in one tube is negated by an equal but opposite change in the other. However...THIS WOULD OCCUR ONLY WHEN THE AMPLIFIER IS OPERATING IN CLASS A, which Dynaco tube amplifiers do for a significant power output..

    What made me think about this was the presence of rather large value, low voltage capacitors bypassing the cathode resistor in some commercial, non-Dynaco amplifiers. These would ground the cathode at AC audio frequencies, so that within the audio band, the negative feedback effect within the output stage would not occur, while DC conditions at idle would not change. Connecting the two cathodes together with a nonpolar capacitor selected for AC bandpass in the audio range would allow the push-pull stage to negate the negative feedback effect, also.

    Bob, or anyone you have any thoughts about this? Would the bypass capacitors to ground work better, or the nonpolar capacitor connecting the two cathodes?

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