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Balance Compensation

I am wondering if it is possible to partially compensate for the intrinsic gain imbalance of the output stage (with no feedback and without resorting to a Futterman phase splitter returned to output), at least in the case of a load equal to the nominal speaker impedance (8 Ohm).

The 6080 (double triode) tubes that I have used have a plate resistance of 300 Ohm and mu=2 (gm = 6.5 mA/V). The parallel of four 6080 tubes exhibits a nominal plate resistance (rp) of 300/4=75 Ohm.

Based on these values and standard formulas the nominal gain of the upper triodes is 0.1616 = mu/((mu+1)+rp/8 Ohm) and the nominal gain of the lower triodes is 0.1927 = mu*8 Ohm/(rp+8 Ohm).
In other words the gain imbalance is 19.2% (0.1927/0.1616).

If one fixed 6 Ohm resistor were added in series with the cathodes of the lower triodes the corresponding gain would go down from 0.1927 to 0.1584 = mu*8 Ohm/(rp+8 Ohm+(1+mu)*6 Ohm) and the resulting nominal imbalance would be less than 3%.

Could it be an alternative (for even harmonic reduction) worth to try?



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  Kimber Kable  

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