[Phil Allison[_3_]]

"Phil Allison"
In order to find the " open loop " THD of a Quad II output stage, I
removed the two EF86s and connected an output tranny primary to
the + supply and both plate pins.

The secondary was then driven by my bench oscillator, which has
very low THD - circa 0.002%.

This gave 52Vrms from grid to grid on the output valves - enough to
drive the stage to 10 W at 8 ohms and 1kHz.

Results: THD = 0.76%, DF = 1.

** Correction, the above THD figure is in error.

Magnetising current in the output tranny caused far more 3H than I had
hunched for the low level and 1kHz frequency I was using.

So I repeated the test with a 100W mosfet amp as a buffer between the
bench oscillator and the tranny - the THD from this set up was checked
at 0.04% at 52V rms.

The corrected figure is a staggering 0.18% THD.

I was so surprised at this result, I repeated the measurement in various
ways several more times.

Peter J. Walker sure knew what he was doing when he went with cathode
feedback for that output stage rather than UL or similar.


..... Phil