"Phil Allison"
"Patrick Turner"
The published data for that UL of operation includes THD curves that
barely
exceed 1% below clipping - with NO feedback applied .
Well, a number of things affect published data which we always
must question about conditions for the test unless fully stated.
** See:
http://www.r-type.org/pdfs/el34.pdf
37W = 1.3% THD class AB ( just )
20W = 0.8% THD class A
Schem and curves are shown half way down the same page.
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** More info:
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.
Also tried were Electro-Harmonix and GE 6L6GCs with closely similar
results, the EHs producing 0.8% and the GEs about 0.7%.
With EF86s restored to their sockets and the OP tranny removed, THD
dropped to 0.036% ( 10W, 8ohms) and DF improved to a respectable 10.
My conclusion is that the two EF86s reduce the THD of the output stage in
line with the level of NFB being used ( ie 26dB ) while adding virtually
*NONE* of their own.
Nice.
..... Phil