"Patrick Turner"
I have very nearly finished re-engineering a pair of Dynaco MkIV monos.
** Question for Pat.
The Dynaco Mk IV uses two EL34s in UL mode with a 470V supply - OK.
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.
-------------------------------------------------------------------
However, Dynaco claim virtually the same THD figures, with circa 20dB of
feedback applied ??
I searched around for data on MkIV and I didn't find
any info saying THD was the same with or without NFB.
** Of course you did not, cos that is NOT what my post says !!!
Is the 7199's pentode gain stage mainly responsible for this anomaly ??
For comparison, the Quad II use two, low cost, EF86 pentodes in
differential mode to provide gain and phase inversion. The output stage uses
local (cathode) feedback, plus overall feedback but is otherwise
conventional PP with KT66s or 6L6GCs performing equally.
The Quad II achieves 0.1% THD at rated power and 0.05% THD at circa 10 watts
or less.
Well all that depends on load conditions, and perfect tube matching, and
almost no Quad-II amps I've tested achieve these wonderful figures you
quote.
** Really??
IME they all do.
My example tests 0.03% at 10W with rather worn Sylvania 6L6GCs and very old
EL84s.
..... Phil