On Apr 2, 6:27 pm, "Bret Ludwig" wrote:
This is very true and interesting. RCA figured out you could strap
the control and screen grids of certain power pentodes and use them as
low or zero bias triodes, and a couple of RCA theater amps do this.
This is also seen in AM era editions of the W6SAI Radio Handbook for
modulators. It is simple. It works. Based on long time experiments it
was concluded it doesn't sound very good at all.

Several of the last of the RCA MI-series cinema amps did this with
6L6s and I believe 6146s. None are desireable to the audiophile in
stock form.

I have never tried it, on the theory if it worked very well it would
be popular.

The 811 is certainly an attractive tube economically. It has a mu of
160 as I remember. The Altec 1570B used them as did the Gotham Audio
cutter head amp using a GR toroid for output.

They showed why toroids are a bad idea for opt's very well, although
there are still a lot of people not listening. With a conventional or
C-core transformer and a revised feedback loop the basic design is
well worth study, though.

The 811 can be substituted in the MI200 McIntosh if the proper
intermediate transformer is put in so as to get the filament voltage
at the 6.3 rather than the 10 volts of the, I think, 8005s. Although
the amplification factor of the 811 is way too high it works anyway.
Distortion still meets Mc specs. We wound one, in fact, where I worked
on a toroid, because we had plenty of scrap surplus transformers we
could dewind (and take the copper home.) And a Gorman winder, which
makes winding toroids pretty fast and simple. The 811s pull more
current but the Mc filament transformers never even got hot.