[AMRadio] 807's triode connection question...

[John E. Coleman]

I think it was RCA labs that found the triode connection using the 20K
resistor from grid to screen and driving the screens in a zero bias
Class B arrangement.  As Class B circuits go I think this was found to
be the lowest distortion.  The plate voltage I think was 750V.  There
was an article in something I read years ago about this.  It may have
been in one of the RCA Tube manuals.  I have used the zero bias circuit
with 807s and 1625s and ran as much as 1800 Volts on the tubes got lots
of audio but would occasionally arc over a tube on the inside ( Don't
say FouooooR into the microphone, HIHI ).  They were 25 cent each back
then.  I went through about 8 of them in a 2 year period, until I went
to four 811A's.  Let me say this, four 811A's, running Class B, with
about a few volts of bias, and 2000 Volts on the plates would produce a
lot of clean audio. And so would the 807s. 

I recommend the 807/1625 triode connection, zero bias using the 20K
resistor from grid to screen and driving the screens.  It's easy, it's
clean, will produce 100 watts or more, if 2 in Push-Pull (or 4 in
Push-Pull-Parallel for 200 watts or more).  You shouldn't run less than
600 Volts or more than a 1000.  The increased current capability of the
Four Tube in PPP arrangement, also will help if you need to work into a
lower load impedance.  Sometimes it's easier to add tubes than it is to
get a different modulation XFMR for Z matching.

Good Luck
John, WA5BXO

-----Original Message-----
From: amradio-admin at mailman.qth.net
[mailto:amradio-admin at mailman.qth.net] On Behalf Of ka1iic at prexar.com
Sent: Thursday, July 04, 2002 5:02 AM
To: amradio at mailman.qth.net
Subject: [AMRadio] 807's triode connection question...

Hi guys me again,

I've been looking up all the info I can get about using 807's in triode
connection....

I have found 3 different cases and was wondering about the benefits of
each....

First off the older ARRL handbooks show connecting the grid to the
screen
through a 20k ohm resistor and driving the screen...

Another publication shows connecting the screen to the plate directly
and
driving the grid as you normally would...

And case three shows connecting the screen to the plate through a 100
ohm
resistor and driving the grid as per the norm...

Any guidance is welcome...  any comments welcome also...

Thanks in advance

73
Vince
ka1iic
-.--.