Date: Sat, 20 Oct 2018 14:21:46 -0400
From: "R.Morris" <robrk@nidhog.net>
Cc: "amps@contesting.com" <amps@contesting.com>
Subject: Re: [Amps] Super Cathode Driven Tetrodes
Message-ID: <7DA69145-8F85-497E-A73A-EA706B9BBDC0@nidhog.net>
Content-Type: text/plain; charset=utf-8
<Whats the Collins 30S1 doing?
## The 30S1 is cathode driven. Requires 70-80 watts of drive. It
uses a PI tuned input, one for each band,
bandswitched, just like a SB-220. Added twist is... the B+ supply, screen
supply, and bias supply are all strung in series.
Collins used this higher drive config, since tube type xcvrs and TXs back
then, had no easy way to vary the power output on SSB
except via alc. Their 100 w pep xcvrs and TX could then be reduced to
the 70-80 watts required.
## If passive grid config used on the same 4CX-1000, or 4CX1500B, only 10
watts of drive required. Grid driven, with
a regulated screen supply. Buddy across town uses this config on his hb
4CX-1500B. 1.5 kw out with 10 watts of drive
on the low bands, 20 watts of drive required on the upper hf bands. Cathode
is RF grounded with a pair of .01 uf bypass caps.
On bigger tetrodes, a 1:4 unun or 1:9 unun is used, to step the input Z up
from 50 ohms to either 200 ohms or 450 ohms. Then less
drive is required to develop the required peak grid voltage, when driving the
grid positive. A 50-200-450 ohm small globar is wired between
control grid and chassis.
## RE the W8JI writeup on his website. That is for the G2DAF
configuration, and NOT super cathode driven. G2DAF is a grid driven
config, BUT rectifies the incoming drive power, to make screen voltage !
Keyed, but with no drive applied, you have NO screen voltage..
and also NO idle current. I have a 2 x 4-400A G2DAF amp, works good, nobody
bitches about imd, but its a silly circuit really..and used
tubes to rectify the incoming drive RF. Local buddy used SS to rectify the
RF, worked on the lower bands. The screen
caps have to be very carefully selected to make it work right. The G2DAF amp
bascily goes from class C to class B, to Class AB and back
around in a circle. I have seen everything from 4-1000s, 2 x 4-400, and
also 4 x 4CX-250Bs using the G2DAF config..back in the 70s.
## Super cathode driven config bonds the control grid to the cathode.
Screen grid bonded to chassis. Those were popular back in the
70s. A local ham, used a 4CX-1000 in super cathode driven config..and also
another ham buddy, when I was up north back in the 80s. Both
amps operated much the same. 160 watts of drive to get 1200 w pep out. You
cant just ground the control grid and screen grid, and operate the
4CX-1000 like a high mu triode...like a 4-1000. Grid diss is ZERO watts
on a 4CX-1000..and only 1 watt on a 4CX1500. The collins config is unique,
in that it gets around this problem, albeit with a semi complicated circuit,
that requires higher drive levels.
## Bigger tubes , like the 4CX-5000A, can have their screen grid and also
control grid bonded to chassis, and operated like a high mu triode, albeit with
the
caveat that the rated control grid diss is only aprx 1/2 that of the eq
triode version of the tube. Same deal on a 4-1000 or 4-400, when used as a
high mu triode.
with both screen and grid bonded to chassis. Grid diss is only 1/2 that of a
3-1000Z or 3-500Z. Higher drive required ....and IMD is not as good.
Jim VE7RF.
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