[AMPS] Cathode Impedance

[Larry Molitor]

The need to add my $0.02 to this thread has become overwhelming. So here is
something that at least a few of you might appreciate.

I copied this chart out of a book some 25 or 30 years ago. I don't recall
the title and recently a search failed to find said book in my library. No
matter, I made copies of this chart and keep one in my desk at work, one in
my shack file, and one posted next to the TP dispenser in the head.

Those of you who are clever enough to keep your email software set to
display a fixed font may actually see this in table format.


TUBE		Ep		Ip		Cathode ohms

811A		1250		175		320
		1700		160		320

572B/T160L	2400		250		215

813		2000		200		270
		2500		200		270

3-400Z/8163	2000		400		120
		2500		400		120
		3000		333		120

3-500Z		2000		400		115
		2500		400		115
		3000		370		115

3-1000Z	2500		800		65
		3000		670		65
		3500		750		65

3CX1000A7	2500		875		41
		3000		800		42

4-125A		2000		105		340	
		2500		110		340
		3000		115		340

4-400A		2000		265		160
		2500		270		150
		3000		330		140

4-1000A	3000		700		104
		4000		675		106
		5000		540		110


Subject to any typos I may have made, this is an abbreviated version of the
chart I copied many years ago. 

Obviously the tetrodes are Triode-Connected-Grounded-Grid. The only claim I
can make is that the data has been spot on for all the tubes I've tried.
It's so close that I no longer put adjustments on the components in the
cathode pi-net.

By the way, leave the damn tube in the socket when you measure the network
match. Why tack in a cap to synthesize the tube capacitance when you have
the real thing. Besides, why put more wear on the socket contacts. I always
have the whole thing together including the entire anode compartment before
doing the cathode network.

Using this method, I can usually turn on the amp and not see the SWR meter
on the exciter even wiggle at full power out. While other methods may have
some theoretical advantage, they have no practical merit in my
not-so-humble-opinion.

I hope someone gets a little use out of this, it took me a long time to
type it all in with two fingers.

73,

Larry - W7IUV
w7iuv@axtek.com
http://www.axtek.com/w7iuv




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