Yes, a small grid current through a 50 ohm load will not do much bias
shifting and impedance change.
73
Peter, DJ7WW
-----Original Message-----
From: Carl [mailto:km1h@jeremy.qozzy.com]
Sent: Montag, 25. Oktober 2021 18:21
To: Peter Voelpel; 'Rob Atkinson'; amps@contesting.com
Subject: Re: [Amps] 1:9 un-un for grid driven tetrode
The National NCL-2000, ca 1964-70
used a pair of 8122 tetrodes with an untuned resistive input.
The bias was AB1 on SSB and AB2 on CW where grid current was used.
Carl
----- Original Message -----
From: "Peter Voelpel" <dj7ww@t-online.de>
To: "'Rob Atkinson'" <ranchorobbo@gmail.com>; <amps@contesting.com>
Sent: Sunday, October 24, 2021 10:47 AM
Subject: Re: [Amps] 1:9 un-un for grid driven tetrode
> That 1:9 unun and 450 ohm resistor is obviously for use with a high power
> linear tetrode amplifier.
> Passive grid would not work with grid current.
>
> Tetrodes requiring a few hundred volts of drive are for instance 4CX10000
> or
> 4CX15000 and similar
>
> 73
> Peter, DJ7WW
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Rob Atkinson
> Sent: Sonntag, 24. Oktober 2021 13:19
> To: amps@contesting.com
> Subject: [Amps] 1:9 un-un for grid driven tetrode
>
> The unun is (my opinion) a waste of time. Just connect your grid
> input network, usually a pi network for tetrodes, to the grid and
> design it to do all the impedance transformation. It can be band
> switched and you'll be fine. I don't know what tube this is but its
> data sheet should give you the driving peak RF grid v. You also
> didn't specify the class of operation, but for class C the grid has to
> be biased around 5 times past cutoff. The data sheet should have
> "typical operating" specs for a certain plate v. and operating class.
> Divide the peak RF drive v. by the grid current given for the class of
> operation. That will give you the grid impedance. Construct a "grid
> dummy load" of carbon resistors that are close to the grid Z value and
> connect them from the grid socket lug to ground. Now you can put a
> vswr analyzer on the RF input jack and futz with your input network to
> get it in the ballpark to get the grid Z to 50J0. To get the drive
> needed, multiply the peak RF grid v. by grid current. You can get
> the needed grid current by dividing the peak RF v. by the grid Z.
> Tetrodes usually do not require a lot of drive compared to triodes.
>
> 73
> Rob
> K5UJ
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