[Amps] 1:9 un-un for grid driven tetrode

[Victor Rosenthal 4X6GP]

What I did with a 4CX1000A AB1 amplifier was to put a 50 ohm 
non-inductive resistor from grid to cathode, and feed it through a 
pi-section lowpass filter (I used 35 MHz as the cutoff). The input 
capacity of the tube was swamped by the output C of the filter. I had a 
1:1 SWR from 160 through 10m. I used the Elsie program that came with an 
ARRL handbook to design the filter. The coil was just a few turns of #22 
wire about 6mm in diameter. I don't recall the capacitor values. It was 
very easy to do.

73,
Victor, 4X6GP
Rehovot, Israel
CWops #5
Formerly K2VCO
https://www.qsl.net/k2vco/

On 25/10/2021 20:13, paul at paulbaldock.com wrote:
> A much simpler solution would be to use a 50Ohm resistor, no un-un, and more
> drive power. I don't know what the tube(s) is you are trying to drive, but
> for example a pair of GU74B's would require about 50W drive in to 50Ohms and
> give 2KW PEP out.
>
> - Paul KW7Y
>
>
> -----Original Message-----
> From: Amps <amps-bounces at contesting.com> On Behalf Of jim.thom
> jim.thom at telus.net
> Sent: Saturday, October 23, 2021 9:43 AM
> To: amps at contesting.com
> Subject: [Amps] 1:9 un-un for grid driven tetrode
>
> A buddy is trying to get a 1:9 un-un to work on his grid driven tetrode.
> Terminated in a  450 ohm globar,  SWR is  1:1   but only up to aprx  20 mhz.
> Above  20 mhz,  it rapidly all goes to hell....reaching  2.5:1   at 29.00
> mhz.   ( this on the test bench)
>
> He needs the  1:9  un-un to work from  80-10m.   What's the trick here ?
>   What  is the ideal material to wind the trifilar onto ?
>
> Once eventually installed into the amp, a small  coil is wired in parallel
> with the 450 ohm globar to  cancel out the XC from the tube,  but that is a
> normal procedure, and will not alleviate the high swr issue on the upper
> bands.
>
> Jim   VE7RF
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