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

[Steve]

Core choice, wire length and winding style all play a part.

https://cromwell-intl.com/radio/9-1-unun/  and
https://g8jnj.webs.com/Balun%20construction.pdf

might help figure out what's happening.

Doesn't a coil in parallel with the 450ohms to cancel the tube's 
capacitance mess things up at lower frequencies? What's the Cin?

Steve G8GSQ

> 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