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@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@contesting.com> On Behalf Of jim.thom
jim.thom@telus.net
Sent: Saturday, October 23, 2021 9:43 AM
To: amps@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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