The fine tuning and mods continue on the twin 4cx250b homebrew amplifier.
I’ve been using it daily on 80-10meters for the last week.
While in QSO with old friend WB6BEE in Virginia on 10m cw…
Don said….your cw note has a “buzz” to it…
After our qso, I listened on a rcvr with a bigger speaker and good low end response Yes,there it was,and it would come and go as I tuned the final..I noticed at peak output, it had the buzz . If I turned the plate cap off resonance losing 10 or 20 watts..The note cleared up.
More testing…then the familiar smell of burning carbon resistors.
Yep, the parasytic choke on one tube plate had the smoking gun.
Those chokes were another thing I had planned on replacing anyway.Too flimsy too big a coil inductance.Three 1 watt, 220 ohm resistors paralleled.
Studying up on desirable characteristics for a plate parasitic choke.Low inductance and don’t use copper.
Art Collins always chose nichrome wire for parasitic chokes.Higher resistance made the inductors very lossy for vhf parasitics.
Not having nichrome…I chose brass shimstock I had.
about 1/4th the conductivity of copper.
Searching on line,I found others that had opted for 1/2 turn flat stock loops with the resistors nested inside.
These are about 1/2 inch wide strips,bent into a hairpin loop 1.5 inches long and about the same height.
Very smooth tuning now up on 10 meters like the other bands.
We will give it a workout this week before putting the amp back in its cabinet.
73’s
Ron
NU6F
