I'm currently playing around with a YC156 and have and have discovered an
interesting phenomena which I'm having difficultly explaining.
On previous amps I have never really bothered tuning the i/p matching
until I've hit it with RF (I build single band amps, so its quite easy to
guess).
Now I have a MFJ I'm able to "play a little" and do a cold test setup
first.
I set my input matching up for 1:0 SWR with the MFJ, at this time the
heaters are not on. Soon as I switch them on SWR shoots up to 2:1, then over
a 6 minute period gradually to 4:1.
So I retuned for 1:0 SWR once things had settled down, after 6 minutes, and
it stays rock solid all day long.
Of course in the real world I'm not going to ever engage the i/p circuit
until minute 6 of warm up with this tube, at which time the i/p SWR is
always now 1:0, but I'm puzzled in what may be happening?
Has anyone else ever experienced this with either the YC156 or any other
tube?
This is how its set up now.
Heaters off swr = 4:1 on i/p match
Switch heaters on, swr immediately goes to 3:1
After min 1 swr = 3:0
After min 2 swr = 2.9
After min 3 swr = 2.7
After min 4 swr = 1.8
After min 5 swr 1.3
After min 6 swr 1.0 ..and stays there until the heaters go off.
This to me looks like a thermal "thing" due to the large size of the
heaters on a YC156 and heat expansion.
What do others think?
Neil G0JHC
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