After several failures of Dow-Key T/R switches and unsatisfactory results
from open-frame relays, I have decided to use an old, tried but true
technology: A knife switch.
I have three here: one is about 2" X 4 1/2" , DPDT. The other two are
larger, about 3 1/2 X 5 1/2", one on a phenolic base and the other on a
ceramic base. I assume the ceramic base would be the best for 1,500 w. r.f.
applications at h.f. My concern is the apparent lack of isolation between
the input switch and the output switch, since the switch bars are only about
1 1/2" apart on the big switches. The amp is a pair of 4CX1500Bs, grid
driven in AB1 and only 8 w. is needed to excite the tubes to produce legal
limit output. While experimenting last week I got the worst r.f. burn I have
ever gotten, but have now wrapped the switch bars in Saran Wrap and I wear
rubber gloves when operating the amp. Feed back caused by coupling between
the switch sections is a big, big problem and it has to be eliminated. I
have put an upright piece of aluminum (about 4" X 2" between the switch bars
but now I can't throw the switch. Anyone have any ideas. Also, my T/R
switching time has gone from 18 ms. to something just under 4 seconds, unless
I stand up all the time.
Thoughts????
P.S. Thanks to all those who helped me solve the knob orientation dilemma.
|