Pete Smith wrote:
I can testify that, even when isolated with a strip of silicon rubber
around its circumference, the HC-1 vacuum relay in my modified SB-220 is
noisier than the original open-frame relay. Before you do radical
surgery, try Dave's suggestion, and maybe substitute some short flexible
braid for any rigid wire connections to the relay, all in an effort to
isolate contact vibration from the chassis, which acts as a sounding board.
See <http://www.somis.org/QSK922.html>, Rich Measures' article on vacuum
relay QSK. He uses a reed relay for the input and an RJ1A or HC-1
vacuum relay for the output. I have used a similar circuit in at least
three amplifiers so far, and it is ABSOLUTELY SILENT if you follow his
advice to use silicon rubber to mount it and thin pieces of copper foil
to make the leads. Foil works better than braid, becasue the braid
wicks solder and ends up rigid.
His circuit also includes an electronic cathode bias switch, operated by
the PTT (NOT by rectified RF) which can be used to cut off the tube(s)
when in standby. This provides some of the function of the additional
relay in the MLA-2500.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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