The most difficult part to find when building an approx. 20db amplifier
is a means of bandswitching the tank L. For example, a typical 8171
amplifier generates c. 40A-rms of circulating current. Due to skin
effect, 40A of AØ/'hammer-down' current at 20MHz heats like c. 160A of
DC. For CW and SSB duty-cycle, the effect is less. Switching such RF
currents requires stern measures.
Multi-tech Industries builds a line of 3/8" shaft-size bandswitches that
are suitable for tetrodes-with-handles amplifiers:
http://www.multi-tech-industries.com/mod_92.gif
- These switches are rated at 100ADC and 24kVDC. Nacherly, the current
rating decreases for RF due to skin effect. Thus, for SSB-CW, one wafer
should switch the tank L for an 8170, or 8171. For an 8281, two,
paralleled wafers should do the job.
The tank L can be built from the ordinary soft copper tubing that's
available in plumbing shops. Unlike as with a roller-coil, this allows
one to step up conductor size in proportion to frequency rise.
Although silver-oxide is a good conductor, copper-oxide is not, so,
after the silver-soldering is completed, the copper needs to be polished
with fine steel wool and given a coat of urethane gloss varnish.
Directing forced air cooling on - and in - the copper tubing is good
engineering practice. Yes, someone at Heath had the right stuff.
[SB-220]
have fun
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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