[Amps] Large Bandswitches

[Richard]

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 AŘ/'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.  
end