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Re: [Amps] Inductance Variations

To: <amps@contesting.com>, "Mike" <k4gmh@arrl.net>
Subject: Re: [Amps] Inductance Variations
From: "Phil Clements" <philk5pc@tyler.net>
Reply-to: Phil Clements <philk5pc@tyler.net>
Date: Tue, 8 Feb 2005 14:20:18 -0600
List-post: <mailto:amps@contesting.com>
Next question is related in that tank circuits in most commercial amplifiers use a shorting switch to select the inductance values. Where as, the amp. circuit designs in ARRL Handbook, etc. use a non shorting switch. Which is the better (electrical) switch method - shorting or non-shorting?

73,
Mike, K4GMH

This problem really becomes evident when you depart from the 5-band tank circuit by adding 160 meters and/or WARC band taps. I built several amps from handbook articles; the last one being an 8877. I used a brand new Radio Switch band switch which was non-shorting, just as shown in the diagram in the article. I immediately burned the switch to a crisp! There is a tremendous voltage build-up in the floating coil section. When the band switch was set to 40 meters, the floating turns for 80 and 160 meters wreaked havoc and all hell broke loose. I now use band switches that are proven in commercial and amateur amps and are the shorting type. I have never figured out why the ARRL or the original builders of the projects in the articles did not have these problems themselves. When I build an 8877 amp, I use the scheme from an Alpha 77DX and duplicate the parts, or use similar but beefier ones.


Again, if you are only interested in 80-10 meters in 5 bands, there is probably not a problem, as long as the band switch rating has some head room for voltage and current.

(((73)))
Phil, K5PC


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