On Sep 10, 2004, at 2:25 AM, Ian White, G3SEK wrote:
Steve Thompson wrote:
Search around for the W2GN design using a pair in parallel, it works
a
treat. I modified the grid circuit to have separate bias control, so
it
will work with unmatched tubes. Personally, I don't like link
coupling -
it's possible to get false indications of optimum loading.
Ooops - what I meant was - tuned link coupling. A variable untuned
link should give you monotonic loading adjustment, the same as a
variable series capacitor.
Should, but probably won't. Most link-coupled VHF/UHF amps suffer from
chronic under-loading, because it's very difficult to make the link
coupling tight enough to load the tube(s) correctly.
Good point, Ian. In the 2, 4cx250R 144MHz amplifier I built, I finally
resorted to making the link out #12 TFE-insulated wire and laying it
against the gnd-end of the tank-L. Since the resulting screen-I was c.
1mA, I knew the loading was adequate.
cheers
In contrast, capacitively-coupled VHF/UHF amps tune and load as easily
as a pi-network.
The W2GN articles appear to be downloadable from:
http://www.newsvhf.com/w2gn.html
(I wasn't able to verify that - please can someone confirm?)
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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