On Wed, 12 Nov 1997 09:25:08 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>> To: amps@contesting.com
>> From: km1h@juno.com (km1h @ juno.com)
>> Subject: Re: [AMPS] Yet more parasitics
To: <amps@contesting.com>
>> Date: Tue, 11 Nov 97 20:31:08 +0000
>
>Hi Carl,
>
>> That is assuming the manufacturers even use bias John!
>> The lack of adequate bias circuits for linear operation of the
>majority
>> of VHF-UHF brick amps is a sore point with many users. It appears
>that
>> only Teletec is using an active circuit that holds up under all
>drive
>> conditions.
>
>That's right Carl, using a series resistance and a shunt diode for
>bias is just plain dumb.
But the manufacturers keep using it even when Motorola and TI at leas
have published good bias circuits.
>It makes a great bias system for a CB "linear" or low power stage,
>but the bias wanders all over the place when any base current
>appears.
I suspect the original "pattern" started out as AM CB for the HF bricks
and FM for VHF.
The "engineering" in either case did not have a clue about linear
operation of those biplolar devices. When SSB started to become popular
on VHF it was "one size fits all" .
>
>IMHO, QST ought to make most VHF amplifier manufacturers stop
>claiming they are "linear amps".
QST ought to do a lot of things...but short of a nuke on Newington
nothing will change.
>
>The "bias" systems often do have LF parasitic paths, but then the VHF
>stabilty is also poor in many designs. Quite frankly, most are junk.
At least the LF parasitics are easy to recognize and cure with modest
test equipment. It's those funny things at 500MHz + that drive me nuts
with some tube cavity amps. The other nasty is TE Systems 70cm amps.
BTW, I took ur advice about posting to originator and group...never
realized it was a problem to some. Tnx for mentioning it. Holler at me
if I retrograde!
73 Carl KM1H
>
>73, Tom W8JI
>
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