[AMPS] L4B

[Ian White, G3SEK]

Bob Marston wrote:
>>
>>For many years we have been using active shunt voltage stabilisers to
>>maintain a very constant cathode to grid bias, irrespective of the cathode
>>current.  Net result virtually no change in cathode-grid voltage with drive
>
>Yes the shunt voltage stabiliser will improve IMD by a few points but at
>what price. More parts in the machine are more parts to break.

"What price" comes into this very literally. In Europe the classic 8.2V
50W zeners are very hard to find. From the big catalogs, the choices are
between a 20W diode at $10 (but probably under-rated in a flashover) or
a 75W diode at $50 - and neither of those has adjustable voltage.

The cost-effective alternatives then are either a string of forward-
biased diodes or something involving a transistor.

One reason for microwavers liking regulated bias voltage is that it
allows more RF output in the common situation where drive power is hard
to find and small tubes are being pushed to the limit. But mostly I
think it's the "follow-my-leader" effect.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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