John Fielding wrote:
>Hi everyone - It seems us "VHFer's" know a bit more about using grounded
>grid triodes than the "DC boys".
>
Hey, that's fighting talk!
>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
>(typically less than 10mV over the operating range). And I am talking
>about "healthy" currents - up to 2A peak. You will need a bipolar
>transistor with good gain (darlington) or better still a Mosfet, a few
>resistors and diodes and something to heatsink the transistor to.
>
>Look in the RSGB Microwave Handbook or the German magazine compendium VHF
>Communications for the basic circuit. You just need to change a few values
>to suit your needs. BTW the circuit is adjustable with a pot. so you can
>set the cathode voltage (hence standing current) to almost any value you
>need. As an added benefit the constant voltage means superior IMD
>performance as the cathode voltage doesn't wobble around as the drive goes
>up and down - which with resistor biasing can be quite alarming!
>
>Also the tube can be cut-off on receive very easily by adding a suitable
>zener diode and a relay contact.
>
>Maybe Ian White G3SEK can point you to other sources for the circuit.
Nobody has a monopoly on those ideas. There have been many circuits
described in both HF and VHF contexts. For example there was an article
in CommQuart by Marv, W6FR, describing an adjustable emitter-follower
bias circuit with very good regulation.
One problem with these circuits is that many of them require an extra
floating power supply. I've recently been playing with a circuit using a
TL431 variable shunt regulator IC with a bipolar darlington inside the
feedback loop to boost the current. This acts as an almost ideal
"variable zener" but does not require its own floating supply.
The basic circuit is in the TL431 data sheet. Haven't tried it with a
tube yet...
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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