[Amps] bias for 3cx3000

[Barry Kirkwood]

Stan,
Sounds to me like you are overdetermining things a bit.
Seems to me that if a suitable power transistor is placed in the cathode
circuit,  then the voltage drop across the pass transistor can be controlled
by the voltage between the collector and base of the transistor.
Consider a transistor with a battery to give forward bias between collector
and base, and a resistor ( say, 50 ohms ) between base and the emitter. If a
supply is connected from base to emitter, then the drop across the
transistor will be the battery voltage plus about Vbe ( probly about 1 volt
or so).
If you had some old type C batteries with clips every 1.4V you would have it
made!
But wait, there is more: If you put a string of low wattage zeners in place
of the battery, then you have an amplified zener, and just short out or open
the zeners to set the voltage you want.
This will give better regulation than the zeners alone, and will be cheaper
than a high wattage zener.
Once the operating condition is found you can leave this hookup in place
forever.
Beats me why people buy high watts zeners to set the bias of amplifiers when
there is a better and cheaper way.
73
Barry


end
Barry Kirkwood PhD ZL1DD
Signal Hill Homestay
66 Cory Road
Palm Beach
Waiheke Island 1240
NEW ZEALAND
ph/fax 64-9-372-5161
www.waiheke.co.nz/signal.htm
----- Original Message ----- 
From: "Stan C." <kf6ikc at sbcglobal.net>
To: <amps at contesting.com>
Sent: Sunday, October 03, 2004 11:40 PM
Subject: [Amps] bias for 3cx3000


> Hello to all. I need some of the experts to give me some advice on a HB
amp I am building. I am using a 3CX3000A7 in GG. I want to ad some bias to
bring down the zero signal current. To do this I want to use a variac,small
transformer,and rectifier.
>  Will this work. I am looking at a 3-5 amp variac. A 24vct transformer
rated for 5 amps. And a bridge rectifier (the square kind you bolt down).
>  Will this work? Do I need to use a small choke in series with the
rectified DC to the cathode? Do I run the B+ to the cathode and B- to the
grid current meter?
>  I hope you can get an idea of what I want to do with this little bit of
info. I would like to be able to vari the bias voltage and that is why I am
not using a bunch of zeners,block diodes,etc. And yes I know you can run
this tube zero bias. But I like a little bias to help the tube run a little
cooler,etc.
>  Any help or ideas would be great. Always like to enrich my knowledge.
>
> Take care,Stan
>
>
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>