[Amps] bias for 3cx3000

[R.Measures]

On Oct 3, 2004, at 4:40 PM, Stan C. wrote:

> 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.

**  Yes, Stan, but you could accomplish this by letting the tube bias 
itself to zero current by putting a 10k to 100k, 2w R in series with 
the cathode, and then shorting it out on Tx - either with a relay or 
with an NPN switch in a circuit like:
http://www.somis.org/HSS-diagram.jpg

> 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?

B+ goes to the anode.  The –HV (B-minus). gets to gnd through the 
grid-current meter shunt R.  Glitch diodes should be connected across 
the shunt R.  the –HV gets to the cathode through a 10uH or so RFC.

>  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.

Running the tube cooler can be achieved by using VOX and an electronic 
cathode bias switch.  However, if you are into broadcasting, 
forgetaboutit.

>  Any help or ideas would be great. Always like to enrich my knowledge.
>
> Take care,Stan
>
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org