[Amps] bias for 3cx3000
R.Measures
r at somis.org
Mon Oct 4 05:41:31 EDT 2004
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
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