Why not use a string of normal diodes in series? They go in OPPOSITE to how
the zener connects. Figure about 0.7-0.8V per diode, string what you need
in SERIES to generate the bias voltage you need. Use diodes with an
appropriate current rating to handle whatever your current load is, probably
3A more than sufficient. Probably talking a couple bucks, most of that for
the bubble pack.
I've been using same in my SB220 for about 5 years now. No problems
whatsoever, and the string allows me to "tap" it to provide different bias
settings for SSB versus CW.
Lose the zener!
Gary W2CS
> -----Original Message-----
> From: amps-admin@contesting.com [mailto:amps-admin@contesting.com]On
> Behalf Of Douglas R Snowden
> Sent: Tuesday, July 16, 2002 3:46 PM
> To: amps@contesting.com
> Subject: Re: [Amps] Bias Zener Diodes In Parallel ?
>
>
> That is what I was sort of thinking would happen. Most of the
> responses to my question are useful, but as usual you can't post
> to this list without someone calling you stupid.
> Gee- at least I am doing some building unlike most hams.
> I did not consider the fact that I was actually pushing the diodes.
> The reason I am running several in series is because the amp is a pair of
> Russian GI7 triodes and needs more bias than usual ~30volts
> or so. Sounds like I need to go with the xsistor/zener. Or a big zener.
>
> Doug
>
>
>
>
> dan hearn wrote:
>
> > Unfortunately the branch which has the lower total zener voltage will
> > take most of the current so that is not a good idea. Fortunately, there
> > is a good simple solution to your problem. A power transistor with a
> > single zener and a resistor will make a good bias source. One I have
> > used with an 8877 consists of a 2n3055 with an 8.2 volt zener from
> > collector to base and a 47 ohm 2w resistor from base to emitter. The
> > zener cathode goes to the collector. The emitter goes to gnd and the
> > collector to the amp cathode. I have seen this design in several
> > published amp circuits. Sometimes a power resistor 5K 20w is shunted
> > across the transistor collector to base. I think this may be to protect
> > the tube in case of failure of the bias circuit.
> > 73, Dan, N5AR
> >
> > Douglas R Snowden wrote:
> > >
> > > I have an amp that I have put several 5.6Volt @ 5watt zeners
> > > in series for cathode bias. They get way too hot with
> > > about 800ma of current. Can I parallel zeners to divide the
> > > current without problems with balance between them?
> > >
> > > Doug, N4IJ
> > >
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