[Amps] Zener Diodes
Jim Thomson
jim.thom at telus.net
Sun May 28 23:37:47 EDT 2017
Date: Sun, 28 May 2017 16:22:09 -0600
From: MU 4CX250B <4cx250b at miamioh.edu>
To: amps at contesting.com
Subject: Re: [Amps] Zener Diodes
Mike needed this as a replacement cathode bias diode in an old
homebrew 8877 amp with 2600V on the plate. Manfred's suggestion is
the more contemporary solution, but in any case a 50W zener is way
overkill. A 10W zener would work just fine, and these are still
available without spending a fortune.
73,
Jim w8zr
## I tried a 6.3 volt at 10 watt zener.......with the correct heat sink. Peak
plate current is 800 ma. Lasted 2.5 years b4 it failed..shorted. That was in
a drake L4B amp.
## Plate current X zener voltage = zener diss. 8 V X 1 amp =
8 watts. You need at least a 25 watt zener for the job. A 50 watt
zener requires a 50 watt heat sink. Zeners will not handle a surge.
## Most reliable bias scheme is a simple series string of 1N5408 diodes..which have
a 200 A surge rating. A 6A10 diode has a 400A surge rating. You can also parallel
diodes to increase both surge rating and also their CCS current rating.
## 1N5408 is good for 3A CCS. 6A10 is good for 6A CCS. With any string of diodes
you can easily vary the bias Voltage, by using more or fewer diodes. IE: you can easily
adjust in aprx .7 vdc increments. For some applications, like more bias for CW or FM, and
less bias for SSB, a simple SPDT or SPDT-center off miniature toggle switch on the rear apron
will provide either 2 positions of bias.....or 3 positions if a center off toggle is used.
## A 3 A CCS rated diode will diss aprx .7vdc X 3A = 2.1 watts CCS..which will run hot.
A 6A CCS diode will diss .7 X 6 = 4.2 watts CCS. Tested on the bench, I found that a 1N5408
will handle 1 A CCS for hrs on end..and run barely warm. A 6A10 will handle 2A CCS for hrs
on end..and run barely warm. Dont use a 1N400X, like a 1N4007. Surge rating is through the floor,
they run too hot. 5408 and 6a10 are dirt cheap.
## if you really want to go crazy, a simple rotary switch will provide all the taps you want.
## For the purists, you can also wire a big electrolytic across the entire string of diodes. Like
5000 uf to 47,000 uf. A 25-50 vdc lytic works fine. The bias Vwill not budge.
The V across each diode will increase by aprx 5-6% when going from normal
idle current to full bore key down plate current. So as you suck more plate current, the V drop
across each diode will increase a tiny bit...which then tries to charge the cap up higher.
## IMO, 50 watt zeners and mating heat sinks are way too unreliable, non adjustable, failure prone,
and way too expensive.
## henry radio used strings of diodes in both their 3 k uktra and also 8 k uktra. I have used the same
scheme on everything from 3-500Z to 4-400 to 4-1000, 3x3, and every other GG triode out there..or triode
connected tetrode. .
## On a side note, for CW, you can reduce the idle current by a huge amount, like down to just 20-40 ma, and the
amp will run just fine..... with way less idle power dissipation and slightly higher tank eff. However, you cant reduce
the idle current down to zero, on any GG triode amp....... or you will end up with key clix on CW. 25 ma idle
produces way less heat vs 150-250 ma....when on cw mode, vox or qsk.
### Zeners have gone the way of the do-do bird.
Jim VE7RF
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