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Author Topic: zener diode for bias  (Read 12431 times)
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Posts: 135

« Reply #25 on: December 20, 2020, 09:10:41 PM »

These are 250 MW. Use a 1N2804 (6.8 Volts) or 1N2805 (7.5 V) . Both are available on ebay. TO-3 case

Seth KC2WE

* 2805.jpg (22.88 KB, 500x323 - viewed 388 times.)

Seth Taylor

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« Reply #26 on: January 02, 2021, 08:42:49 PM »


FB on the rotary switch and grouped increments. I do the same here.

I have also had problems with zeners shorting in this service - as biasing in the CT leads, and they are always a short.  I thought it was just me.  Individual diodes in a string never crapped out. The BIG amp stress takes them out, whereas a docile 3-500Z amp may survive for years.

FB on adding the elec cap across the string.  I have done that on some amps here but will check to be sure I did it on all.  The stable key up is important.

From all the amps you tried, did you find a better efficiency using cut-off bias on AM as discussed?

I have not tested ur  concept on AM..yet.   Here's  what I want to know,  what happens to the anode dissipation, with  just the Un modulated  CXR... with the amp biased to class C, or  at least  cut off ??

In 'normal'  AB-2  biasing,  an UN-modulated cxr  typ results in  25 %  tank eff.  (and  65-70-% at max pep output.... amp tuned for max pep output).
IE:  375 watts  UN modulated cxr  and  25%  eff =  1.5 kw  dc input.  1.5 kw - 375 watts  = 1125  watts  anode diss !       1125/375 = 3

IE:  anode diss is a whopping triple the  UN modulated CXR.

Ok, what happens when the  GG triode is now biased to  cutoff..or  just beyond  cut off ?   What happens to the anode diss, when UN  modulated?   How much  current  does the  typ  2 x 3-500Z  amp  sit at when UN  modulated  with  'normal' bias, like  180 ma..... vs  idling at  0 ma ??

The  11m  folks may well have figured out the class  C bias scheme  eons ago, dunno.   Some of em used an adjustable  wire wound..with the  slider on the side.   Slider set the bias..and idle current.  Problem is... when driven hard, and way more plate current, the  V drop across the  resistor  increases, putting the amp more towards class C.... or well  into class C.     It amounts to a sliding bias scheme, except they are sliding in the wrong direction.   Some will wire a cap across the  adjustable resistor, so the  V drop hangs up higher/longer on voice peaks.   

  Years ago, I tried. and failed, to  cook up a sliding bias scheme for ssb use, where the idle current was really low..and increased, as more drive applied. In the end, I used the adjustable  bias scheme, with the  rotary switch and  string of diodes..and just reduced the idle current   to where  IMD  was still acceptable.... like  100  ma on my drake amp...( ssb position, 1290 watts pep out)...and just  40 ma ( cw position, 625 watts pep out).

Jim   VE7RF 
AC0OB - A Place where Thermionic Emitters Rule!
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« Reply #27 on: January 02, 2021, 10:45:20 PM »

On my Henry 2K-4 GG amp with 2, 3-500Z's, I have a 12V 50W zener in the bias circuit.

The measured cathode bias is 28.25V in STDBY, 12.5V in Transmit.

See the bottom center of the schematic.

Phil - AC0OB

* Henry 2k-4 HV Power Supply Only pdf.pdf (231.65 KB - downloaded 239 times.)

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