Very good your last words, hi.
What about noise from the rectifiers modulating the PA? And RF getting into
these rectifiers in series? The discs are maybe there to short out the RF?
The HEATHKIT SB1000 even had a RF choke in series with the hv diodes. For
the same reason? And did not Pat Hawker have something on this some years
ago?
Interesting. 73 de Hans SM5KI
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>Från: "Ian White, G3SEK" <G3SEK@ifwtech.demon.co.uk>
>Till: amps@contesting.com
>Ämne: Re: [AMPS] Diodes
>Datum: tis 22 feb 2000 19.42
>
>
>Peter Chadwick wrote:
>>Bot not identical voltages across them.
>>
>>If you have branded diodes from the same date code, and/or guaranteed
>>controlled avalanche characteristics, then probably it's OK. If not, I still
>>maintain voltage equalising resistors are a good idea. But not the cheap any
>>source resistors - a voltage equalising resistor across a 1kV diode should
>>be rated as a 1kV resistor. (I think Rich has made this point in the past,
>>saying that Mouser do suitable resistors at low price). Disc ceramic
>>capacitors across each diode do need matching in capacity - the +80/-20%
>>tolerance typical on a 1000pF 1 kV disc could make things worse, rather than
>>better. Do such disc caps drift the same way with time and temperature, does
>>anyone know? If not, they should be avoided.
>>
>> > The 1995 to 2000 Handbooks advise to not use "equalization".
>>
>This is another of those recurring FUQs (Frequently Unanswered Questions
>- what else?).
>
>When it came around about two years ago, I checked direct with the
>author of the revised Handbook chapter (a power supply engineer to MIL
>standards). He said that "equalization" became necessary because the
>first rectifier diodes had relatively low PIV, were too expensive to
>throw in lots more diodes in series, and failed catastrophically by
>arcing over or through the silicon die. All of that has changed. Modern
>rectifier diodes have higher PIV, lower cost and also have avalanche
>breakdown characteristics which are not necessarily fatal.
>
>Therefore in his professional view and that of the diode manufacturers
>he consulted, it is not necessary to use equalization components - so
>that's what the Handbooks now say.
>
>For me, the whole question about how close you can come to breaking down
>a string is not relevant. If in doubt, use more diodes - they're cheap
>enough! We amateurs cannot insist on diodes that have been specially
>high-temperature-reverse-bias tested, but so long as the diodes are
>reasonably well matched, eg full-spec brand-name diodes from the same
>bandolier, we should be pretty safe. It *has* to be worth the extra few
>cents.
>
>However, someone also pointed out that the economics are different for
>large amplifier manufacturers, who are very cost-conscious but can get
>well matched ceramic disc caps much more cheaply than we amateurs can.
>Then the economics may tip back towards using fewer diodes with
>equalization capacitors.
>
>There are also EMC considerations where capacitors can helpful in
>reducing reverse breakdown spikes going back through the mains, so
>perhaps capacitors may be making a comeback in these more EMC-conscious
>days.
>
>>They don't advise nichrome parasitic suppressors either. Just because it's
>>the ARRL Handbook, it's not guaranteed to be correct.
>>
>Depends who's writing it... and also who's reading it :-)
>
>
>73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
> 'In Practice' columnist for RadCom (RSGB)
> http://www.ifwtech.demon.co.uk/g3sek
>
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