[R-390] Solid State rectifiers vs Tubes

[Helmut Usbeck]

I've wanted to bring up diodes in power supplies for a while now.  For
the most part, the usual 1n400X doesn't produce any noise that one can
hear.  If you put a scope on the high voltage line you might see 50
millivolts and switching spikes.  Older power diodes from 20-30 years
ago where more prone to this.  I designed digital ckts for quite a few
years and noise problems from power supplies where common, there was no
going back to tube rectifiers.  The way the noise is generated  is due
to a mismatch between the secondary of the transformer and  the diode
impedance.  Papers were written about this way in the fifies.  Anyhow
the old "put a ceramic .01 cap across the diode" doesn't work.  We also
can't rewind the power xformers for 390's either, so what I've been
using is either a fast recover diodes, or Hexfred diodes, both designed
to be hash free. 

1N4937 1amp 600volt, fast recovery type
HFA08TB60 1 amp 600 volt Hexfred
Both available from Mouser, Digikey and such.
Don't cost more than a buck or two.

The old .01 cap thing is for keeping RF generated by the radios
oscillators and such out the power supply to prevent "hum" modulation.
Transmitter power supplies usual have these. 

Another way of knocking out the hash is to put a resistor of low ohmage
in series with the diodes, but in order for this work correctly the
impedance of the secondary and other data of the diode needs too be had
and along with some math, hash can be also gotten rid of.  That resistor
that one sees in some older solid state design wasn't just there just
for surge protection.  

That's it in a nut shell.

--Helm.  WB2ADT

Norman Ryan wrote

> 
> Hello, Ken,
> 
> Interesting that solid state rectifiers are noisy-- never ocurred to me.
> 
> What capacitance disk ceramics would you use across, say, a pair of SS
> rectifiers replacing the 26Z5W's?
> 
> 73 de Norman KG4SWM
> 
> Kenneth G. Gordon wrote in part:
> 
> > SS rectifiers cut off so sharply that they
> > generate a goodly amount of noise, kind of like a spark-gap. I
> > ALWAYS bypass for RF any I install. Disk ceramic caps both
> > "across" every diode, and sometimes, depending on the noise
> > problems I have with them, from each end to ground.
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