[R-390] Solid State rectifiers vs Tubes

[Scott, Barry (Clyde B)]

I checked the manual's "change pages" for the SS-diode to tube modification and, sure enough, the distortion figures were changed for the better -- at least in part of the frequency ranges.  IIRC, it went from 0.2% to 0.1%.  Now if I just had a method to measure this.  I don't have a distortion analyzer :(

Barry - N4BUQ

> 
> 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
>