[R-390] Solid State rectifiers vs Tubes...a reply...

[Kenneth G. Gordon]

> 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.

Hmmm...perhaps you are correct. I have never looked at my HV 
line with a scope nor tried to track the noise back with a scope.

None-the-less, when I by-pass the SS diodes with a .01 MFD 1KV 
disk capacitor, the noise goes away. I have had this problem in 
several receivers: ARC-5s with built-on and separate power 
supplies, HW-16 (two of them), home-brew receivers (two of them).

>  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.

I am certainly not going to argue the point with you since you 
obviously know more about it than I do. I arrived at my solution 
empirically and only guessed at its cause. Again, none-the-less, my 
solution worked 100% of the times I had to use it.

>  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. 

So the hash problem has been recognized by the industry, then, 
and a solution devised?

> 
> 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.

That definitely works for regenerative receivers, which are most 
susceptible to that problem.

> Transmitter power supplies usual have these. 

Yes, but in that case, they are used primarily to keep the "switch-
off" transient from blowing the diodes. Voltages in transmitters are 
usually higher than in receivers, and are usually switched on and 
off more often... My RSGB handbook addresses this issue at some 
length and suggests a series combination of capacitor and resistor 
across transformer primary and secondary and across any filter 
choke, in addition to by-pass capacitors across every diode.

> 
> 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

Thanks, Helm.

Ken Gordon W7EKB