From: "Barry Ornitz" <ornitz@usa.net>
Newsgroups: rec.antiques.radio+phono
Subject: Re: Discussion topic - full wave rectifiers
Date: Sat, 6 Jan 2001 19:28:25 -0500

"John Byrns" <jbyrns@enteract.com> wrote in message
news:jbyrns-0601010739440001@207-229-151-64.d.enteract.com...
> In article <3A5772BC.60FC@xs4all.nl>, tgale@xs4all.nl wrote:
>
> > When you replace such a rectifier with semiconductor diodes, at the
> > moment of switch-on there is (a) a potentially damaging H.T. inrush
> > current into the reservoir / filter capacitors, followed by (b) these
> > capacitors, since there is yet no 'load', are operating at well above
> > their design voltage, and (c) H.T. is applied to the other stages
> > before they are properly warmed up, with possible side-effects
> > depending up the set's design.
> >
> > Silicon diodes can be substituted, but my recommendation is to include
> > (a) series resistors as appropriate and (b) some form of time delay
> > (e.g. a diode, resistor, electrolytic and a relay) before the H.T. is
> > applied.
>
>
> I agree that a series resistor should be used, but is a time delay really
> necessary?  Look at all the sets, both American and European that were
> built with selinum rectifiers instead of vacuum rectifiers, and they seem
> to suffer no ill effects even though their design is otherwise identical
> to sets using vacuum rectifiers.


One common argument used by tube rectifier aficionados is the so-called
cathode stripping effect where high voltage applied to the plate of a tube
before the cathode has warmed up can strip the cathode of emitting
material.

Unfortunately, this effect only occurs at high voltage, typically above 10
kilovolts.  It is not a factor in small receiving or transmitting tubes.
If it really were a problem, it would destroy the tube rectifiers which
have plate voltage applied immediately at turn-on.

The only purpose a time-delay on turning on the high voltage can provide is
for the remaining tubes to be warmed up so they draw some current
immediately from the power supply.  This slightly reduces the stress on the
high voltage capacitors.  It is actually far superior to use properly rated
capacitors and forget about the time delay.  Besides most electrolytic
capacitors have surge voltage ratings well above their normal operating
voltages.  This is typically 50 volts or more.

        Dr. Barry L. Ornitz     WA4VZQ     ornitz@usa.net

From: "Barry Ornitz" <ornitz@usa.net>
Newsgroups: rec.antiques.radio+phono
Subject: Re: Discussion topic - full wave rectifiers
Date: Sat, 6 Jan 2001 22:17:44 -0500

"Dick Carroll" <dixie@townsqr.com> wrote in message
news:938hgr$43d$0@208.207.71.141...
>  I *have* seen this occur in rectifier tubes on a few occasions when I
> happened to turn on a piece of gear which had a hard shortcircuit that
> loaded the rectifier in the power supply to the maximum, usually a
> shorted filter cap.
>  If you happen to be watching the rectifier tube (5y3, 5u4 etc. style)as
> it heats you can sometimes actually see small chunks of the filament
> coating popping off. That is your cue to turn it off, quick!
>  Afterward you can see those small white pieces of filament coating
> inside the glass envelope, lying loose in the bottom. Of course not all
> the coating comes off the filament, and the tube still functions, so it
> isn't desgtroyed, But pieces of the filament coating will come off.
>  I'm sure you could demonstrate this for yourself if you wanted to.

This is not the same process as high voltage cathode stripping.  This is
due to localized overheating, and the flecking-off of oxide is caused by
differential thermal expansion.

If you had used solid-state diodes, you would have blown the fuse (you
ALWAYS have one, don't you?).

           73,  Barry     WA4VZQ     ornitz@usa.net