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From: peterca@ento.csiro.au
Date: Wed, 27 Apr 94 15:14:10 EST
Subject: Re: Tube vs Solid-State rectifiers

>I hear tube rectifiers are better than solid state besauce they warm up
>slowly and softly and prevent startup transients from wearing out the
>rest of the amplifier.  
Yes and no.  
Solid state rectifiers work as soon as you throw the on/off switch so the
rest of the circuit gets the full supply voltage very soon and before any
of the cathodes have got hot.  This has two effects:
1)When tubes are called upon to produce a current which requires all the
free electrons from the cathode damage can occur to the cathode known as
"cathode stripping".  If the cathode is hot before the high voltage is
supplied the free electrons surrounding the cathode (space charge) are more
abundant than are required at any time (unless the tube is overdriven).  I
gather there is disagreement about whether the reduced life expectancy of
the tubes from having high voltage applied cold is significant or not.
2) If the circuit originally had tube rectification solid state
rectification may cause the supply rail voltage may be higher than intended
and exceed the ratings of circuit component/supply caps untill the tubes
start to conduct and draw significant current.
Also even when the circuit is all warmed up the supply rail will be higher
if a solid state rectifier is simply substituted for the tube because the
internal resistance of the tube is higher and so a voltage drop occurs
across it.  This latter problem can be easily fixed by installing suitable
resistors in series with a solid state rectifier.  
>I also hear that tube rectifiers are noisier
>than solid state rectifiers.  Any opinions?
I don't know about that.  Perhaps the higher internal resistance together
with the supply caps form an RC filter with a bigger R.  On the other hand
the C can be larger with solid state.  (I plan to go the whole hog and have
a fully regulated supply for my amplifiers)
It is possible to slow the turn on of solid state rectification by various
tricks including a power resistor in series with the transformer mains
which is bypassed automatically (by a relay) or manually (just the operator
throwing another switch).  With a separate DC heater supply you can arrange
for a slow cathode warm up to a full voltage.  The heater supply can then
also turn on a relay to connect the high voltage transformer's primary.  
These arrangements may turn out to be more complex than the amplifier
circuit and achieve no more than the tube rectifier operating with the
circuit from a shared transformer.  

Peter Campbell (peterca@ento.csiro.au)
C/O Div. Entomology, C.S.I.R.O.
Box 1700 Canberra A.C.T. 2601    Ph.61-6-2464158 (w), 61-6-2516213 (h)
Australia                           61-6-2464173 (fax)