[Boatanchors] Re: Copper Cap Recitifers, Zener diodes, solid-state replacement, R-390A, etc.
Barry L. Ornitz
[email protected]
Thu, 15 May 2003 03:49:59 -0400
I realize that this newsgroup is for Boatanchor radios, but
the subject of solid-state rectifier replacement has come up
many times.
>From what I have been able to learn, the "Copper Cap
Rectifier" is nothing more than solid state diodes with a
resistor wired in series to approximate the plate resistance
of the tube it replaces. This resistance provides the
equivalent forward voltage drop of the tube AT ONLY ONE
PARTICULAR FORWARD CURRENT RATING. At lower currents, this
combination will produce higher output voltages than the tube
does and at higher currents it will produce a lower output
voltage than the tube does
This is easy to understand when you consider that the plate
current of a vacuum diode is proportional to the plate voltage
to the 3/2 power (Child's Law). [To be completely technical,
the filament must be operating with normal voltages here so as
to not be temperature limited. Also at very high currents,
some deviation from this equation will occur.]
With a solid-state diode, such as silicon or germanium, the
current through the diode is exponentially proportional to the
voltage across the diode. At very low currents, a vacuum tube
will show a lower voltage drop than a semiconductor diode, but
the reverse is true at higher currents.
The basic equations, neglecting non-ideal ohmic resistance, are:
Vacuum Tube: I = a * V^1.5
Semiconductor Diode: I = Io * exp[ V/(n * Vt) +1]
where: I = current through rectifier
V = voltage across rectifier
a = constant for a particular tube
Io = reverse leakage current through
diode
n = a constant, usually between 1
and 2 for most semiconductors
Vt = k*T/q = thermal voltage, about
25.69 millivolts at room
temperature
I have regressed these models for the 6AL5, the 26Z5 (of
interest to R-390A folks), and the 5AR4/GZ34 (of interest to
vacuum tube audiophools), as well as for the 1N4007 and 1N4148
silicon diodes. [I also have some models for copper oxide,
selenium, and germanium rectifiers.] These models are suitable
for SPICE simulations and include non-ideal resistance terms.
Contact me for more details.
I have also plotted the currents for various voltages for the
above devices. If you need a copy, contact me. I can provide
GIF or JPEG formats.
I am not sure if the so-called inrush protection of the
"Copper Cap Rectifiers" is anything in addition to the series
resistor they use to simulate the plate resistance. I doubt
if it is from what I have read.
I would certainly not use one for replacing a 6AL5 if the tube
is used as a signal detector or for automatic volume control
detection. The sensitivity will be less. But occasionally a
6AL5 may have been used as a bias rectifier and a solid-state
replacement would be fine here.
The 0A2 is a voltage regulator rated at 150 volts. It is not
a diode but rather a gas discharge tube. It can be replaced
with a 10 watt Zener diode like the 1N3011 (properly heat
sinked, of course) which will provide better regulation than
the tube.
To correct a small point, WBob wrote:
> Well that isn't too true.. All silicon diodes will zener at
> about 2 or 3 x their voltage ratings. They don't make very
> good zeners and it is an uncontrolled parameter so even two
> diodes of the same name and manufacturerer will not have the
> same zener voltage, but they will all zener somewhere.
Technically, this is not Zener operation which is a low
voltage phenomena (less than 5 to 7 volts). What he is
talking about here is avalanche breakdown. All modern silicon
diodes made after about 1980 or so are controlled avalanche
rectifiers, meaning that avalanche breakdown will not destroy
the diode as long as the diode does not overheat. This is why
paralleling modern diodes with resistors and capacitors is no
longer necessary when building high voltage rectifier stacks.
Glen Zook, K9STH, then wrote:
> Actually, a germanium diode can be used under certain
> circumstances as an approximately 0.3 volt zener and a
> silicon diode can be used as an approximately 0.7 volt
> zener.
I have written about this before in this mail reflector. What
you are doing here is using the forward voltage drop of the
diode. As the above equations show, this voltage is not very
constant with changing current levels. In the 1N4000 series
of diodes, for example, the following data is representative:
Current, mA Voltage, V
0.1 0.449
1.0 0.565
10.0 0.682
100.0 0.800
1000.0 0.931
In my opinion, it is almost always beneficial to replace
vacuum tube power rectifiers in old gear with modern solid-
state silicon diodes. The 10 to 15 watts you save in filament
power puts less strain on the power transformers. These are
some of the most expensive components to replace, and are also
very sensitive to temperature. Their lifetime approximately
halves for every 13 degrees F rise in temperature.
If you are concerned about the higher voltages resulting with
silicon diodes, you can add series resistance like that found
internally in the "Copper Top" products. This generates heat,
of course, but no more than the tube would have with its plate
resistance (and you still save the filament heat).
You will get "instant" high voltage, of course, and without
the other tubes warmed up and drawing current, the voltage
will be higher than normal for a few seconds. If you have
properly sized electrolytic capacitors, however, this is
normally well within their surge ratings. If not, go up to
the next higher voltage capacitor. This is much cheaper than
replacing a transformer. Also applying high voltage to the
plates of receiving (and typical ham transmitting) tubes
before the filament is warmed up is not a problem. The so-
called cathode stripping some people worry about does not
occur at voltages below about 10 kilovolts.
73, Barry L. Ornitz WA4VZQ [email protected]
Copyright 2003 B. L. Ornitz
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