NoSPAM wrote:
"k3hvg" wrote in message
...
Many, if not most, of the smaller (read: 5Y3, 5U4, 5V4, etc.)SS
replacements do not have a dropping resistor molded inside the unit. I/we
use a series of SS replacements up to and including the 3B28 and 4B32.
The latter two SS replacements do have the resistors owing to FAA
purchase specs, however.
Could you explain why the 3B28 and 4B32 xenon-filled rectifiers would need
a series resistance to be added?
These tubes as well as the mercury-vapor rectifiers (like the 866 and 872A)
have a relatively constant voltage drop (typically 10 to 15 volts depending
on the exact tube). Quite often silicon replacements for these tubes have
about the same or even larger voltage drops than do the gas-filled diodes
due to the use of many silicon diodes in series to achieve the necessary
reverse voltage rating.
High vacuum rectifiers do have a significant forward resistance which
depends on the plate current. The classic journal article by O. H. Schade
discussing this issue is "Analysis of Rectifier Operation" (Proc. IRE, Vol.
31, No. 7, July 1943). Using the curves generated by Schade (a partial set
of which is reprinted in most recent ARRL Handbooks), you can calculate the
required resistance to be placed in series with the diodes to replicate the
voltage drop of the 5U4 originally used.
In case anyone is wondering about the heat dissipated by this series
resistance, it is essentially identical to the heat generated as plate
dissipation by the rectifier tube. However, the filament heat is no longer
produced. Since transformers are one of the most expensive items in a
radio to replace, this savings of filament current is beneficial.
73, Dr. Barry L. Ornitz WA4VZQ
Actually, that's a very good question. In addition to providing a fwd
resistance more or less equal to the hard tube, the resistor provides a
safety valve, so to speak, to prevent a true, dead short that these
rectifiers could present. This is in addition to normal circuit
protections, of course.
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