AKSWL wrote:
Is there a soild state replacement for the rectifier tube 5U4 in a
HQ-145xc?

Seems I remember such a tube being available for around $20 from an
internet source.

73

John

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. Just be sure and include resistors in your
replacement design. As far as filament "stripping", as alluded to by
another posting, some say its a big deal. If the unit is not being
turned on and off many times a day, every day, I can't see it being a
problem.
The simplest thing to do is get an old octal tube base and solder in two
1N4007's. If it pleases you, fill it up with some Home Depot epoxy or
the like, just like the commercial jobs. Remember to hook the diode
outputs up to the correct DC output filament pin, as the Hammarlund
schematic indicats. Leave the other pin vacant, of course. While your at
it, try and keep the a.c. input voltage down to where the mfgr says it
should be, or a few volts less. High line voltage can really add to the
spectre of failures of vintage equipment



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Later models of the HQ-145 had hard wired diodes in place of the 5U4
family. I found that out when I had bought a schematic and found some
discrepancies between what my HQ-145 had and the schematic. Then I
happened to find on the web a schematic that had the 5U4 in it, so
this was undoubtedly an evolutionary change in the production runs.

My schematic doesn't specify what diode was used and I'll have to
check to see if a dropping resistor was added. All my schematic gives
is a Hammarlund part number. Of course, with no Hammarlund Radio Co.
that is now meaningless. But I agree that garden variety high voltage
diodes would work. This certainly would cut down on heat in the radio
as the 5U4 is the biggest tube in it.

W3JT





On Sun, 30 Nov 2008 06:58:28 -0500, k3hvg wrote:

AKSWL wrote:
Is there a soild state replacement for the rectifier tube 5U4 in a
HQ-145xc?

Seems I remember such a tube being available for around $20 from an
internet source.

73

John

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. Just be sure and include resistors in your
replacement design. As far as filament "stripping", as alluded to by
another posting, some say its a big deal. If the unit is not being
turned on and off many times a day, every day, I can't see it being a
problem.
The simplest thing to do is get an old octal tube base and solder in two
1N4007's. If it pleases you, fill it up with some Home Depot epoxy or
the like, just like the commercial jobs. Remember to hook the diode
outputs up to the correct DC output filament pin, as the Hammarlund
schematic indicats. Leave the other pin vacant, of course. While your at
it, try and keep the a.c. input voltage down to where the mfgr says it
should be, or a few volts less. High line voltage can really add to the
spectre of failures of vintage equipment

** Posted from http://www.teranews.com **

"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

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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k3hvg wrote:
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. Just be sure and include resistors in your
replacement design. As far as filament "stripping", as alluded to by
another posting, some say its a big deal. If the unit is not being
turned on and off many times a day, every day, I can't see it being a
problem.

Stripping is a big deal with some big power tubes... I have never seen it
be a problem with small signal tubes and any receiving tubes that are
rated for series-string operation will not have any issue.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."