Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Darrell -
The "mod" has replaced the 6X4 rectifier tube with solid state diodes.
The tube is serving no purpose and can be left out. Most likely the
modifier left the tube out, and the "seller" inserted the tube to keep
from answering the question "why is this tube socket empty".
There are at least two schools of thought here. One is that the solid
state diodes are more efficient (less voltage drop), don't consume
filament power (and produce attendant heat), and eliminate the need for
a tube! The 100 ohm resistor is just to limit the current when those
"zero drop" diodes with 60 A surge current capabilities see those
discharged filter caps that look like a short circuit! On the negative
side, the resultant B+ is higher than the design was intended to have
(resulting in additional heat dissipated throughout the receiver). One
of the reasons the 2-B (and Collins 75S-x receivers are so quiet is the
low B+ (reduced thermal noise from the tubes). Also, the rest of the
tubes in the receiver are subjected to full B+ before the tubes have any
chance to warm up and generate the space charge (electron cloud) around
the cathode to protect it from back bombardment. There is some evidence
that this causes the cathode to be damaged, and shortens the life of the
tube.
Drake and most manufacturers went to solid state rectifiers en masse,
starting with the R-4, albeit with a lower voltage transformer to
compensate. The two diodes were considerably cheaper than a tube and
socket and filament winding and associated assembly costs, tubes were
almost all around a dollar, and so nobody cared. Personally, as long as
6X4 tubes are plentiful and available for a buck or two, I'll stick with
the tube.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line & TR-4/C Service Supplement CDs
<www.k4oah.com>
Darrell Bellerive wrote:
I have found two mods that have previously done on my 2-B:
1) A second headphone jack has been added to the back panel. It is wired in
parallel with the headphone jack at the front. It is wired for connection of
an external speaker as it will not break the connection to the normal speaker
output. I will simply remove it.
2) A 100 ohm resistor and two solid state diodes have been added in the high
voltage section. R55 (470 ohms) was lifted from pin 7 of V10 (6X4 power
rectifier) and a 100 ohm resistor inserted in between. Two diodes (1N2071A)
are attached from the junction of the new 100 ohm resistor and R55 and
connect to each of the two plates in V10. The anodes of each of the solid
state diodes are attached to the anodes of V10.
Attached is a picture showing the high voltage mod. The diagram on the left is
the factory schematic, and the diagram on the right show the mod in red.
What would have been the purpose of such a mod and should it be reversed?
73, Darrell VA7TO
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