Darrell Bellerive <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- Garey,
Thanks so much for all of your assistance! I never thought about removing the tube. I guess that was what was confusing me. I've been running the receiver with both the tube and the diodes since I acquired it. Hope that didn't damage anything. I will remove the mod and put it back the way that Drake designed it. I like the advantages of the lower noise due to lower B+ voltages, and the delay in B+ reaching full voltage. I would expect the delay and lower B+ voltages to have a positive effect on the life of the tubes. Also been thinking about the double pole slide switch for the AVC. If I can't find a replacement switch, a small DPDT relay might be the answer. Use the existing switch to control the relay and let the relay do the switching. A relay with a low current 6.3 VAC coil would be ideal as it could be powered from the filament supply. 73, Darrell On Thursday 24 July 2008 19:28, Garey Barrell wrote: > 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 -- Darrell Bellerive Amateur Radio Stations VA7TO and VE7CLA Grand Forks, British Columbia, Canada ---------------------------------------------------------------------- Submissions: [email protected] Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net ----------------------------------------------------------------------
