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The only reason for splitting up the 24 hours into cycles was to
thermal-cycle the relevant tube components. It may be mythology, but I've heard that the hot/cold cycle "shock" will help release any gas molecules that may be right at the surface. Even if it is mythology, it seemed the kind that's relatively harmless when translated to superstitious action. ;-) Thanks for your comments on the heater supply. I'll chew on that a while and see if any implementation ideas come up. 73, Jim Barber, N7CXI Ian White GM3SEK wrote: Jim Barber wrote: Thanks, Ian. That's a very clear and concise description of the issue. This may be overkill, but since it costs nothing I'm thinking of four six-hour cycles, allowing the tube to cool to room temp or so between cycles. What would be the reason for dividing the 24 hours into four parts (bearing in mind that the 24h is already little more than a guess)? Also note that the tube must be cooled anyway, to avoid overheating of the base seal. 5V at 10.5A is a lot of heat input, and all of it must be removed somehow. Since you're here, Ian, I'll slip in an unrelated question. What do you think about using regulated DC for the heater on an 8877 ? I ask because I'm thinking of using a couple of spare controller I/O pins and a home-made R2R network to ramp up a series-pass regulator as part of the overall controller startup. On the other hand, I suppose a relay and appropriate power resistors would be simpler... ;-) There are two separate issues the the need for a regulated heater supply; and the need for a slow ramped startup. The need for a regulated heater supply depends on the regulation of your mains voltage. Eimac specify +/- 5%, so if your mains regulation is worse than that, a separately regulated supply could be worthwhile. Mostly you need to prevent the heater voltage dropping too low, because electron emission from a chilled cathode may not be able to deal with the peak anode current. If you ever plan to use the amplifier from a Field Day generator, then the answer changes to a definite "yes". Otherwise you can get a nasty scenario where you switch to TX, the increased demand pulls the generator voltage down, and the heater voltage along with it... and then you're trying to pull peak anode current from a chilled cathode. With two or three stations sharing the same generator, there will frequently be a "double low" or "triple low" when stations are transmitting at the same time. You can't do much about the effects on anode voltages in this FD situation, but you can prevent unnecessary stress on the cathodes by regulating the heater voltages. As it happens, the 8877 has a 5.0V heater, and low-cost 5V switch-mode supplies are very readily available. A PC power supply would do, but is much larger than needed for an 8877 (10.5A). Nominal 5V power supplies can generally be modified for other voltages in the 5-6V region by jimmying the master voltage divider. Switching noise should not be an issue if you choose a supply that is shielded, and filtered at both input and output. But this raises an issue related to your second question. It is not normally necessary to avoid prevent inrush current surges with *indirectly* heated tubes such as the 8877... BUT... a voltage-regulated heater supply MUST be modified to prevent it from driving a very large current into the low resistance of a cold heater at startup. I haven't tried this personally, but some people have created a ramped startup by simply adding a large electrolytic capacitor across the right half of the master voltage divider (you can check this using a 6V light bulb). By the way, I learned a lot of this stuff by reading the mail on the AMPS list over the years (http://lists.contesting.com/mailman/listinfo/Amps). There are also searchable archives. |
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