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.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek