Back on the original topic, I found what appears to be an updated
3CX1500A7/8877 data sheet on the CPII site. Updated, that is since the last
time I looked there... (!)
https://www.cpii.com/docs/datasheets/213/3CX1500A7-8877.pdf
*My original question was answered on page 4:*
"CATHODE WARM-UP/COOL-DOWN TIME - It is recommended that
heater voltage be applied for a minimum of
three minutes* before anode voltage* and drive voltage are
applied, to allow for proper conditioning of the cathode
surface. It is also recommended that after all voltages are
removed from the tube that air cooling be allowed to run
for several minutes to allow for proper cooldown."
*Other interesting tidbits:*
"HEATER OPERATION – The heater power supply should
be isolated from ground (ie.; no center tap on the secondary of a
transformer if used)."
*I happened to set my current amp up that way because the heater xformer
didn't have a center tap, not because I thought it was bad practice. I also
tied the cathode and heater elements together at the socket with 0.01uF
caps to prevent RF "overdrive" of the heater. Right or wrong it seems to
work fine. That trick is described in a project from Bill Orr's handbook.
Point being, lots of 8877's out there with no heater choke and one side of
the heater tied to ground.*
"INPUT CIRCUIT - When the 3CX1500A7/8877 is operated as a cathode driven rf
amplifier, the use of a resonant circuit in the cathode is recommended. For
best results with a single ended amplifier, it is suggested that the
cathode tank circuit operate with a “Q” of 5 or more."
*Hmmmm why? 4 has worked the best for me. What sin am I committing?
Potential stabliity? Some difference in IMD?*
Anyway - sometimes answers do appear, even if some of them raise more
questions. :-)
73,
Jim N7CXI
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