Dear Ron,
You will find a mix of opinions on this reflector regarding the
application of
Anode Voltage at turn-on.
However, there is a general agreement that running just filament voltage
for a
while "conditions" a tube.
Although 3 minutes as a minimum warmup period is no where near a
conditioning cycle, holding off the Anode Voltage during filament warmup
does no harm.
Several commercial broadcast transmitter manufacturers agree with this
policy, and several do not.
It's cheaper just to slam everything on, but if you are interested in
preserving the life of an 8877 triode, such considerations as
mentioned above, and strict control of the heater voltage to
it's rated standard may be beneficial in the long run.
Every amplifier I have ever built or modified contained
a means to monitor filamant voltage at the tube socket pins,
a cooldown blower and a High Voltage Interrupter.
Hal
W4HBM
On Thu, 28 Jul 2005 09:14:49 -0400 w2cqm@juno.com writes:
> Can anyone help with specific knowledge about the 3 minute time delay
> and
> the application of HV on the Eimac 3CX1500A7/8877? The 8877 tube
> data
> sheet only specifies a 3 minute filament warmup. However it does
> not
> mention whether HV can be applied when the tube is turned on (no
> excitation of course) and filament voltage applied. Ameritron
> (AL1500)
> holds off application of HV for the full delay period. Dentron did
> not
> in their 8877 amp with no apparent problems! I hasten to mention
> that I'm
> not too confident about the general level of Dentron circuitry and
> that
> testimonial. The 3CX800A7, smaller version of the 8877, date
> sheet
> specifically admonishes the ham not to apply either RF excitation
> or
> high voltage "...for a minimum of three minutes (of filament
> warmup)...to
> allow for proper conditioning of the cathode surface". At this
> point, I'm
> not certain if the 8877 is materially different from the smaller,
> yet
> similar tube. If the application of HV makes no difference with the
> 8877
> during the warmup period, it saves a lot of control circuitry and
> a
> separate filament transformer; especially when both windings are
> from
> the same xmfr. Consequently, the delay can be incorporated in the
> antenna
> relay circuit only. In the 8877 amps I've built, I've always delayed
> the
> application of HV just to make certain there'd be no damage. Any
> comments based on first hand engineering data would be appreciated.
> Ron
> W2CQM/3
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