On Jul 28, 2005, at 6:14 AM, w2cqm@juno.com wrote:
> 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.
Ron ------ 3-500Zs have a filament, the 8877 has a heater.
> 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!
To avoid stripping off flakes of the emissive coating, do not attempt
to draw any cathode-I until the cathode temp is semi-normal. According
to an ex-Eimac employee, this takes about 200-seconds of pre-heat. He
also told me that it was a marketing dept decision to reduce the warmup
to 3-minutes. Presumably, this was done to enhance sales of
replacement tubes?
RE: The Dentron DTR-2000: It had faster than average warmup because
the heater potential was 6v instead of 5v. Unfortunately, this
condition caused evaporation of barium from the cathode, which
condensed on the grid -- thereby causing grid-emission.
> 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".
HV is no danger to the emissive surface unless cutoff bias is removed
during the preheat period.
> 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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>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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