Subject: [Amps] RE: [Amps] 8877 Max Grid Current
> A brand new and optimal 8877 (years ago, rarely later) was typically 40-
> 50mA for 1.8Kw of output power at VHF (broadcast) with 4Kv HV (loaded).
> As the tube was aging, the RF power decreased and drive was often
> repeatedly increased by guys of the maintenance to obtain again the
> same output and step by step the grid reached levels over 100ma.
> I've also seen 8877 working well over 100ma for different reasons (for
> days, months) without any abrupt failure.
> I'd set with confidence the trip to twice the normal predicted current
> (mostly depend by the applyed HV), beeing aware that some amateur use
> 2nd hand tubes that may require high grid to give the desired power.
> Hams have the bad custom to run low HV (or PS with big drops) thinking
> that this is wise not to stress the tube, unluckily this is not always
> true.
Since the original question was VHF related, take my observations for what
they may be worth.
I was charged with testing 33- 8877's, with date codes from late '70's to early
"90's. I used an RF generator I had on hand operating at 13.56mhz for the
testing. With a new 8877 in the socket, I tuned it up; 3900 volts Ep, 850ma Ip,
and 62ma Ig gave a power output of 2120 watts. I then tested the 33 old tubes
without touching any controls on the generator. All of them produced from 2050
to 2120 watts out, with the Ig's varying from 60-68ma. Since then, I have set my
grid-trip circuits on several projects to 100ma, with a little built-in
hysteretic to
prevent nuisance trips when the loading cap is not properly preset for tune-up.
I found that Ig varies greatly with plate loading but power output does not. By
adjusting the loading cap. for minimum Ig without decreasing power output can
buy you a several-ma reduction Ig with less distortion Truly a win-win
situation.
I too have seen and reported here the phenomenon that occurs on an 8877 when
Ep is increased above 3700-3800 volts Far less Ig and Ip for the same power
out. Just like shifting a car into overdrive.
(((73)))
Phil, K5PC
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