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.
73,
Mauri I4JMY
> Remember there's a difference between the maximum grid current you
> would normally expect to run, and the maximum level you *could* run
> without damaging the tube.
>
> Depending on the speed of the trip circuit, it might make sense to set
> the trip level about 50% above the maximum you'd normally expect to ru
n.
> That leaves the tube well protected, without causing too many nuisance
> trips during tune-up.
>
> The same question could also be asked about a suitable full-
scale value
> for the grid current meter. I generally make it so the normal reading
is
> about 2/3 of full scale, and then set the current trip at full scale.
>
> BTW, all this can be done by pulling out the tube and using an externa
l
> power supply to force current through that part of the circuit. The id
ea
> is to get all the metering and protection circuits calibrated and
> working *before* the first time you light the tube,
>
> In my experience of running this tube on 50Mhz expect to see around 10
0mA in
> practice when the max recommended cathode emission is reached. I would
set
> the grid trip higher than Ian suggested at around 225mA. The important
thing
> is not to run more than an amp (average) of cathode current, the tube
also
> likes 3.5kV when it really comes alive. It is a wonderful tube in my o
pinion
> and beats the pants off the Russian tubes despite claims of the Russia
n
> tubes being similar. I run both a GS35B and a 8877, give me a 8877 eve
ry
> time.
>
> I have had no problems with golden balls :-)
>
> Conrad G0RUZ
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