[Amps] RE: [Amps] 8877 Max Grid Current

[Steve Katz]

In my 8877 homebrew amps, I've always found that with high enough Ep and
heavy loading, I could produce "legal limit" output with very little grid
current, typ. 50mA.  Lower Ep, more Ig.  Looser loading, more Ig.  If I had
to set a trip-out of some sort, I'd probably set it at 150mA if Ep = 4kV and
200mA if Ep = 3.5kV

WB2WIK/6

"If everything seems under control, you're just not going fast enough." -
Mario Andretti

> -----Original Message-----
> From:	i4jmy@iol.it [SMTP:i4jmy@iol.it]
> Sent:	Tuesday, June 18, 2002 8:48 AM
> To:	g0ruz@btinternet.com
> Cc:	amps@contesting.com
> 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.
> 
> 
> 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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