>
>Hello I am just about to start building a 8877 amplifier for 6m.I have
>looked at lots of designs for 6m all of which are using the standard Pi
>tank.
It is my opinion that it is not a bad idea to locate the Tune-C close to
the anode.
>This part is no problem.What I find confusing is the correct bias
>voltage and optimum eht volts.Eimac give the max as 4kv I am using all
>vac variable so I was thinking of runing about 3800 off load which will
>probably drop to 3500 on load.I can run any EHT volts up to about 5kv at
>1.5 amps.
>Now comes the bias bit.In Europe it is very important to have a clean
>signal.The density of amateur population is at times incredible
>particularly in IO93 and IO91 in the UK.6m is a very popular here and I
>wish to happily co-exist with my fellow amateurs,so I am after optimum
>IMD performance.Here are my concerns.
>1.Do variations in the bias voltage severly degrade the IMD performance
>of a 8877?
Not nearly as much as with tubes that do not have a dispenser-cathode.
The 8877 is cleaner than any transceiver that I am aware of. A zero
signal anode current (ZSAC ) of between 90mA and 180mA should result in a
total IMD of better than -40db.
>My experience with tetrodes has taught me the importance of
>keeping the screen regulation absolutely rock solid (look at G3SEK's WEB
>site for an explanation)
The TS-830S uses an unregulated screen supply for the 6146Bs. It appears
to have the lowest total IMD of any amateur radio transceiver.
>How does this
>translate to Triodes?I have never
>run one and therefore don't know.Is a 50 watt zener adequate or should I
>use a shunt stabaliser like the OE9PMJ one?This is also variable so you
>can set the bais for the optimum standing current.
Better to use a string of >1A fwd-biased rectifiers mounted on perfboard.
They are much more tolerant of glitchs and much cheaper than 50w zeners.
With diode string cathode bias, the bias potential is adjustable in 0.7v
increments by shorting out unneeded diodes. .
>2.What is the optimum standing curent for AB1 operation on a 8877?
The 8877 is unsuitable for Class AB1. Its forte is Class AB2
cathode-driven. The only triodes that are suitable for Class AB1 have a
Mu of 5 or less.
>
>Most designers say use a 12 volt 50 watt zener and say that the idling
>current will vary depending on EHT volts available! Surely that can't be
>right can it?
The correct 8877 bias potential for any given anode supply potential is
whatever it takes to obtain a ZSAC of 90 to 180 mA
>3.What is the maximum grid current that should be expected in normal
>operation with 3500 volts on the anode.
Depends on how heavily the tube is loaded, the particular characteristics
of the tube, and whether or not the cathode is being overdriven. There
is no maximum grid current rating. . From my experiences doing
autopsies on kaput 8877s, the greatest danger to the grid is not from
fundamental grid current. It is gold sputtering from intermittent VHF
and UHF parasitic oscillations. [see the Eimac Letter on my Web site].
> I intend using a grid trip and I
>would like to know where to set it?
>
Based on a 20w grid diss. rating, and the normal grid/cathode RF
potential of c.50vRMS, probably c.200mA for a 100% safety factor. I
know it sounds bonkers, however, based on my observations, I would design
an 8877 grid trip circuit to be capable of interrupting 50 amperes at
50v, and I would connect a 1500w transient voltage suppressor across the
grid trip circuitry. The added cost is relatively small.
- later, Conrad
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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