> >
> >The draw backs of this tube. The filament is of the Oxide Coated
> >Unipotential type. 15 Volts at 14 amps with a 6 Minute warmup time and no
> >one rebuilds them. Secondly they have a fairly high output capacitance
> >(36pf) So even though they have a max freq spec of 110Mhz your
> >not going to match them up with a pi-network on 10 Meters.
> >
> ? At QRP levels, this is undoubtedly true because RL would be abnormally
> high. Increase the anode current to normal, RL drops to a reasonable
> level, and a match is easy. One of the greater old-wives' tales of
> amateur radio is that adding another tube doubles C-out which renders
> the amplifier unusuable on 10m. However, when a second tube is added,
> RL is cut in half, and that doubles the C needed for C-tune. Thus, it is
> not the anode-C / Cout which causes a match prob. on 10m, it is the ratio
> of Cout to RL.
>
This is the third (in addition to the larger filament power and cooling
requirements) problem with using "tubes with handles" at amateur power
levels. When underdriven the RL goes quite high - a 3CPX5000/YC-156
will be about 10K Ohms and the 8171/8170 may reach 15 K (assuming 6500
and 7500 volts respectively on the anodes, 60% efficiency and 1500 watts
out). As I recall, tube geometry is optimized for design voltages and
running the tubes with significantly reduced anode (and screen) voltages
will result in lower efficiency and higher IMD.
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