>> >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
? One of the advantages of using the 8171 at amateur power levels is
that its 7" (178mm) anode-cooler needs only a 1700-rpm blower instead of
a 3400-rpm blower, which makes Much More Noise.
>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
? For an 8171 or 8170, the optimum RL is approx. 1100-ohms. When I
ran one, I would tune-up at full throttle using a tuning-pulser, and then
back the microphone gain off until the tube showed no increase in anode
current during ssb modulation -- i.e., Class A operation. Under these
conditions, the power gain was roughly 10db and eff. was probably c. 33%.
If needed to maintain communications during episodes of deliberate
interference, or whatever, the microphone gain could be advanced
>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.
? In Class A operation, total IMD is in the neighborhood of -50db below
- cheers, Joe.
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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