On Tue, 23 Feb 1999 09:28:23 -0800 Rich Measures <firstname.lastname@example.org>
>>> >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
>>> >one rebuilds them. Secondly they have a fairly high output
>>> >(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
>>> high. Increase the anode current to normal, RL drops to a
>>> 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
>>> the amplifier unusuable on 10m. However, when a second tube is
>>> RL is cut in half, and that doubles the C needed for C-tune. Thus,
>>> not the anode-C / Cout which causes a match prob. on 10m, it is the
>>> of Cout to RL.
>>This is the third (in addition to the larger filament power and
>>requirements) problem with using "tubes with handles" at amateur
>? 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
>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
>>and 7500 volts respectively on the anodes, 60% efficiency and 1500
>? 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
>back the microphone gain off until the tube showed no increase in
>current during ssb modulation -- i.e., Class A operation. Under these
>conditions, the power gain was roughly 10db and eff. was probably c.
> 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)
>>will result in lower efficiency and higher IMD.
>? In Class A operation, total IMD is in the neighborhood of -50db
And as you asked in another post....who has an exciter capable of -50dB
>- cheers, Joe.
>R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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