[Amps] 200-ohm tank and "un-un" in HF amp design?

[Manfred Mornhinweg]

Hello,

it should be remembered that the values of loading capacitance, tuning 
capacitance and the inductance of the tank coil not only depend on 
impedances and frequency, but also on Q. The designer has a wide degree 
of control over these values.

For example, if we need to match 4000 ohm to 50 ohm, on 160 meters, and 
we have a loading capacitor of just 1000pF, and we don't want to add 
capacitors in parallel, we can pick a Q of 10. This leads to 215pF 
tuning capacitance (including tube and stray capacitance), a 35.8uH 
coil, and 882pF of loading capacitance.

If that's still too high, just let's decrease the Q to 9.5. That gives 
us 204pF, 37.2uH, and 645pF.

Still too high? Use Q=9. Then we get 194pF, 38.4uH, and just 272pF of 
loading capacitance!

In practice this would be done by tapping the tank coil at a spot where 
the Pi circuit tunes up with the loading capacitor at the center of its 
range, into a dummy load. Regardless of how much maximum capacitance 
that capacitor has! That will give us the best matching range possible 
for real antennas with non-perfect SWR. Of course, the smaller the 
loading cap is, the narrower this tuning range becomes.

The actual penalty of having a very high plate impedance is that we 
cannot use a lower Q. With 4000 ohm, we cannot go below a Q of 8.89. 
This makes tuning a bit touchy, and we need a VERY good tank coil to get 
acceptable losses. The loss depends on the ratio between the natural 
(unloaded) Q of the components, mainly the coil in practice, and the 
loaded Q. If the coil has a natural Q of 300, which is typical, and we 
are running it at a loaded Q of 10, it will loose one thirtieth of the 
total RF power. At 1500 watts, that's 50 watts heating the coil, which 
may be a problem.

If the plate impedance instead is just 2000 ohm,  the minimal Q we can 
use is 6.4, in practice we might use 8 or so. That way the loss is a bit 
lower, and tuning is a bit broader.

Of course we cannot go too low with the loaded Q, because that will 
increase harmonic output to an unacceptable level. Old handbooks say 
that actually we shouldn't go lower than Q=10 or so.

On the highest bands, the problem is another one: All that tube 
capacitance and stray capacitance forces us to use a Q higher than we 
would like! For example, on 10 meters with a 4000 ohm plate, a Q of 10 
would need a tuning capacitance of just 14pF. Good luck finding a tube, 
and a construction method, that keeps the tube and stray capacitance 
lower than this! Likely that capacitance will be higher, forcing us to 
use a higher Q. If the total tube and stray capacitance, plus the 
minimal capacitance of the tuning cap, is 40pF, we need to use a Q of 30 
(or higher). At Q-30, the coil becomes only 0.83uH. With a natural coil 
Q of 300, we would have 150 watts of loss in that little coil! That's 
why many amps use coils wound from thick, broad silver plated copper 
strip for the high bands. Absolutely highest possible natural Q of that 
coil is a strict need! And of course tuning gets much more touchy than 
on the lower bands.

For these reasons, most amplifiers don't use the same loaded Q on all 
bands. They use higher Q on the higher bands, to accomodate the tube and 
stray capacitances, and lower Q on the lower bands, to keep the tuning 
and loading caps from growing too much, and to allow the use of smaller 
loading coils. High Q coils for low frequencies are large.

Well, that was a quick burst of antique tube technology, from my side. 
Since it's monday morning, I hope I may be forgiven. Now I switch back 
into 2013.

Manfred

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