[Amps] SB-200 input.

Manfred Mornhinweg manfred at ludens.cl
Fri Jul 25 12:53:42 EDT 2014

Hi all,

adding to Carl's comment:

> Tuned inputs provide the flywheel effect that makes the tubes easier to 
> drive and improves IMD 5-10dB.

That's indeed to biggest reason to use a tuned input. The impedance of a class 
AB grounded grid tube's cathode varies dramatically throughout the RF cycle. 
During a good part of the cycle, the tube is cut off entirely, and the cathode 
impedance is essentially infinite. During the rest of the cycle, the tube 
conducts, and the cathode has an impedance that changes depending on the 
instantaneous current. The curves of triodes are pretty crooked, and this 
reflects on the cathode. Having a big flywheel there, meaning a relatively high 
Q network of some sort, helps a lot in keeping the waveform clean, and in 
providing a huge drive current during that part of the cycle when the cathode 
needs it, while accumulating energy from the driver not needed by the cathode 
when it's in cut-off.

The point is that with any grounded grid amplifier, you need a reasonably high Q 
network between the driver's output tube or transistors, and the amplifer's 
cathode. If the driver radio has a tuned output network, such as the PI tank of 
a tube rig, that's really quite enough, and a WELL DESIGNED broadband drive 
matching circuit can work OK. Same thing if you have a solid state radio with an 
antenna tuner inserted between the radio and the amplifier. In that case the 
tuner provides the required Q and flywheel effect - or at least we can hope the 
Q is high enough! But if you have a bare bones solid state radio, whose only 
flywheel effect comes from a 5 pole lowpass filter having a Q of unity, you will 
see trouble with a broadband cathode driving circuit. In that case, a tuned 
drive circuit works wonders.

We can also see it this way: When you load a radio with a correctly tuned 
antenna, or with a plain resistor (dummy load), it sees a constant load all 
through the RF cycle, and that load is ideally 50 ohm, making an SWR of 1:1. But 
if you connect it to the cathode of a grounded grid tube, it sees a terribly 
varying impedance, with the SWR being far above 1:1 most of the time, and at 
infinity for a part of the RF cycle. No amount of broadband matching can fix 
that! Any matching transformer can only change an impedance by a fixed ratio, it 
cannot match to a wildly varying impedance. But if you insert a tuned circuit, 
which acts as a flywheel, this tuned circuit evens out the varying impedance of 
the cathode, and presents the average impedance to the driving radio, smoothly 
and cleanly. This impedance may still be wrong, but at least it's constant 
throughout the RF cycle, and that allows matching it to a nice 1:1 SWR with 
either a broadband transformer or a resonant network. Usually, when you go to 
the trouble to install such a resonant circuit, it's very easy to tap it for 50 
ohm, or to elaborate it into a PI section having 50 ohm input impedance. So 
that's what's usually done, rather than combining a broadband transformer with a 
set of simple tuned circuits.

Please note that there is yet another problem: The tuned cathode matching 
circuit can very well even out the cathode impedance over the RF cycle, but it 
cannot do anything when the average impedance over the RF cycle varies according 
to the amplitude of the signal! So, even with a tuned matching network, the 
input SWR can change according to the exact instantaneous drive level. That is, 
an amplifier might present a 1:1 SWR to the radio at 50 watts CW drive level, 
but at 100W or at 20W the SWR will be higher. During SSB transmission, the SWR 
will be varying all the time, according to the signal's envelope. Some radios 
cope better with this than others. If an amplifier needs far less drive power 
than the radio can deliver, it's a good idea to insert an attenuator, so that 
the radio runs at full power, and the attenuator dampens the SWR changes of the amp.

But the best way to avoid all this trouble is to avoid grounded grid amplifiers! 
If you use a grid-driven amplifier in class AB1, the tubes almost don't load the 
driver at all. A dummy load inside the amplifier loads the driver smoothly and 
cleanly. No tuned drive circuits are required, no drive distortion happens, and 
many tubes can be driven without needing any impedance transformation, and just 
a low drive power at 50 ohm. The disadvantage, of course, is that the tubes have 
to be tetrodes, at least, with an additional screen power supply. Triodes aren't 
linear enough to use them in this way.

When using MOSFETs rather than tubes, the situation in this regard is way better 
than with grounded grid tubes , but not as good as with grid-driven tubes in 
class AB1. By using proper circuit design, with enough negative feedback and 
resistive gate swamping, normally the performance is pretty good without needing 
any tuned input circuit. But in extreme cases, when milking a marginal MOSFET 
for all the gain it can provide, leaving no room for gate swamping nor negative 
feedback, a tuned input circuit can be required!

Many of you of course knew most or all of this, which is all pretty old 
technology, but perhaps somebody learned something new to him. And I had fun 
writing it...


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