[Amps] MLA-2500B Question

[Roger Parsons]

I will probably be sorry, but I am going to stick my
nose into this one:

1. It is certainly true that a Class B grounded grid
amplifier will have a varying input impedance over the
driving cycle.

2. If a matching network is placed at the input, it
can be designed to transform that (instantaneous)
impedance into some other value.

3. If the matching network is lossless, the
transformed value will vary in just the same way as
the drive impedance, but the values will have been
transformed.

4. If a lossless length of coax is connected between
the amplifier input and the matching network, then the
impedance will still vary over the drive cycle, but
the values will have been transformed again.

5. Therefore, with perfect components, there is no
difference in principle between the matching network
being at the amplifier or separated from it.

6. Pefect components do not exist, so the matching
network/coax will introduce some loss. This loss will
depend on the network configuration and type/length of
coax. The variation in the drive impedance will
therefore be marginally less, but good quality real
components should not be dramatically different from
perfect ones.

7. If a transceiver with an automatic ATU is used and
it hunts without an input matching network, it will
hunt with one, assuming the circuit losses are low. It
is also conceivable that it may not be able to match
the impedance seen at the end of a coax connected to
the amplfier input.

8. Although grounded grid amplifiers are often
considered to be unconditionally stable, that is not
always true (believe me, I know!), and a capacitor
across the input may help stability. This is
automatically achieved with a pi network and may be a
good reason to use one close to the tube.

9. The only way that I know to make the drive
impedance appear effectively constant over the driving
cycle is to swamp it with a dominating admittance.
This increases the drive power required, probably by a
significant amount.

10. If the feed impedance of the amplifier is
significantly different from the drive impedance of
the exciter, then a matching network can reduce the
driving power required by providing better matching.

11. Any flywheel effect from a matching network at the
input to the amplifier will be no different from a
flywheel effect from a matching network some distance
from the amplifier. The commonly recommmended Q for
input matching pi networks is so low as to produce a
negligible flywheel effect.

12. It is a lot more convenient if all impedances
within a station are kept constant. No retuning is
then necessary when the amplifier is/is not in
circuit.

I can see no mechanism by which the IP performance of
the amplifier can be improved by the use of a matching
network physically at its input, rather than slightly
separated, always provided that the amplifier/system
is stable.

Hard hat firmly in place.

73 Roger
VE3ZI






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