COLIN LAMB wrote:

More sensitive tubes can get away with using a 50 ohm resistor and no tuning
on the input. That would probably not result in sufficient drive (I have
not worked with the numbers). However, you could use something like a 300
ohm resistor to load the grid then use a 4:1 step up broadband transformer.
One less control.

My thoughts exactly but what is a 4:1 broadband transformer, other than
the obvious?

I was talking to someone today about not tuning the input and he said it
can't be done.

js

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However, you could use something like a 300
ohm resistor to load the grid then use a 4:1 step up broadband transformer.

My thoughts exactly but what is a 4:1 broadband transformer, other than the
obvious?

I was talking to someone today about not tuning the input and he said it can't
be done.


One can certainly use a 50 ohm load for the exciter and a broadband transformer
with an impedance ratio sufficiently high so as to get the required RF voltage
on the 813 grid, but:

- if the required turn ratio is high, chances are that neutralization will
anyway be required, because the plate-to-grid RF voltage feedback (due to
plate-to-grid capacitance) will not see the grid as a sufficiently low impedance
point any more.

- it may not be easy to realize a broadband transformer properly working on the
whole 160meters - 10m- range and also showing a high reactance value at any
frequency across that range. In particular on the lower frequencies (e.g. 40 to
160 meters) its reactance may not be high enough to show the exciter a
purely-resistive 50-ohm load. A resonating parallel capacitor (different for
each band) may then be needed if one does not like the SWR meter to show some
reflected power.

73

Tony I0JX