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Bob[_18_] · September 22nd 08, 08:50 AM posted to rec.radio.amateur.homebrew

wrote:

What circuit and values did you use, Bob? Voltages, balanced vs.
unbalanced, output and input circuits, etc.?

Balanced throughout! Supply is 130 V, and the deflection plates have a 10 k
preset (to give DC balance) with the wiper going to the supply, and the
ends connected to potential dividers (68 k and 10 k) to give 15 V at pins 8
and 9. The anodes are connected directly to the primary of a 10.7 MHz IF
transformer, with the centre tap of the transformer going to the positive
rail through a 220 ohm resistor. There is a fixed capacitor (of 100 pF)
from one anode to ground, and the other anode has a parallel combination of
68 pF fixed and a 65 pF trimmer to ground to allow phase balance. Pin 3
(grid 1) has - 1.9 V bias fed through a 470 k resistor and is coupled to
the top of an at-frequency tuned circuit - the high input impedance of the
valve allows the tuned circuit to be connected without a tap. The bias
line has to be well-decoupled (I used a 470 uF electrolytic) to prevent
hum. The control grid (pin 2) is connected to the positive supply through a
470 ohm resistor, and is decoupled to ground with 10 nF.

The balanced local oscillator (a pair of 2N3819 FETs) is buffered by a pair
of common emitter bipolar transistor stages (I used 2N3866, because I have
a lot of them) and fed to the deflection plates in antiphase. IF rejection
is roughly 60 dB, conversion gain is 25 dB. You can tune right through the
IF frequency /without/ /a/ /whistle/ - which is unique in my experience!

I found the circuit values in the original valve specification, and they
seem to be well optimised - I tried various alterations, but always to the
detriment of performance.

The Kallitron oscillator is stabilised (in 10 Hz steps) with a huff-and-puff
unit based on a PIC, which also provides a frequency readout on an LCD
display.

Over the years, I've tried /every/ possible mixer topology, including the
modern H-mode switching types (which are astonishingly good), but the 7360
can't be beaten.

The receiver is simply the best that I've ever used, even including some of
the high-priced exotica that I saw and used at the BBC Caversham Monitoring
department!

Bob

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