Bud Wireless Phono Osc

Hue Miller kargokult at PROAXIS.COM
Wed Sep 29 04:20:06 EDT 1999

Hello, pipsqueek broadcast veterans: i just traced out the wiring of a BUD
Phonograph Oscillator, and here's my comments.

The device uses a 117L7 tetrode - diode tube. The diode is the power
and the other is the osc., with the G2 element going to the center tap of the
tuned circuit coil between the plate and B+ ( RF ground). The coil saves money
and space by just being wound around the ceramic holder of the mica
trimmer. Output is by means of just some turns around the same coil, with only
one side of this winding connected, and that goes to the antenna connection.
Audio input is to the G1 and power ground with a 600kR load across it. This
all fits in a perforated metal box about 1.5 x 2 x 5 ins.

There's a couple points to have reservations about: the lytic cap (20+20/150)
is right parallel to the tube - nice supply of heat to help dry it out.

The shield of your audio input cable goes effectively, right to one side of
wall plug. I wouldn't use this with my CD player, or anything! this might make
for sparks in the youngster's broadcast studio!

With this minimal output coupling, probably less than 50pF equivalent to the
antenna from the tuned circuit, i do not think any kind of antenna tuning unit
would have been very effective. You could put another hi-impedance circuit
the ANT connection to ground, and then use that to match to the (capacitive
reactive) antenna, but then, this becomes part of the same components that
set the frequency of operation - that would be a challenge to tune up for
and set on frequency at the same time.

The oscillator current and operating point is set thru a 600R resistor from
cathode to ground. Lowering this resistor would increase the oscillator's
power, and that's probably how that previous raconteur achieved the remarkable
range he got - and why the tube shortly went away!

Since having one side of AC power right on the ground connection doesn't
to me, i don't think i'll be using this one to broadcast CDs soon - I think it
needs really to be FETized!
Hue Miller

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