[HBR] more w9vc local oscillator

[Walt Hutchens]

Bill said:
> First I was able to hook up my scope and see a clean sine wave on
> the grid, but a not so clean pattern on the plate. The signal on the
> plate looks like a series of "M" s, there is a spur that drops down
> between the peaks of each sine wave.

This is normal.  Such an oscillator will operate class 'C', that is,
with the tube cut off over most of each cycle.  So what you see on the
plate over most of the cycle is voltage coupled from the tank circuit.
No plate current is flowing.  Then comes the pulse of plate current
where the grid is driven positive and the plate voltage will fall as
current flows: This is the (brief) period when the energy is coupled
into the tank circuit for the next cycle.

> Second I learned that its hard to take a picture of a scope pattern
> the flash evidently messes things up.

Don't use a flash for scope pictures. Start with the shutter speed
roughly equal to the scope sweep time and use the aperture setting
to get the trace to look right in your picture.

If the trace is clear and stable then you can use a longer shutter time
(lower speed) to brighten the trace if needed.

I wouldn't make a bunch of changes, let alone try a whole different
circuit until you've fixed the raspiness. Problems like that have a
message and you want to be able to QSL that, before going on, so
you'll know the next time you encounter the issue. Three to one it'll
be either the parasitic, too much feedback (controlled by the plate
coil number of turns and spacing), or a part (most likely a cap across
the tank) breaking down.

Often you can diagnose a parasitic by touching the grid or plate with
the lead of a pencil.  This advanced diagnostic tool will often
absorb enough VHF energy to kill (or nearly) the parasitic.  Of course
it will also somewhat reduce the amplitude of HF oscillation, so it might
also hide a part that's breaking down, so you can't really be certain.
But after you fool with enough oscillators that have VHF parasitics
you'll learn the signs.  For example, touching one end of a wire (that
has VHF on it) with the pencil lead will have one effect but touching
the other end of the same wire will do something different.

For HF a wire is a wire; for VHF that same wire is an inductor that's
part of a tuned circuit. Making a stage operate at HF WITHOUT VHF
parasitics requires that you make the gain low enough that it won't
oscillate at the frequencies determined by the parasitic tank circuits
that are found in every circuit. This is usually not a problem with
tubes like the 6J5 but with modern tubes -- 6BZ6's, the TV UHF
oscillator tubes like the 6AF4, the 6J6, both sections of the modern
triode-pentode tubes (6CL8, etc.), and the like -- you might as well
start with a 100 ohm resistor in series with the grid.

Oscillators don't have parasitics as often as amplifiers do because
the tube is cut off most of the (HF) cycle and the grid is drawing
current for the rest of the time, which damps the VHF circuit.  But it
can happen.

Be careful of high voltage issues when probing around a vacuum tube
circuit with a lead pencil that you're holding ... In a receiver
circuit this test is pretty safe with one hand in your pocket, dry
floor, etc., but when voltages get higher, not so

One question: How are you hearing this raspy sound?  Listening to the
oscillator in a receiver, or ???

You MAY be able to SEE the raspiness on the scope if you slow the
sweep down enough and fool with the triggering.  Or connecting the
scope may make the bad sound go away -- if so, it's most likely the
parasitic.

Yep, I got leaves to do, too.  I'm at about 15% of the annual chore
...  But it's good exercise.

Walt
KJ4KV