Hartley oscillator question - LC tank and 12AT7

[6SN7WGTB]

Have been tinkering with my HK RF-1U RF oscillator and although it really isn't 'important' was looking at the AF modulation of 600Hz vs. the supposed 400Hz.

Attached is the modulation circuit, which I believe is a Hartley with LC tank.

Where I am confused is that I thought the frequency was determined by the L and the 'surrounding' C - i.e. the 0µ1 C12 in the schematic.

But shunting C12 does not vary the freq - whereas shunting the 5,000p C13 does. If I put 2,200p across it I get about 400Hz.

Can someone illuminate my misunderstanding(s) please?

EDIT: what I don't want to inadvertently do is somehow undesirably load the overall RF circuit when MOD is on.

[Radio Wrangler]

That's not really a Hartley.

There are two capacitors trying to tune with the same inductor. This would be OK for an untapped inductor, but one is on a tap, one is on the full winding. With the inevitable leakage inductance of the two windings, trouble is likely.

Secondly, signal is taken from an anode load. The Hartly idea relies on cathode follower action so th anode should be a signal ground. Lots of Hartleys use anode output, but you get a better waveform from a follower on the main tank.

David

[6SN7WGTB]

Thanks David, so given I am where I am, how should I tune this in a way not to compromise downstream RF?

[Bazz4CQJ]

In post #1 , is it over-modulation you are worried about? I think I'm right that most sig gens only modulate to a level of ~30%, so I doubt if over-mod will be a problem. Doubt if it will affect the RF circuitry or output in anyway.

B

[kalee20]

Originally Posted by Radio Wrangler

That's not really a Hartley.

The circuit does look like a Hartley to me, to be honest - despite the output being taken from the anode (if the anode load is small it should hardly affect things) and the 'extra' tuning capacitor C14.

However, we don't have details for the tapped inductor.

Based on what the OP says, shunting C12 ought to reduce the frequency. If it doesn't, something else is happening - I'm wondering if coupling is so great that relaxation oscillations are happening - it's a blocking oscillator - and the Hartley oscillation is a very small fraction of the overall period?

Scope waveforms would be very revealing!

[6SN7WGTB]

Originally Posted by kalee20

Scope waveforms would be very revealing!

Here you go. A reasonably good sine wave, at least for the purpose intended.

To answer a previous question, the modulation depth is around 50%, but no, I'm not concerned about that.