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Originally Posted by Neil Purling
I have followed the reccomendations by member 'radioman'.
Look at the attached image. It is a modified version of the circuit I first posted.
I have managed to stabalise the amplifier by putting a resistor 'X in. The existing pentode grid resistor was left where it was. The thing doesn't motorboat now. The value is presently 2.2 Meg. I think further experimentation is warranted.
However I have been using the output from my turntable as a test signal.
What is the rated output of a Sonotone 9TA.
I would like to know if any of the TRF builders on the forum have measured the peak-to-peak signal output of the detector with a scope. I wondered how it compares to my test signal?
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When a resistor in the grid line stops the motorboating, it is most probably RF oscillation in the VHF band. It happens more. ECC83 and 82 valves are notorious for that problem. A lot of times (most times actually) you do not notice, but the amps distorsion is higher. Normally a 10 k resistor will be more than sufficient, 2.2 M is very high. I should not take tha value, it may increase the sensiivity for hum.
In order to check for VHF oscillation, measure the current (cathode voltage) and ground the control grid with e.g. a 1 nF ceramic capacitor. When the current changes slightly, you have oscillation. An RF oscilloscope does not always show because the tip capacitance may influenece the oscillation.
Sorry for the long response, I work in RF valve circuits and bin there a lot of times.
Regards,
Nico