04-28-2012, 08:08 PM
the answer to your #1 question is:
Your det-osc. is an Autodyne type. The coil in the cathode is the Osc. positive feedback coil. You're simply varying Cathode relative to the Control grid, which is the same as varying Control grid relative to the Cathode.
Due to the negligibly small value of the active resistance of the coil relative to the 10K resistor, the DC bias is determined by that resistor, and the coil only impresses the AC voltage on the Cathode.
When designing this circuit, engineers realized that the operation was not very reliable with various tubes, and it was also found that the 10K resistor provided the compromise between the most uniform oscillation, ability to handle very powerful signal and the most detector sensitivity.
This is why you will find that value of around 10K in many Philco det.-oscillators. (it is there in 80 jr). In your case it is 15K.
Your det-osc. is an Autodyne type. The coil in the cathode is the Osc. positive feedback coil. You're simply varying Cathode relative to the Control grid, which is the same as varying Control grid relative to the Cathode.
Due to the negligibly small value of the active resistance of the coil relative to the 10K resistor, the DC bias is determined by that resistor, and the coil only impresses the AC voltage on the Cathode.
When designing this circuit, engineers realized that the operation was not very reliable with various tubes, and it was also found that the 10K resistor provided the compromise between the most uniform oscillation, ability to handle very powerful signal and the most detector sensitivity.
This is why you will find that value of around 10K in many Philco det.-oscillators. (it is there in 80 jr). In your case it is 15K.
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