By Don Kang
When you put S-line radio in upside down service position, the PTO tube may hit the table top. The tube can be damaged. Build your own low profile 6AU6/7543 This is a fun project. Only 4 parts are used. The circuit, prototype, and finished unit are shown in the picture.
N-channel J-FET operates like a vacuum tube. For T1, N-channel J-FET 2N5485 or 2N5486 is selected. 2N5486 is a higher Idss version of 2N5485. Both are made for RF applications. Other J-FET, 2N5457, 5458 and 5459 made for AF/low frequency applications also work in this circuit. Again their differences are Idss. Idss is a drain current measured at a pre determined drain voltage when gate is shorted to the source terminal. With J-FET made for RF, the PTO frequency goes up about 2khz, on the other hand with J-FET made for AF, the frequency goes down 2-3khz. These frequency differences are coming from the junction capacitance of the J-FET. T2 works as a drain voltage extender. The J-FETs mentioned here do not have high drain voltages. High voltage NPN transistor 2N5551 or MPS-A42 is good for T2. The Zener diode Zd sets T1 drain operating voltage. It can be 8-15V. I used 12V 1/4W Zener diode.
A typical “R”value for 2N5485 is 200 ohms. However, because of wide range of Idss, other J-FET requires resistor value adjustment. A 500-ohm variable resistor will be used to adjust the “R” value. Connect an oscilloscope at the VFO output connector on the radio chassis with a 100-ohm shunt resistor. Set 500-ohm VR for 1.5V peak to peak output reading on the scope. Output of as low as 1V peak to peak seems acceptable. If the output is less than 1V even with zero resistance, check the output with original tube. If the tube gives higher output, try different J-FET. Higher Zener voltage can increase output also.
When RF J-FET is used, higher output frequency can be corrected by adding a small capacitor between pin #5 and pin #7 made by two insulated wires twisted together. In the picture two AWG #30 kynar wires were twisted. About two twist turns gave right frequency correction. For lower output frequency, the correction should be made at the PTO.
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