verting from tubes to FET s, I used a cautious three-step approach. The idea being that if something didn't work, 1 would have a better idea where to look for the trouble. The 6J6 oscillator tube was removed first and a TI3-34 soldered to pins 2, 5 and 7 of the tube socket. 1 had expected that the injection voltage from the new FET oscillator might be a little low for a tube mixer but this did not seem to follow. The converter worked as well as ever with improved stability.
Next, I removed the 6AK5 mixer and soldered another TIS-34 to pins 1, 5 and 7 on the mixer tube socket. It was necessary to change the grid leak and cathode (source) bias resistors. Operating again as a hybrid with tuJo FET's and the 6AK5 rf amp., the old VHF-152 performed better than it ever had in its life. I was almost tempted to 'leave well enough alone/'
The final step was to remove the 6AK5 rf amp and replace the tube with two FET's, In this type of cascode circuit, the FET's are in series so it is desirable to increase the volt age to some value between 18 and 24 volts. What I actually did was to add a small 9 volt transistor battery in series with my 12 volt regulated power supply. In the original VHF-152 circuit, the bottom of L2 was grounded (refer to Fig, 1), It was merely necessary to lift the ground, bypass it. and supply the 21 volts as shown in Fig, 2. The ideal situation would be to have the voltage drop across Q1 and Q2 approximately equal. If you use transistor sockets, you might try swapping Ql and Q2 back and forth to get the best arrangement . The gain of the stage is determined almost entirely by QL You can even use an NPN bipolar transistor such as the 2N706 for Q2 if you wish. The voltage divider for the base bias should consist of two 6K resistors instead of the pair of 100K resistors shown in Fig, 2. No other change is necessary.
Nov/ comes the part that is a real pleasure. The power transformer, choke, rectifier tube and VR tube can be removed from the VHF-152 chassis and deposited in the junk box. The new VHF-152 is ten pounds lighter than
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