With the advent of VIIF FET's, such as the TIS-34, at $1 or under, it becomes practical to remove the tubes from old style converters or receivers and replace them directly with Field Effect Transistors. The unit that I chose for my first experiment along these lines was a rather ancient VHF-152, 10, 6, and 2 converter (see Fig, 1). I decided to start with the ten meter band but in order to be certain of having plenty of signals for tests I moved the lower limit down to 11 meters, the GB.'eis being very active in these parts. A 4 pf capacitor across each of the three ten meter coils did the trick, (dotted lines in Fig- 2).

It can be seen from the schematic that the VHF-152 is not wired like a conventional receiver front end. I don't say the VHF-152 is better, just different. Note the 1 8K resistor which serves as a plate choke for the 6AK5 rf amp. Obviously that will have to go before we can substitute an FET for the 6AK5-

Next comes the problem of neutralization. A typical VHF FET has about 1.5 pf capacity between its input and its output as com pared with the .02 pf capacity of the 6AK5. There would seem to be two possible routes. Either use a conventional tapped plate (drain) coil and a neutralizing capacitor, or employ 2 FET's in place of the one 6AK5 in a cas-code arrangement.

Because of the band switch consideration, it was decided to use the cascode approach. LI will serve as the gate inductance for Q1 and L2 will serve as the drain inductance for Q2- It will be necessary to lift the bottom end of L2 from ground and feed the plus 21 volts through it as in Fig. 2

This particular model converter never did work well on 2 meters. One look at the band change switch will explain why. But, it was thought that the transistorized version would cover 10 and 6 meters in good style. Therefore, the band change switch was preserved. For 2 meter operation, a separate rf preamp which I already had on hand (refer to 73 Magazine, July, 1967, page 48) can be wired into the band change switch as shown in Fig. 2.

Because this was my first attempt at con-

0 0

Post a comment