Notice: This article and accompanying schematic are Copyright 1997 by Robert Pierfelice. Individuals are free to use this circuit for their personal, non-commercial use.

About a year ago I acquired my first TransOceanic -- an H500 model with but one non-repairable flaw: a weak 1L6. Being in no hurry, I waited until the 1996 Dayton Hamvention to seek out a couple of spare tubes. At the swap I thought myself lucky to find two 1L6 tubes for a mere 5 bucks each. A bargain, except when I returned home I found that neither tube was very healthy. This was the first straw. The last straw to actually motivate yours truly in this project was reading that 1L6s are now retailing for $30 from a well-known supplier. This is probably a fair price considering there is a finite number of remaining 1L6s, but not for me to stuff a $30 tube in a radio I picked up for $40. The 1R5/ 1LA6 options did not appeal to me as a real solution and modifying my radio was not about to happen so that is how this little project got rolling.

My goals for this project were a plug in 1L6 replacement with no radio modifications, equivalent performance, cheap construction and no re-alignment. Well, I got two and a half out of four on this one. Modifications to your radio will not be required and overall cost (presuming a well-stocked junk box) should be less than $15. Re-alignment will be required but assuming you have access to the equipment this should not be a big deal as you don't need to touch the IF stage alignment. Sensitivity seems to be on a par with or better than the 1L6 once you have re-aligned your radio. Microphonics typically generated by the 1L6 are eliminated. And now for the bad news. Re-alignment could not compensate for the 200kHz error I now experience on the 16 Meter band. The additional capacitance of the local oscillator transistor gate (Q1) as compared to the 1L6 grid is part of the problem and is not recoverable through normal alignment of the TO. I suppose careful circuit board design using a surface mounted, low input capacitance FET might help, but I personally find very little of interest in the 16 Meter band so I simply left 16 Meters aligned for the tube.

The RCA tube manual does a fine job of explaining how a pentagrid converter such as the 1L6 actually functions so I will not repeat any of it here. I will make a brief explanation of how my circuit emulates a 1L6. Transistor Q1 is used as the local oscillator (LO) and is connected to the TO's LO tank through pins 4 & 3. Transistor Q2 is a buffer amplifier to increase the LO signal level provided to the mixer transistor Q3. Q3, a dual gate MOSFET, mixes the LO signal with the incoming RF signal at pin 6 and feeds the result through C5 to the IF transformer via pin 2. The Zener diodes pad down the tube level voltages to FET range. C1 & C3 provide an RF short around the Zeners and also shunt their diode noise. C6 provides a bit of additional gain at higher frequencies by reducing the negative feedback at Q3's source as the frequency rises. Note that heavier bypassing of R10 will cause Q3 to oscillate quite nicely. The reason I used two diodes in the drain line of Q3 is that I had no 90V Zeners in my junk box so I used what was available. The filament circuit of the radio is maintained via R11 & R12 and a virtual ground for the "solid 1L6" is picked off at their junction.

In conclusion, the 1L6 solid works, is quiet, cheap and will probably outlast my Trans Oceanic. Tube purists may not appreciate what I have wrought, but hey, these 1.5 Volt filament tubes do not even glow nicely so I say be gone with them. Other easy candidates for solidifying should be the 3V4 (maybe a 100 Volt Darlington transistor) and the 1U5 (one detector diode and a FET or two). The 1U4 may prove a bit more of a challenge with it's sharp-cutoff characteristics. As far as building the solid 1L6 goes, if you have no experience with static sensitive FETs and RF circuit layout, get help from a friend who does. The photos should give a pretty good idea of how I have successfully constructed two of these. The only real problem I had on my final version was carelessly connecting D2 & D3 backward and blowing one of my MOSFETs.

Photos

The test bed.

The schematic (be patient, this is a big file).

Shows 1L6 solid final version on the left and the prototype on the right. You know who in the middle. For the prototype I cut the base off of a tube using heated nichrome wire wrapped around the tube near it's base. Then I soldered wires to pins inside of the tube base and filled the base with ordinary hot glue.

Shows rear of final version of 1L6 solid and type of prototyping board I used. Note that the center path of the board is the main ground for this circuit.

A closer view of the final version. Transistor Q3 is shown at the right of the picture, Q1 is located closest to the socket.

1L6 solid shown installed in my TransOceanic. Looks right at home, eh?

Credits: thanks to Henry B. for the digital photography and Alan P. for scanning the schematic.