Using the LM-13 Frequency Meter As A VFO
by Greg Latta, AA8V
2 Transistor Buffer Amplifier and 1 to 2 Voltage Step Up Transformer Page

Two Transistor Buffer Amplifier Click on the image for a larger view.

1:2 Voltage (1:4 Impedance) Transformer Click for a close up view.

Buffer Amplifier:
The output of the LM-13 is quite low, about 200mV-300mV peak-to-peak. This is fine if you want to use the LM-13 for its intended purpose, but it is far to low to drive vintage transmitters, which typically require up to 10 or 20 Volts peak-to-peak. To use the LM-13 with a vintage transmitter, the output must be amplified.

It turns out that it is very easy to build a two transistor amplifier using a couple of 2N2222 transistors. The amplifier circuit below yields a peak-to-peak output of about 5V, which may be enough to drive some vintage transmitters. I got this from a web site by N3ZI. I have instituted the changes he suggests to maximize the output of the buffer amplifier up to 7 MHz. It rolls off above 7 MHz, but still gives useful output even up to 14 MHz.

Two Transistor Buffer Amplifier

Though RCA jacks would have been better for the input and output, I had a small 1.5"H x 2"D x 2 3/4"W minibox that I found at a hamfest that already had two 1/4" jacks mounted on it, so that is what I used. I added an RCA jack so I could connect power to it from a 12 VDC wall wart transformer. I built my amplifier on a small piece of PC breadboard and mounted it inside the minibox as can be seen in the photos below:

1 to 2 Voltage (1 to 4 Impedance) Step Up Transformer:
The output with the two transistor amplifier is about 5 volts peak-to-peak, and this is sufficient to drive many vintage transmitters. However, some transmitters, such as my Eico 720, need more drive. To drive such transmitters, you will need to use a 1 to 2 voltage (1 to 4 impedance) step up transformer.

The output of the two transistor amplifier is low impedance, and it is possible to get more output by using a transmission line transformer to step up the voltage. It is very easy to make a 1 to 2 voltage (1 to 4 impedance) step-up transformer and it is pretty much foolproof. All you need is some small wire and the proper toroidal core.

My transformer is wound on an FT-50A-61 ferrite toroid core (AL=75 mH/1000 turns). Cores are available from several sources such as hamfests or on-line at Amidon Associates:

Specific Link FT-50A-61 Toroid Core
General Link To Amidon Associates

Do not use a powdered iron core. These won't provide enough inductance. The core is not critical in size or material as long as it is a ferrite core with an AL value of at least 75 mH/1000 turns. If you have an unknown core, it is easy enough to try it out. If it works, fine. If not, you will need to try another core.

I used #24 enameled magnet wire to wind my transformer, but just about any insulated wire that is small enough so that 11 bifilar turns can be wound on the toroid will work. Double the wire up, and then carefully wind 11 turns on the toroid, keeping the wire neat and parallel to itself. The first pass through the toroid counts as turn number one. If you wind an extra turn or two the transformer will still work fine. Strip the ends of the four leads. To strip enameled wire, burn about 1/2" with a lighter and sand off the ash. Use an ohmmeter to identify the two windings and then wire up the transformer as shown in the schematic and photo below:

1 to 2 Voltage Step Up Transformer

I built my transformer into a pill bottle I got from my local pharmacy. For the input to the transformer, I mounted a female RCA jack on the top of the pill bottle. This is visible at left in the photos below. I used a short length of RG-174 coax with a male RCA plug on the end ("pigtail") for the output from the transformer. This is visible at top center in the photo below. This piece of coax must be kept as short as possible. The value of the blocking capacitor is not critical. Anything from 0.001uf up to 0.01uf will work fine.

Notated Picture of the 1 to 2 Voltage Step Up Transformer
Click for a close up view.

Below is a picture of the completed transformer mounted inside the pill bottle:

Completed Transformer Mounted Inside A Pill Bottle
Click for a close up view.

Use 50 ohm coaxial cable to connect the output of the buffer amplifier to the input of the transformer. This cable can be any convenient length, but good practice says to keep it as short as possible. Though the cable from the buffer amplifier to the transformer can be any convenient length, the cable from the transformer to the transmitter must be as short as possible, no more than a few inches, as shown above. This side of the transformer is high impedance and any excess cable will capacitively load the transmitter input and reduce the output. In fact, it is perfectly acceptable to mount the transformer inside the transmitter for best performance. I decided to do this with my Viking Ranger transmitter by mounting the transformer inside the external VFO input I added to the Ranger.

Using More Than One Transformer:
It is perfectly acceptable to use more than one transformer to step up the voltage even more. For example, the output of one transformer can be fed into the input of a second transformer to get a voltage step up ratio of 1 to 4. I found that when I added a second transformer to the input of my 6AG7 VFO amplifier that I got much better drive when using the buffer amplifier on 40m, so I now use two transformers all the time.

The connection between the transformers must be kept as short as possible to prevent any capacitive loading. Whether an additional transformer will help depends on the signal level and input impedance of the stage being driven. The only way to know whether a second (or third) transformer will help is to give it a try.

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All images, designs, and materials on these web pages are the property of Gregory P. Latta and are ©2017 by Gregory P. Latta. You may use them for personal purposes and for educational purposes, but any commercial or other use is strictly prohibited unless written permission is obtained from the author.

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