VG-1 VACUUM TUBE VHF FM GENERATOR - This VHF Generator has been built almost 10 years ago, when I needed a VHF generator for repairing some receivers. I had something around 17 years old. Unfortunately some details were not finished at that time, like the dial. I did some marks on its front panel, which is made with a stainless steel plate, but nothing similar to a dial. These marks were made with the help of an analogue receiver, far away from a frequency meter like my cute HP5314A, I bought some years later. When I thought about doing it, I had in mind to make it Frequency Modulated, so I could apply a saw tooth wave in its input and trace some IF response curves, with the help of my “super” VOLTIX 420 C-5 oscilloscope (the one I had at that time). One of the stupid things I have done in the VG-1 , is that I had cut the tuning capacitor axis, and was difficult to find a knob to use on it.
The chassis came from an old Colorado TV set, I reduced its size to fit inside an old voltage stabilizer cover I had found in the junk. After a sand brush and a zinc treatment to the chassis and electrostatic painting on the cover, the rig was ready to go. The mechanical design has a lot of bugs, the most notorious is that you need to remove the tube shieldings prior to take out the chassis from the cover and only put it back when the chassis is inside again.....
The final dial was made many years later, with help with a degree scale, that I place on front of it and marked each degree and frequency with a frequency meter, in an xls file, to draw the dial. The linearity was good for me. Finally my brother did the work. And, as the output has not filters, I put a “Calibrated Harmonics” scale. It helps sometimes.
The working principle is simple, follow the schematics: two tubes, a 6AF4A for RF oscillator, in a typical Hartley circuit, and a 6AV6 for the audio oscillator, R-C phase rotation type, that has a nice sinusoidal wave of 390Hz (almost 400Hz) with 35Vpp at the plate. Since the tuning capacitor I used had two sections with 410pF and two with 36pF, I used them in separate way, to ease the adjustments.
The different tuning capacitors allowed me to make the modulation to be done with separated BB119 varactor diodes for each scale (sorry tubes, nothing is perfect!), and the linearity and distortion is reasonably low, for a home made rig like this. Obviously the values of the divider network in front of each diode were obtained empirically, to achieve the better modulation deviation/distortion ratio. No pre-emphasis or pre amplification is done. I had provided VG-1 with a modulation level potentiometer, that, after pulled enables the external modulation. My mistake I didn’t thought about using the 6AV6 as preamplifier when it is not used as LF-oscillator.
I hope to think about it next time…
Here you can see the degree scale used to create the Dial of the VG-1. The accuracy is quite great. The frequency stability is very poor, without modulation the oscillator runs, after 30 minutes warm-up, into around ±500ppm variation on 100MHz. In 10MHz this values goes down to less than 50ppm. In this photo the unit is inside its case. Luckily it don't fear any disturbances around it. You can place other equipment or put your hands over it that you won't affect the stability.
This generator has a high output level and even by using its attenuator (not constant impedance-my fault) it can disturb many devices near it, since it has not an efficient shielding at all !!! It don't occurs with the LSG-532, which has a very good shielding. If you turn the LSG-532 output to zero, almost no signal is detected outside the unit.