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Practical vacuum tube signal generator

  • Hello everybody

    I've been out of touch on valves due to time restrictions, but I want to share with you a small project that I just recently started.

    I have always had plans to build a second valve oscilloscope and a powerful audio amplifier (with 807s). Mr Time had been implacable, so I've been a little distanced from valves for a while. But now I want to work on a project which is a device to be used with those both bigger (and future) projects.

    It is my intention to build a signal generator with two fixed outputs: one at 1kHz and the other at 25kHz. The first one will be to test valve amplifiers while the second it's intended to help in the design of the EHT generator for the scope.

    2018.11.07

    This way I can work with valves (which is something I love), and at the same time I can build a useful tool for future projects. It's a win-win situation. On the other hand as Mr Time is always flying away from me, I want to use parts that I have built before for other abandoned (or modified) projects (Picture 2).

    I also want to experiment with new circuits (for me) so, I want to build the 1 kHz signal with a phase shift oscillator (Picture 6), while the 25kHz will be an L-C oscillator (Picture 5). This way a can "play" with valves too. I have tested those circuits in my valve tester board (which I call "High Tension Bread Board" or HTBB) (Picture 1)

    The attached pictures show the chassis in which I will mount the generator, and the circuit tested on the HTBB.

    I mounted the circuit PSU (Pictures 3 & 4). It will not be a standalone device as the high tension (200V AC) and heaters supply (6,3V AC) will be taken from my bench (Picture 3). The PSU has a bridge rectifier (1N4007) and two STR100/80 regulator tubes in series (Picture 7).

    I also mounted the LC oscillator section and tested several tubes there. It oscillated with ECC83 and 6N2P. I also tried ECC81, ECC82, ECC85, 12BH7 (with 12,6V on the heaters), and the Soviet 6N1P and 6N23P. None of them oscillate. It seems that this circuit requires huge gain in the oscillating valve.

    2018.11.09

    Picture 8 shows an oscillogram of the LC oscillator output at 25 kHz. What seems to be a crossover distortion is a flaw in the oscilloscope that I've been too lazy to fix.

    2018.11.19

    The phase-shift oscillator is also mounted (Picture 9) and the output has been adjusted to 1 kHz with some modifications to the original circuit. (Picture 10). Both signal are appearing on the other so I will need some decoupling to eliminate this effect.

    2018.12.03

    By applying decoupling to both oscillators independently, the super-imposing signals disapperaed or were reduced to negligible levels. See picture 11 for details.
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