Audio modulating a VTTC
Andrew L., Fri Feb 16 2007, 10:29AMHello
Quite some time ago I built a VTTC and I tried to audio modulate it by puttinga a transistor betwen grid1 and R2 C3 but then it didn't even oscillate
. I haven't yet tried modulating the power supply I also haven't tried connecting a staccato controller. The schematic that I used is CTC labs valve no.2 and instead of the GU-50 tube I used a PL519. It works just fine with the PL519 tube. So if you could please help me because I'm kinda new at these things.Andrew
Re: Audio modulating a VTTC
Bjørn, Sat Feb 17 2007, 04:25PM
What kind of transistor did you use and how did you couple it to the audio source?
Bjørn, Sat Feb 17 2007, 04:25PM
What kind of transistor did you use and how did you couple it to the audio source?
Re: Audio modulating a VTTC
Andrew L., Sat Feb 17 2007, 04:52PM
I baisicly frankensteined these two circuits together
. But it didn't work.
Andrew L., Sat Feb 17 2007, 04:52PM
I baisicly frankensteined these two circuits together
. But it didn't work.
Re: Audio modulating a VTTC
Steve Conner, Sun Feb 18 2007, 02:06PM
It won't work with grid 1 because grid 1 expects an AC signal superimposed on a negative DC bias. The transistor will kill the AC that's trying to get through, and stop the circuit from oscillating.
You have to modulate the DC supply to grid 2 as shown in the second schematic, or make a different circuit that modulates the negative voltage on g1 without killing the RF signal.
Steve Conner, Sun Feb 18 2007, 02:06PM
It won't work with grid 1 because grid 1 expects an AC signal superimposed on a negative DC bias. The transistor will kill the AC that's trying to get through, and stop the circuit from oscillating.
You have to modulate the DC supply to grid 2 as shown in the second schematic, or make a different circuit that modulates the negative voltage on g1 without killing the RF signal.
Re: Audio modulating a VTTC
WaveRider, Sun Feb 18 2007, 02:07PM
Grid modulation should work. However, plate modulation is often more common (I seem to remember it behaves more linearly..
Another technique that I've seen with low voltage transistors is cathode modulation. Low voltage (<100V) transistors can modulate Vgs by modulating the cathode potential with respect to the control grid. If you want, I can try to dig out the reference...
Bill
WaveRider, Sun Feb 18 2007, 02:07PM
Grid modulation should work. However, plate modulation is often more common (I seem to remember it behaves more linearly..
Another technique that I've seen with low voltage transistors is cathode modulation. Low voltage (<100V) transistors can modulate Vgs by modulating the cathode potential with respect to the control grid. If you want, I can try to dig out the reference...
Bill
Re: Audio modulating a VTTC
Andrew L., Sun Feb 18 2007, 06:00PM
That would be great if you could find that WaveRider.
Andrew
Andrew L., Sun Feb 18 2007, 06:00PM
That would be great if you could find that WaveRider.
Andrew
Re: Audio modulating a VTTC
WaveRider, Fri Feb 23 2007, 09:03PM
Andrew,
I could not find the reference. However, the a popular way to modulate AM transmitters in days of old was plate modulation... This requires basically a big fat audio amplifier with a big heavy audio power transformer. Grid modulation is somewhat less linear, but clever use of feedback can reduce non-linearity. Cathode modulation, like grid modulation, can use low voltage devices without the need for transformers. The benefit of cathode modulation lies in its ability to be easily used with grounded grid triode PAs or oscillators. I am not sure about tetrodes or pentodes, tho'. Triode tubes can often be modeled with modified JFET models in Spice... For more complex grid arrangements, I can't say... Sorry!
WaveRider, Fri Feb 23 2007, 09:03PM
Andrew,
I could not find the reference. However, the a popular way to modulate AM transmitters in days of old was plate modulation... This requires basically a big fat audio amplifier with a big heavy audio power transformer. Grid modulation is somewhat less linear, but clever use of feedback can reduce non-linearity. Cathode modulation, like grid modulation, can use low voltage devices without the need for transformers. The benefit of cathode modulation lies in its ability to be easily used with grounded grid triode PAs or oscillators. I am not sure about tetrodes or pentodes, tho'. Triode tubes can often be modeled with modified JFET models in Spice... For more complex grid arrangements, I can't say... Sorry!
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