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Registered Member #113
Joined: Fri Feb 10 2006, 01:40AM
Location:
Posts: 49
Hello everyone I'm working on several projects right now (and have been for a while), and one of them is a VTTC. I was planning on dual 811s. I was wondering if anyone has actually successfully made an audio modulated VTTC. I saw on Dan's site that he had an audio modulated VTTC "coming soon". Would it be possible to somehow modulate the audio in a separate tube after the oscillator (where just the oscillator would be a normal VTTC)? It seems so simple - why has nobody done it? Thanks!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, a VTTC operates in Class-C, so modulating the grid drive will hardly do anything. The usual approach with Class-C transmitters is high-side modulation, where you superimpose the audio waveform on the plate supply, but that requires an audio amplifier that can deliver hundreds of watts and thousands of volts. Probably the most practical way is to build your 811s into a circuit something like a ham radio linear amplifier, and drive it from a low-powered RF oscillator which you modulate.
Registered Member #113
Joined: Fri Feb 10 2006, 01:40AM
Location:
Posts: 49
Thanks for the info! That actually makes perfect sense.
...But, would it be possible to simply audio modulate the plate of the 811a as it would be in the normal circuit? It seems like if there were no input, the input could be 0V on the plate, and full volume would be full voltage and so forth. That would seem easier, no?
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
could you just use a staccato going at a high frequency to disrupt oscillations to make the noise, sorta pwm like? My coil is running at 1.5mhz, so it seems that if you selectively killed some oscillations if would make a decent sound? I wouldn't expect too much from them because if you are running cw there is not going to be very much plasma... I am running a single 811a pretty much to the max on half wave rectified (by the tube's nature) and the 60hz buss is noticeable but not what I would consider loud...
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Liam: Yes, what you called "modulating the plate" is what I called "high side modulation" in my previous post. The advantage is it's very easy to understand conceptually how it works. The disadvantage is that it needs an audio signal of high power and voltage.
... what do you mean by a statco? Oh right, I get it, a staccato. I guess there's no reason why not. You could put a big MOSFET in the cathode circuit and PWM it. The only problem might be beating between the PWM frequency and the oscillation frequency of the VTTC itself.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Yes I meant staccato, kind of forgot a syllable there...
I think that modulating the plate would be major pain in the ass... Probably the easiest thing to do would be to (instead of using a feedback coil) use an oscillator running at the resonant frequiency of the coil, that has the audio encoded in the amplitude of the waves... I am thinking like a 555 running at the resonant frequiency, switching into a mosfet. The mosfet is powered by your audio, and feeding the grid. It seems like that would be much more efficient way of doing things...
Looking at my coil, the frequency seems to stay pretty much constant....
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Modulating the plate supply isn't too bad, although it may require some specialized components. In fact, the best way to modulate the plate is to build a tube based modulator.
Basically, you can get (2) more 811 tubes and run them in push-pull fashion driven by an audio signal. The outputs of these tubes will then drive a modulation transformer (again, these are tough to find) which will modulate the plate supply.
Works great and the quality ends up being top notch.
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Audio modulated VTTC
