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Registered Member #4052
Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69
I want your opinion on if this will work or not.
VT1 and VT2 (GMI-6 double beam tetrodes) are in parallel. The grids are going to be ran as a normal grid leak circuit. The screen will be modulated using VT3. R1 and R2 will set the CW screen bias just below the maximum (+800v). Then VT3 will be ran as a class A amplifier to raise and lower the screen voltage for modulation.
I will probably need a pre amp to drive VT3, However I still do not know much about tube amps so any sites with good information would be greatly appreciated. It seems hard to find useful information on designing vacuum tube circuits.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi sir
I've been playing with something similar lately - though I used direct base feed instead of an Armstrong type oscillator, with 3 6146 tubes.
Your tubes look a bit smallish but can stand fairly high voltage so they might be even better for base feed.
You also want the frequency to be over 4Mhz or so in order to get quiet, stable plasma flame, and designing a tank and feedback circuit for this frequency might end up being troublesome.
You'll also need to balance the tubes which I did by using separate feedback antennas for each tube and this worked well.
I honestly didn't try audiomodulation yet, but there are two distinct possibilities:
- Modulate the G2 and use antenna feedback on G1
- Modulate G1 (could likely be done with audio signal directly) and use G2 with a feedback winding and a grid leak network.
I'm not sure how wold these compare but you could perhaps try out both! (I will once I get home)
Registered Member #4052
Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69
I have never built a base fed coil before, so I was thinking that I would just try to make a really small grid leak system.
Could I balance the tubes by putting resistance on the grids? It would be nice if I did not need to build four seperate feedbacks.
I was thinking that with a decent sized amp driving the screens that it would be possible in the future to add more tubes to increase the power, however if the tubes prove difficult to balance then it will not be worth the effort.
I will keep the different posibilities in mind, however I still have a lot more research to do before I actually build this thing.
EDIT: Sorry Dr. Spark I didn't see your post, we must have posted at the same time. What kind of system was used for the coil in the video, it is not explained in much depth.
Also how much wattage would it take to drive the screens?
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Audio Modulated VTTC
