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Registered Member #1316
Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365
The 4-400c looks fine. Put in in the coil and see how she runs. A good screen bypass is essential to oscillation and other nasties. What value did you use? In my 4-400a plasma tweeter I used two 1200pf 5kv doorknobs in parallel. You want the lead path as short as possible. On some tetrode sockets, the screen bypass capacitor is built into the socket itself.
I don't think a bypass on the control grid would be a good idea. It would interfere with the feedback.
You should make a DC supply for the coil so you can control the screen voltage precisely with a separate DC power supply or audio modulate it.
Registered Member #3505
Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108
How did you go about audio modulating your 4-400A coil? As for the screen bypass I used 4x1600v/.1uf polys in series and that gave me about 5 inches more. I figured since they are only bypassing high frequency garbage the polys would survive. Would making a DC supply drastically change its performance?
Registered Member #3505
Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108
@ Weston I dropped the 4-400 in and cranked it up and nothing happened. I checked everything out with ~50V on the variac and all the sudden it started working. Then after ~45 sec it stopped again. I tried different taps on the primary and played with the grid and nothing. It did the intermittent operation thing to me a couple times. If I throw the 4-125 in the whole system works like a charm. Got any ideas on what might be causing this? @JCM Check out Steve wards website it has all the newbieish info you need to get started on your VTTC.
Registered Member #1316
Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365
The 4-400a might require higher screen current (lower screen resistor). I audio modulated a 4-400a with a 6l6 cathode follower driven by a 12au7/6cl6 preamp. I think I had issues with negative screen current boosting the screen voltage. A better method would be connecting the 6l6 between the screen and ground and used it as a clamp modulator.
Registered Member #3505
Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108
Is there any real benefit of making a dc power supply for the screen vice using the dropping resistor? I have a step up transformer that will give me ~460VAC which I can rectify and smooth.
Registered Member #3505
Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108
I understand the relationship and function between the screen,grid and plate. What I don't know is if it worth obtaining the required DC transformers. So is there any real(significant) benefit in supplying the screen with an adjustable DC transformer vice using the dropping resistor? I assume its easier to adjust the gain but cant this same thing be done with an adjustable resistor?
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4-125A VTTC
