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Yes you are correct. That voltage was on Cfeed and not the grid voltage.
I have cleaned up all the high voltage and other basic circuits. The filament transformer now has 18nF caps across it that can take 3 amps each. Way too big but I had them on hand. I have added grounding to all the parts and cases. Also just cleaned up some other wiring. I know have two working 811 and two new 572B tubes that all basically test the same. The 811s are pretty beat up but good for experimental stuff. I’m working on a new primary capacitor since the old one was not that greatly made.
I have found the the primary coil form is fairly easily removable or replaceable so changing the primary inductance and the location of the feedback and primary coil is not too difficult.
Do you think the feedback coil is ok above the primary coil or should it be moved under it?
Does the inductance of the feedback coil matter or is it just a voltage thing to get to 1/10 of the anode voltage?
I can also change Cpri and Lpri if there is any advantage?
Just running with the old 811 tubes at very low power but it is purring like a kitten now!
The tube plates are cold despite how that pic looks...
I just need too check the anode voltage limits now and the grid currents and voltages.
Then plug in the normal running 572B tubes >:D
It no longer feels like it is going to blow up in my face :D
It happily busts out corona right of the minimal variac twist now!
I have learned with this coil that the primary "Q" is super low (0.3) so precise primary tuning is a don't care unlike spark gap things... These coils pretty much tune on the fly at every cycle!
A lot has to go into the care and feeding of the tubes though....
Registered Member #64746
Joined: Sat Sept 08 2018, 09:22PM
Location: Italy
Posts: 27
wow... very good! Regarding the feedback coil, yes it is most a voltage thing related to turns ratio. I would place it on the bottom to avoid risks to the grids in case of sparks and to avoid electric rather than magnetic coupling (electric field is quite high above the primary).
I have not figured out a really good model equivalent for the tubes yet… But going with the computer modeling army I got. LTspice is doing very well :)
Just updated a few values slightly for the new C2 and Cpri caps and Rfeed measures slightly lower. But nothing significant.
I was working on the unloaded case of Vrail traveling up and down 150V over several minutes… No model predicted that and when actually scoped it seems to be just switching noise off D1 or prevailing wind noise charging C1 with a DC offset. Pretty darn weird actually… I was worried it was an LC resonate thing on the MOT and C1 but no. It has always done it. I thought replacing C1 And D1 would get ride of it – but no… Very weird, but very real….. That could use more investigation to track down since it really could damage something……
So I pulled out some HV probes out of my very dusty toy box…
I have not used some of these in 20 year so I had to verify that they and “me†were still sane. Had to remind my self of the voltage ratios… When working with 10kV you don’t want to mess up by a factor of 10X :D
So after the voltage probes were figure out I put a big one on the anode voltage.
But the voltage and stuff seemed super high at low variac setting.
I have turned it up far higher without actually looking at the voltages… Maybe, I was overloading something, again :D I am thinking once the streamers get high the loading takes the voltage way down...
So that is where I am at. I have a lot of knowledge about Tesla coils, high voltage, test equipment, theory…
But rest assured, when dealing with tube coils I am definitely playing with things I do not understand at all! :D
Registered Member #64746
Joined: Sat Sept 08 2018, 09:22PM
Location: Italy
Posts: 27
well, peak anode voltage without loading should be close to double the power supply. The power supply architecture is a doubler, so 1900 Vpk RF mean 950 Vpk from the power supply and 425 Vpk at 60 Hz. Don't know how your variac was set, but seems reasonable. May be too much for those tubes, but this is another problem... Didn't understand the first part, the weird problem...
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Rebuilding VTTC
