GU-81M Vacuum Tube Tesla Coil / Hartley VTTC. Update and Schematic

[@opii9163]

Babe, wake up, new janno upload!

[@Ulysses-t2c]

Super awesome

[@tuopeeks]

I was initially a bit concerned with the GU81M having internal capacitors but so far I haven't had any issues interchanging it.

[@janno288]

@@tuopeeks The GU-81 has those not the GU-81M. If you see a kink in the top grid 3 connection its a GU-81 (only for soviet made tubes and not from the svetlana factory (really early gu-81 makers) All non soviet copies are GU-81 equivalents Polish GU-81 and Chinse FU-81 have capaciors inside. Only the GU-81M doesnt have capaciors

[@tuopeeks]

@@janno288 Double checked, you're right 👍. I just mixed them up my GU-81M is in my TC and the spare is GU-81 with the kink in the grid feed & caps. Didn't know so many others copies this tube, although I heard the Soviets copied it originally from a German design.

[@janno288]

@@tuopeeks Original one is the RS-384 originally built 1938 (1939) There are a lot of midwar versions too made by philips for example AS 1000 but the soviets first copied and improved it in 1955 as the GU-80

[@egg99]

Nice. Btw it could be a good idea to use a high frequency suitable ferrite material on the choke for the current meter.

[@janno288]

@@egg99 might be a good idea you are right. I think that ferrite is "good enough"

[@Farben-X]

Would you happen to have any book recommendations for building VTCC? Where did you find all this info? Also, did you leave the MOT magnetic shunt?

[@janno288]

@@Farben-X What an interesting question. There is a book called "chapter twelve vacuum tube tesla coils" but its not in print anymore and no pdf Version exists even if you are willing to pay. There is the "VTTC FAQ" Page on steve wards Website. Google "VTTC FAQ" first result that is often refered to as "the bible for vttc building" Most of the knowledge comes from online posts people who have built vttcs and share their experience. Joining discord servers like te styropyro one also helps since there are a lot of people building stuff like that. For Vacuum Tube knowledge i recommend the RCA RC-30 Tube Manual and Seelys book Electron Tube Circuits. The shunts were left in, the shunts only really start limiting after 0.5A from my experience. I tried to nock one shunt out but I didnt have a suitable tool so only 1 is half out kind of.

[@mykedoes4099]

👍

[@Wtfinc]

I noticed a big issue when putting a capacitor across the output of the MOT and the output of the voltage doubler. A whole amp flows doing nothing. Almost lost my MOT. The tube is really sensitive when you put 600 volts on G2. G1 needs to be driven by a smaller tube at high voltage. 300 volts I think? I always put resistors or cap or both on g3 if its not in use. I think i might use my tubes for radio and not risk it for little return. Should probably be triodes. Wish i had a glass lathe, id make one. I found a big source of bjts of fets, idk what they are yet, but imma put em all in series parallel. High voltage, high current. Seeing that graphite glow like that is fucking wild. Can’t wait to see the new tube and txfmr

[@janno288]

Wait what type of setup were you running? GU-81M in pentode mode with a voltage doubler? Yes the capacitance across the diode should only be a couple of nano farards for decoupling.

[@Wtfinc]

@ it was a labcoatz schematic. I did run it in pentode mode but all i could do was light a neon bulb LOL! I was trying to keep it in spec but I didn’t have near enough current. Or something. I felt like i was dumping in so much power for little return, where was all the power going? So sketchy. So im putting it on hold until i can finish building a good high voltage oscillator so i can eliminate a bunch of variables. Heck. I should probably just use an Arduino so i can get that staccato. If that works, then ill go back to feedback control. I think I noticed, at like even 2kv and the cathode is off, there is a bit of leakage. Gotta get a good negative swing in there to shut it down. I wonder if using G2 as an accelerator gets in the way. Maybe 2 and 1 should be tied and g3 accelerate. Or maybe g2 is negatively biased. I’ll find out soon enough. I love tubes

[@janno288]

@Wtfinc Use triode mode, i have too tried pentode mode with only limited success its more efficient to use these tubes as triodes by the looks of it.

[@rogerbrowne5514]

What is your total capacitance with your microwave capacitors and how do you have them wired . Regards Roger.

[@janno288]

3 in parallel, 2 strings of those in series. So (1uF + 1uF + 1uF) / 2 = 1.5uF

[@LutzSchafer]

Just wondering as to why in the tesla coil community almost always consists of a simple oscillator. Wouldn't it make more sense to go with a traditional transmitter design consisting of an oscillator, driver and finals? This way you avoid most of the instabilies that you report.

[@janno288]

​@@LutzSchafer it is simpler to get going since in self excited oscilation the tube generates its own bias i guess there is less to go wrong and the parts are easier to get, maybe someday i will make an extermal oscillator vttc