Vacuum tube tesla coil HELP - Page 1
Forum section: electronics / projects

author: Epiccow400
author_rank: Newbie
timestamp: October 09, 2023, 08:23:55 am
content: Hello, i built a dual 811a vacuum tube tesla coil using steve wards VTTC 1 design. My build does not work. Can someone please look over my circuitry and check it against the schematic below? however shoddy my connections look they are solid and tested. I am inexperienced with electronic schematics but know how to be safe with high voltage. Anyone with tesla coil experience could please help me with this? My vacuum tubes light up and my primary seems to be getting power, but no sparks on the secondary. I have tried swapping connections on all the coils and have ran out of ideas. Any help is welcome, thanks.

author: Psi
author_rank: Super Contributor
timestamp: October 09, 2023, 08:38:16 am
content: Only thing I could see on first look is that the schematic has 140V from a variac on the HT transformer, but you have both transformer inputs in parallel from 120V.The fact the schematic says 'variac', and is a higher input voltage, makes me wonder if you need to tune the voltage to get it to work.

author: jonpaul
author_rank: Super Contributor
timestamp: October 09, 2023, 09:43:22 am
content: Haywire connections, alligator clips no tube sockets.Check for correct filament voltagesChinese clone 811A may be junkReverse ONLY ONE wdg polarity eg grid at one timej

author: Weston
author_rank: Regular Contributor
timestamp: October 09, 2023, 07:33:13 pm
content: I built a (single) 811A tesla coil as my first tesla coil. Its a fun project.Build wise, I highly recommend you get sockets for the tubes. There are some pretty cheap Chinese sockets you can get off ebay. I would also recommend you connect the microwave oven transformer though a variac (but not the filament transformer) for testing. It can be easy to melt the plates at full power if everything is not tuned correctly.Debugging wise:Are the filaments glowing? They should visibly glow.This design is a free running oscillator. You need the right phasing between the primary and feedback coil for it to oscillate. If nothing is happening try reversing the connection.The primary also needs to be tuned to the secondary resonant frequency. Its pretty common to use a primary coil with tap points. You adjust the tap for maximimal spark length. If its badly tuned you might not have any sparks but you should be able to get a neon bulb or fluorescent tube to light up.Your primary capacitor (and possibly the grid leak capacitor) are very much the wrong type of capacitor. The primary capacitor sees a peak voltage close to Vin, and a peak to peak voltage of 2X Vin. So ~ 1.4*2kV volts peak. It also has a (relatively) high current. It looks like you are using the capacitors you get from those cheap kits off ebay/Amazon or similar, which have a class II dielectric with high loss, a capacitance that changes strongly with temperature and applied voltage, and very poor tolerance. It's going to overheat and make tuning very difficult. You want a mica capacitor with a suitable voltage rating, a Murata doorknob capacitor with a N4700 dielectric, or one of the USSR surplus doorknob capacitors that have a KVAR rating.

author: TimFox
author_rank: Super Contributor
timestamp: October 09, 2023, 07:38:05 pm
content: What voltage, if any, do you see across the grid-leak resistor R2?

author: floobydust
author_rank: Super Contributor
timestamp: October 10, 2023, 01:10:53 am
content: OP cross-posted:https://forum.allaboutcircuits.com/threads/vacuum-tube-tesla-coil-help.196482/https://maker.pro/forums/threads/vacuum-tube-tesla-coil-help.299808/https://highvoltageforum.net/index.php?topic=2661.0

author: Epiccow400
author_rank: Newbie
timestamp: October 10, 2023, 01:25:08 am
content: Quote from: Weston on October 09, 2023, 07:33:13 pmThanks for the response, Yes the filaments are glowing, ive tried reversing the connections one at a time as well. Where should i put the neon bulb to see it light up? On top of the toroid? Ive purchased this capacitor on ebay, is this the type you said would work?https://www.ebay.com/itm/124972124910?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=kgvm1_YQR0C&sssrc=4429486&ssuid=RUPyuIWlTo2&var=&widget_ver=artemis&media=COPY

author: jonpaul
author_rank: Super Contributor
timestamp: October 10, 2023, 04:41:59 am
content: The Murata HV caps are NOT capable of a Tesla resonant tank current.Will overheat and fail. Use RF TX rated T mica.The Detail and photos of secondary? Materila of sec, eg lucite, PVC, cardboard tube?Wire details, insulation, varnish?Perfect layer?Find the ARRL,1954..1960 Tesla Coil articles in Popular Electronics, Radio-TV experimenter,1959 Electronics Illustrated. (built this one)https://www.worldradiohistory.com/Archive-Electronics-Illustrated/Electronics-Illustrated-1959-10.pdfGreat ham designhttps://www.qsl.net/w8ujx/tesla.htmlNote construction, components.Read the many books and sites on SAFE and SIMPLE Tesla coil design and construction,Jon

author: Weston
author_rank: Regular Contributor
timestamp: October 10, 2023, 07:34:35 am
content: Quote from: jonpaul on October 10, 2023, 04:41:59 amA transmitting capacitor with a kVAR rating would be better, but the N4700 dielectric doorknob capacitors work. Its pretty close to a type I dielectric. Look at the datasheet:https://www.mouser.com/datasheet/2/281/Murata%20DHR_DHS%20Series-1186158.pdf. Other "doorknob" capacitors with cheaper dielectrics would not work. Dissipation factor for the N4700 dielectric is 0.3% and capacitance only slightly changes with applied voltage and temperature. Widely reported to work for tesla coils.https://www.pupman.com/listarchives/2003/December/msg00725.htmlhttps://4hv.org/e107_plugins/forum/forum_viewtopic.php?p=1&id=173096Quote from: Epiccow400 on October 10, 2023, 01:25:08 amSame location as where the sparks come out, the E-field is highest closest to the topload / discharge point.That capacitor might work, the voltage rating is a bit on the low side. What capacitance do you need? Are you trying to copy the Steve Ward design exactly in terms of primary and secondary? (so ~1nF?)The old USSR surplus capacitors with a kVAR (reactive power) rating work really well, but the shipping from eastern europe can take a while.https://www.ebay.com/itm/202557133513?hash=item2f295892c9:g:uAQAAOSwjthcL-Ophttps://www.ebay.com/itm/275687634560?hash=item4030437680:g:GnYAAOSw4CFYtFvP

author: floobydust
author_rank: Super Contributor
timestamp: October 10, 2023, 03:56:55 pm
content: For anyone interested, lol apparently it was a wiring error, grid not connected, wrong pin on base.