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Registered Member #4104
Joined: Fri Sept 23 2011, 06:54PM
Location: Uk .
Posts: 122
Hi ,
I recently made a small 811a vttc with a 2" x 2.5" secondary I managed to produce 8 - 9 " arcs which I was very happy with .
Moving on I started to make a GU81m VTTC in hopes of getting longer arcs .
My secondary is 2.5" x 11" with 10" worth of windings . The supply is a Level Shifted MOT with 4uF worth of capacitance . The tank capacitor is 780pF and primary is 30 Turns on 3" form .
Feed back is 25 Turns under primary , 60W lamp as grid resistor and capacitor is 1.5 nF
So far My output is 8" or so , which is not very good considering the Size of secondary and Tube . Grids 2 and 1 are connected together with Grid 3 connected to cathode .
I have tried Moving primary numbers up and down from 20 to 35 turns but am unable to get any more output , I understand tuning is half the battle with a VTTC but I don't seem to be getting anywhere . So after looking on the net and seeing a few others using a supply for the grid 2 , I wanted to ask how do people implement it and does it have a large affect to output from a pentode such as the GU81 tube ( max G2 voltage is 600 v )
Also , Would it bet better to have a large tank capacitor and less primary windings or vice versa ?
Im not sure what im missing , my 2" x 2.5" 811a coil produced longer arcs than this :P .
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
just connect all the grids together, the grids need a bit higher current but not as big voltage swing -> grid resistor dissipation is acceptable (around 2K 100W is OK). 20" sparks can be obtained this way on a half wave doubled MOT.
Registered Member #4104
Joined: Fri Sept 23 2011, 06:54PM
Location: Uk .
Posts: 122
Dr. Dark Current wrote ...
just connect all the grids together, the grids need a bit higher current but not as big voltage swing -> grid resistor dissipation is acceptable (around 2K 100W is OK). 20" sparks can be obtained this way on a half wave doubled MOT.
Ok thanks , Ill give that a try and see how it goes . Hopfully ill get 20" arcs :D
Registered Member #4104
Joined: Fri Sept 23 2011, 06:54PM
Location: Uk .
Posts: 122
TwirlyWhirly555 wrote ...
Dr. Dark Current wrote ...
just connect all the grids together, the grids need a bit higher current but not as big voltage swing -> grid resistor dissipation is acceptable (around 2K 100W is OK). 20" sparks can be obtained this way on a half wave doubled MOT.
Ok thanks , Ill give that a try and see how it goes . Hopfully ill get 20" arcs :D
Plus I would guess the primary capacitor quality will affect the output , recommended ones would be ones like the Russian doorknob style with good Kamp ratings ?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Yes, the capacitor primarily must not overheat. The best caps are the RF ceramic ones, Russian ones have KVAR ratings printed on them. The K15U Russian caps are the ones you are looking for. MKP/FKP can be used but will heat more than RF ceramics. Note that most ceramic caps are unsuitable because they are made from a lossy dielectric, you can guess a proper RF cap by its size - it is huge for its capacitance and voltage rating and usually with big screw or ribbon terminals.
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GU81m VTTC , Grid 2 Setup .
