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Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Hello all, on a whim, i bought an 811A JAN vacuum tube by Cetron. I need some help with calculating the values i need for the various parts such as grid leak components and all...I plan to run it with a MOT via a variac as to not overpower, nor overvolt it. How do i calculate the various resistors and capacitors needed?I may have 2 CDE Mica caps coming rated for 5000vac .001uF. I assume the tuning goes the same as with a standard coil?How do i calcuate the feedback coil?
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Nice!Looks exactly what im looking for...A small coil to "play" with...With all my others being so dangerous, id like something more controllable....So i thought the 811a was rated for max 1500volts? Heres what I have: 2700volt MOT Plan to use a variac to control this to 1500vac... 6.3volt 4 amp filament xfmr... 811a tube-Need to get a socket... 1.3nF 10kv cermic capacitor(quite large!) Just need to amass some resistors and some ceramic bypass capacitors. I plan to wind a smal secondary. I got a 6in spool of free magnet wire from Essex Just for asking nicely...
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I have extra 811a sockets/top caps... They are not the twist lock kind, just spring clips for the pins; but work great. Also have extra 4k resistors and 2.2nf bypass caps...
I wouldn't worry about turning down the mot voltage; I have run the tube you have at >4kv and it didn't even think of arking over. Although if you are using that large of a tank cap you are going to need to turn down the power to keep the pate dissipation safe. 0.6nf at 2.2kv was just about right for the tube to glow a nice orange color throughout the plate... I guess that running it off a lower voltage would allow you to do short runs at a higher power and then turn it down for the longer runs, or add a stattco and run at high power all of the time...
I like my 811a coil because the streamers are so benign, they are almost silent, and you can touch them!
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
... wrote ...
I have extra 811a sockets/top caps... They are not the twist lock kind, just spring clips for the pins; but work great. Also have extra 4k resistors and 2.2nf bypass caps...
I wouldn't worry about turning down the mot voltage; I have run the tube you have at >4kv and it didn't even think of arking over. Although if you are using that large of a tank cap you are going to need to turn down the power to keep the pate dissipation safe. 0.6nf at 2.2kv was just about right for the tube to glow a nice orange color throughout the plate... I guess that running it off a lower voltage would allow you to do short runs at a higher power and then turn it down for the longer runs, or add a stattco and run at high power all of the time...
I like my 811a coil because the streamers are so benign, they are almost silent, and you can touch them!
Good luck!
Hmmm....Interesing about the voltage...
Im using a large capacitor simplyb/c its all i have ....I wonder if an MMC would suffice?I could series some Film/Foil caps.They are physcially large as well...Wima...
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
pm replied too...
The ideal thing for you to do would put 2 of the caps you have in series, but if you only have one then that won't work... Probably the next easiest thing would be to buy one (here is a cheap source)
I wouldn't recomend a mmc type thing, the tank cap seems to be quite critical to correct operation. I tried replacing the 650pf one I had with a string of 3 2.2nf ceramics and the coil didn't even want to oscillate
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
I think i will try what i have before buying any capacitors...Im only using one 811a tube, so the power level wont rise too much....I think the plate can only take like 100ma or so?
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I believe they are good for 120ma continuous anode current, but in a vttc there is only current passing like 1/4 of the time... I have not taken measurements for plate current yet, but I know that the tank current in my coil is almost 600a unloaded and 200a with streamers I see why a good tank cap is so important now...
Back to the original questions of how to calculate the feedback; so far as I can tell you can't. Just stick to around 20 turns (I found 18 works good for mine by having the tube ark over at the metal/glass seal between grid/cathode on the outside of the tube with 20 turns at 4.5kv, so I removed 2 turns and it hasn't happened since...) For the tank cap; mica caps should be good but I do not know. I do that the doorknob caps sold by hrhdavid on ebay do work.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
... wrote ...
I believe they are good for 120ma continuous anode current, but in a vttc there is only current passing like 1/4 of the time... I have not taken measurements for plate current yet, but I know that the tank current in my coil is almost 600a unloaded and 200a with streamers I see why a good tank cap is so important now...
Back to the original questions of how to calculate the feedback; so far as I can tell you can't. Just stick to around 20 turns (I found 18 works good for mine by having the tube ark over at the metal/glass seal between grid/cathode on the outside of the tube with 20 turns at 4.5kv, so I removed 2 turns and it hasn't happened since...) For the tank cap; mica caps should be good but I do not know. I do that the doorknob caps sold by hrhdavid on ebay do work.
I tried a quick lashup last night....Got 1.5inchers... Boy does that tube get HOT, just the filament... I put on 12 turns of wire for the feedback(i ran out), and used a 3.3kohm10watt resistor for the grid. Nothing got warm but the MOT, 811a, and the primary. The tank cap stays cold actually.... Kinda strange thing happens-When i crank the variac to close to 75% the sparks get big(2.5inches) but then fade out quickly.Any ideas?
Plate gets red rather quickly as well.... Now that i see it works, i will have to build a proper setup for it
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811A VTTC
....I wonder if an MMC would suffice?I could series some Film/Foil caps.They are physcially large as well...Wima...
