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Registered Member #4108
Joined: Sun Sept 25 2011, 11:43PM
Location:
Posts: 149
this is my gu-81m vttc I was getting about 8 inch sparks (watch the video) I was using 8 soviet voltage doubler caps in series for a tank cap, some one at ham radio club said that was lower my Q factor (so I wasn't getting very big sparks), and that I should switch to ceramic door knob caps to make it ring longer. so I did, I put it on and got no out put, but at least it was oscillating, it lit up a light bulb at about 2 inches away, I thought... well its probably just out of tune I TRIED EVERY SINGLE TAP with the same results, after awhile I started to notice smoke coming from near the tube, so I shut every thing off. my RF choke was melted to waszu!!! Was it just a fluke that it failed with the new cap, or is it something else wrong associated with putting on the new cap.
Registered Member #4107
Joined: Sun Sept 25 2011, 07:30PM
Location: London
Posts: 53
Hmm, that looks like a wirewound ceramic resistor. The resistor has unwanted inductance because it's wirewound. This will hamper the design of your circuit. I can see that the insulation on your turns heated up somewhat but your resistor also heated up a lot - the one probably caused the other. Your resistor looks completely blackened there. Can you post a closer-up photo - just as you have there but showing any damage to the resistor?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The capacitor was probably not intended for RF operation. If you use a ceramic cap, you must use one intended for the job. Such as or , preferrably with the kVAR rating marked on it.
Registered Member #4107
Joined: Sun Sept 25 2011, 07:30PM
Location: London
Posts: 53
Very surprised to see that component fail - it's voltage rating is more than adequate and this is a rugged component totally fit for this application. What was the value of the capacitor before you fitted this one? You've got just a 1.3 nf cap there now. How many turns on your primary/ secondary? The circuit may wildly untuned.... just a hunch. That would explain a higher current through your valve/electron tube. Was the anode orange or red when your chokes failed? Lighting up a bulb just two inches away suggests feeble oscillation, maybe just a harmonic frequency.
Registered Member #4108
Joined: Sun Sept 25 2011, 11:43PM
Location:
Posts: 149
I never had it on long enough to see if it got orange, before the new cap the anode would get orange with it running for 2 minutes. I know the primary wire is too thin, I'm building a new cabinet,and I will make a new primary with thicker wire,that should raise the q and drop the current going through the Valve.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
astralhighway: Most ceramic capacitors are not suitable for rf operation. I have a similar one with a green case which I tried in a TC. I also tried a red disk ceramic capacitor. Both caps resulted in very little sparks and they quickly heated up. Capacitors suitable in a TC duty must be special ceramic caps, usually intended for tube transmitters or induction heaters. Sometimes a "non-specialized" cap will work, but most times it will contain just a standard dielectric which is very lossy.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
search on ebay, try "russian doorknob", "russian capacitor", "plate capacitor", "rf ceramic" etc... After you have seen a few caps, you can guess if the cap is suitable or not.
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what would cause a rf choke to fail on vttc?
I TRIED EVERY SINGLE TAP with the same results, after awhile I started to notice smoke coming from near the tube, so I shut every thing off. my RF choke was melted to waszu!!! Was it just a fluke that it failed with the new cap, or is it something else wrong associated with putting on the new cap.
