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24th Apr 2022, 9:04 am | #1 |
Hexode
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EHT reservoir cap - anything wrong with small ceramics?
Hi,
Is there any reason why ceramic caps such as these cannot be used as EHT reservoir caps in sets with 8-10kV EHT? Perhaps larger dissipation factors compared to doorknobs? The latter are significantly more expensive, larger and harder to find. Thanks, - Helder |
24th Apr 2022, 9:49 am | #2 |
Moderator
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Re: EHT reservoir cap - anything wrong with small ceramics?
They get used in EHT for small CRTs in scopes. For larger screens, you may need to consider ripple current ratings.
David
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24th Apr 2022, 10:16 am | #3 |
Heptode
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Re: EHT reservoir cap - anything wrong with small ceramics?
Hi Helder
I have used ceramics in the past without any problem. Frank
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24th Apr 2022, 10:26 am | #4 |
Hexode
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Re: EHT reservoir cap - anything wrong with small ceramics?
Thanks David. The tube in question - MW22-16 - is still relatively small, although larger than a scope's.
That's great news Frank. At this stage I merely want to experiment with some additional capacitance to complement that of the tube's aquadaq with the hope of improving EHT "regulation" in my TV22 Mk.2. I mostly wanted to be sure that the small ceramics would not blow up right away and once an ideal value was found, I could replace them with a doorknob. But based on what you mention, it seems they can actually be used as a more permanent fix too, which is very good news. - Helder |
24th Apr 2022, 11:43 am | #5 |
Dekatron
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Re: EHT reservoir cap - anything wrong with small ceramics?
When you spread the load across a dozen or more of those in series/parallel configuration, they probably won't be overloaded. Just make sure to overdimension the voltage rating appropriately, as differences in capacitance can result in uneven voltage distribution. For example use 5 x 3kV or 8 x 2kV in series for 10kV EHT.
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24th Apr 2022, 11:52 am | #6 | |
Hexode
Join Date: Sep 2019
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Re: EHT reservoir cap - anything wrong with small ceramics?
Quote:
- Helder |
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24th Apr 2022, 7:50 pm | #7 |
Heptode
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Re: EHT reservoir cap - anything wrong with small ceramics?
Helder,
The MW22-16 has an aquadag layer, which is intended to act as the EHT reservoir capacitor. So, if you have an EHT at line frequency, you don't need any additional capacitor. Just make sure the aquadag layer is properly connected (to GND of course). A few of the mentioned capacitors in parallel would not hurt though. Hope this helps. Jac |
24th Apr 2022, 8:18 pm | #8 |
Dekatron
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Re: EHT reservoir cap - anything wrong with small ceramics?
I hadn't noticed those capacitors are rated 15kV. I'm only used to seeing a 3kV max rating for the higher capacity values and 6kV for the lower capacity values.
I wouldn't trust the 15kV rating though, since they're cheap and Chinese - and assume you need 2 or 3 in series to operate them within reasonable parameters. |
24th Apr 2022, 9:15 pm | #9 |
Octode
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Re: EHT reservoir cap - anything wrong with small ceramics?
I have had a couple of the Chinese ones go short, the Vishay 615R150GAD10 SLCC are very good.
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24th Apr 2022, 9:26 pm | #10 |
Heptode
Join Date: Feb 2006
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Re: EHT reservoir cap - anything wrong with small ceramics?
CPC did sell 15 kV ceramics.
Frank
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25th Apr 2022, 7:41 am | #11 |
Hexode
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Re: EHT reservoir cap - anything wrong with small ceramics?
RS have a good selection of high voltage capacitors. Some of which I use to refurbish Static Generators for Paint Booths.
https://uk.rs-online.com/web/c/passi...135,4291323258
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25th Apr 2022, 8:11 am | #12 |
Heptode
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Re: EHT reservoir cap - anything wrong with small ceramics?
I am not familiar with the UK sets, but the U.S. ones used "door knob" capacitors and often the HV rectifier socket was mounted to it I have them from 10KV to 40KV in my junk box, IIRC., and a few NOS as well. Ranged from 500PF to 1000pf, or possibly more.
My biggest concern with using a ceramic disc is arc over unless the leads were sufficiently spaced, especially since there is no real insulation distance between the leads if any dust were to build up on the ceramic cap. I have seen HV caps arc over, blow apart & burn in color TV sets on the FBT. |
25th Apr 2022, 8:24 am | #13 |
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Re: EHT reservoir cap - anything wrong with small ceramics?
What's wrong with small high voltage ceramic capacitors is that their legs are a bit too close together given the voltage they can run at, so PCB layout and cleanliness become critical to reliability. In scope EHT systems it's common to have a slot routed in the PCB between their pads to increase creepage distance. On the whole, the PCB seems less reliable at the spacing imposed on it then the capacitors themselves.
David
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25th Apr 2022, 9:38 am | #14 | |
Hexode
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Re: EHT reservoir cap - anything wrong with small ceramics?
Quote:
- Helder |
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25th Apr 2022, 9:55 am | #15 |
Heptode
Join Date: Jun 2006
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Re: EHT reservoir cap - anything wrong with small ceramics?
Hi Helder,
I've used several ceramic EHT capacitors, although I must admit that so far I used only TDK and Murata types. A few years back I bought a batch of Chinese 3.3nF-2kV ceramic capacitors (they were very cheap) and I tested them at 3.5kV DC (the limit of the PSU I had handy), and they could easily withstand that (only tested for an hour or so). I wouldn't immediately be too suspicious about the quality, certainly not for a short test. If you can obtain capacitors of a renowned brand, it would be a lot safer though. Not good good practice to risk damaging the LOPT. If the CRT is not too good, the source impedance of the EHT could well be too high in order to increase the supplied current above a normal value. Usually a CRT does not perform better at higher beam currents. Focus is best at low currents of course. Good luck with the experiments! Jac |
25th Apr 2022, 2:00 pm | #16 |
Dekatron
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Re: EHT reservoir cap - anything wrong with small ceramics?
I bet all those blue ones originate from the same place in China. I've got one that I obtained from a UK supplier that I sometime get other capacitors from, for fitting as an EHT smoother in the TV shown in my avatar at the side of this post. These same capacitors are also good for using as output stage coupling capacitors in old valve radio transceivers. The original Visconol branded oil filled unit does (or did) actually work perfectly, but I decided to disconnect it as I didn't want it to suddenly give me a nasty surprise one day, particularly as it's mounted on its side and going off with a bang would cause an awful mess within the set. I've actually fitted a Johnny Birkett un-marked special from decades ago, which tests at around 700pf - working voltage unknown, other than guessing it's high! The actual value needed is 1,000pf, hence thinking about fitting the appropriate blue ceramic one, although it seems quite happy with the old Birkett special that's fitted at the moment. These sets will work without smoothing, but with no aquadag coating on the tube I found the bent verticals on door frames etc. rather annoying, and 700pf clears it up rather nicely, so I probably won't ever bother to fit the 15KV 1,000pf blue one.
Does anyone recognise the red high voltage ceramic shown below? There's a splash of yellow paint on one side, other than that they're unmarked and unbranded. I think I still have one or two in stock somewhere. I was wondering where they originated from and what their actual specifications were, anyone know? |
26th Apr 2022, 6:54 pm | #17 | |
Dekatron
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Re: EHT reservoir cap - anything wrong with small ceramics?
Quote:
They are working well in three of my restored sets, I fitted one inside an emptied Visconal casing. Mark |
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13th May 2022, 7:35 pm | #18 |
Hexode
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Re: EHT reservoir cap - anything wrong with small ceramics?
I finally tried a couple of caps across the EHT output and the result was basically nothing: I observed no increase in regulation for up to a 40% increase in the capacitance - 400 pF - and so I stopped there.
The good news is that the 15kV-rated cheap ceramic caps didn't blow up. - Helder |
13th May 2022, 7:39 pm | #19 |
Heptode
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Re: EHT reservoir cap - anything wrong with small ceramics?
Thanks for reporting back Helder.
Exactly as predicted also. Always good to see thoughts confirmed of course Best wishes, Jac |
15th May 2022, 9:35 pm | #20 |
Dekatron
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Re: EHT reservoir cap - anything wrong with small ceramics?
I knew you wouldn't. You've just added additional smoothing.
There's a difference between 'regulation' and 'smoothing' and adding capacitance across the output isn't going to give any extra regulation. Lack of smoothing causes a sort of cogging, well, more correctly bent verticals on high beam current. As you saw earlier I used 700pF to replace a 1,000pF Visconol type and it worked just fine, but that's not regulation. You didn't add very much at only 400pF. You can get larger values of that type you were using cheap enough. Below is a picture of a 1,000pF type that I bought a while ago especially to fit in that TV, but to be honest the random red one seems to work well enough, so I probably won't bother. I can't remember what that one cost, as I bought it with some other capacitors, but it was something like a couple of quid, I think. |