Tesla 308U capacitor - UK Vintage Radio Repair and Restoration Discussion Forum

petervk2mlg · 6th Apr 2015, 12:29 pm

What type of capacitor should I use to replace the circled one in the attached image?

Thanks
Peter

julie_m · 6th Apr 2015, 1:13 pm

In that position, across the rectifier, a high-voltage ceramic.

petervk2mlg · 6th Apr 2015, 10:17 pm

Thanks AJS
I can get 2KV rating. Is that over the top?

P

julie_m · 9th Apr 2015, 2:33 pm

It will work, but it won't need to be rated 2kV. The most that capacitor is ever going to see under fault conditions is the peak-to-peak voltage of the supply, or about 3 times its RMS value.

petervk2mlg · 10th Apr 2015, 10:14 am

Thanks for clarifying that, AJS.

P

G8HQP Dave · 10th Apr 2015, 3:14 pm

Instead of a 2kV (or whatever) DC rating, should it be a Y-rated AC cap? The original won't have been, but the replacement could be. That way you get fail-safe operation.

Herald1360 · 10th Apr 2015, 5:11 pm

Since its failure isn't a safety issue, an X cap would be adequate. Its galvanic connection is between line and neutral rather than line and ground/accessible metal.

julie_m · 10th Apr 2015, 6:34 pm

It's not a safety-critical part, so it doesn't need the Y rating. It's just providing some RF suppression.

Interesting, because I always thought of diode switching noise as a problem only with fast semiconductors.

turretslug · 10th Apr 2015, 8:51 pm

I expect that it's there to prevent modulation hum as much as anything, as a set with "AC/DC" power supply arrangements won't have the incoming (power lead) RF isolation of a set with classical primary/secondary screening in its mains transformer.

"Y" rating might not be necessary- but 4n7 Y ceramics are widely available and inexpensive and, as well as purely mains voltage considerations, will have kV-amplitude spikes taken into consideration at design. The thickness of ceramic between the faces does inspire confidence.

6th Apr 2015, 12:29 pm	#1
petervk2mlg Heptode Join Date: Jan 2007 Location: Parkes, NSW Australia Posts: 877	Tesla 308U capacitor What type of capacitor should I use to replace the circled one in the attached image? Thanks Peter Attached Thumbnails

6th Apr 2015, 1:13 pm	#2
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Tesla 308U capacitor In that position, across the rectifier, a high-voltage ceramic. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

6th Apr 2015, 10:17 pm	#3
petervk2mlg Heptode Join Date: Jan 2007 Location: Parkes, NSW Australia Posts: 877	Re: Tesla 308U capacitor Thanks AJS I can get 2KV rating. Is that over the top? P

9th Apr 2015, 2:33 pm	#4
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Tesla 308U capacitor It will work, but it won't need to be rated 2kV. The most that capacitor is ever going to see under fault conditions is the peak-to-peak voltage of the supply, or about 3 times its RMS value. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

10th Apr 2015, 10:14 am	#5
petervk2mlg Heptode Join Date: Jan 2007 Location: Parkes, NSW Australia Posts: 877	Re: Tesla 308U capacitor Thanks for clarifying that, AJS. P

10th Apr 2015, 3:14 pm	#6
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Tesla 308U capacitor Instead of a 2kV (or whatever) DC rating, should it be a Y-rated AC cap? The original won't have been, but the replacement could be. That way you get fail-safe operation.

10th Apr 2015, 5:11 pm	#7
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Tesla 308U capacitor Since its failure isn't a safety issue, an X cap would be adequate. Its galvanic connection is between line and neutral rather than line and ground/accessible metal. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

10th Apr 2015, 6:34 pm	#8
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Tesla 308U capacitor It's not a safety-critical part, so it doesn't need the Y rating. It's just providing some RF suppression. Interesting, because I always thought of diode switching noise as a problem only with fast semiconductors. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

10th Apr 2015, 8:51 pm	#9
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,394	Re: Tesla 308U capacitor I expect that it's there to prevent modulation hum as much as anything, as a set with "AC/DC" power supply arrangements won't have the incoming (power lead) RF isolation of a set with classical primary/secondary screening in its mains transformer. "Y" rating might not be necessary- but 4n7 Y ceramics are widely available and inexpensive and, as well as purely mains voltage considerations, will have kV-amplitude spikes taken into consideration at design. The thickness of ceramic between the faces does inspire confidence.