The AM Forum

   I note that several of my BA receivers have AC line bypass caps rated at >1000 V. And I seem to remember reading a recommendation to this effect here on the forum.
   My question is, are not Y rated (250 V) caps appropriate for this purpose?

Thanks,
Ed

yes, the safety caps rated Y cap class, at 250 V AC in which they are encased, for safety,  would be more appropriate then the higher voltage DC rating disc caps. In a Johnson Ranger that I restored,  the 600 WV disc caps used in several of its TVI filters, had flashed over to the chassis and blew the line fuse.  I witnessed the event, quite a nice POP. The cap actually survived with only a small chip of ceramic blown from the top of the cap and a trail of black dust on the chassis pointing to the culprit.

I've seen micas, paper, paper-n-oil, ceramic, bathtubs, just about every type of cap out there engineered at one time or another into receivers, transmitters, test gear, eTc, for line bypass.

Some were set for 200V, some over a thousand.

I suppose it boils down to whatever looks like a really low impedance to RF does the job.

I use .02 @ 630V paper/oil Russki jobs now, easier on the wallet than Vitamin Q's and the like. ;)

73DG

AC line bypass caps have a pretty hard life. I have witnessed old tubular paper caps go off like an M-80 after checking good. The line bypass caps in ANYTHING vintage that uses them should be replaced for safety sake.

I would reccomend ceramic disks with at least a 1Kvdc rating for this job!!

Somewhere some years ago, I picked up some .01@1400vdc disk caps with an extra thick epoxy (or whatever it is) coating. They were a pretty robust looking disk cap, and noticably larger than most disks of anything near that rating. IIRC they were designed especially for use as line bypass caps. I used them in every rig that I repaired until I ran out of them.

I have seen quite a few of the 500v rated caps burn up as line bypasses.

Also keep in mind that most caps are rated in DC voltage and not AC, which means you have to at least tripple the voltage rating for AC use.
120V RMS = somewhere around 370v P-P.....................

Why ceramics?  I always use X1/Y2 which are designed for this purpose.  I think this means if they fail they don't catch fire or explode. They may be ceramic or maybe something else

http://www.justradios.com/safetytips.html

With the popularity and availibility of ferrite line chokes is there any reason to continue use of a line bypass which raises hell with GFI circuit breakers?

Mainly because I have bazillions of them, and they seldom fail under normal circumstances.

Those thick blue disks look like the ones that I had. Also I had some thick orange ones with the 1400v rating. (which I thought was a bit of an odd voltage) I honestly didn't know that they made "Safety caps" specifically for that purpose. I have never seen a failure of the 1kv rated disks of the what I now think may have been safety caps in line bypass service. (Although, I have seen the 500v ones smoke out)

And for most applications, I prefer ceramic dialectric caps over the plastic or any other dialectric caps from a reliability point of view. They just dont go bad in anything approaching normal operation. I have seen mylar and polyprop caps fail, although not often

If you want to buy "safety caps", go for it. But why bother if you have a s-load of 1kv and higher rated disks allready in the drawer.

All the saftey caps offer is that they wont short when they fail and considering the typical source I wouldnt bet the farm on it 10-50 years down the road.

Ive been using .01/2000V discs marked "buffer" for decades since I got a huge bag full of them for real short money back then. Nobody wanted automotive vibrator caps!

With a 3 wire line cord I also connect a .1 between hot and neutral which really cleans up line crud according to the scope. You can also build a basic 3 cap filter inside the wall outlet box, inside outlet strips and anywhere else it may help.

Carl

They really need a small resistor in series with them to reduce surge current. I hav 3 KV disc caps on the big rig the they also went off llike M80s once when I turned off the breaker due to inductive kick of 2 CG310s in parallel

The surest safety precaution is to use an isolation transformer in addition to bypass caps, especially with ac/dc sets.  Some iso transformers have an internal Faraday shield between primary and secondary to reduce noise pick-up.  In addition, the transformer provides natural surge suppression, since the core will saturate at some point and limit the peak voltage and current that can transfer through the transformer.

I would use a main cut-off switch at the primary of the isolation transformer so that it is not left running all the time when it is not in use, to prevent the possibility fire in case the insulation in the transformer breaks down or lightning hits it while it is unattended. For the same reason, wall warts can be hazardous when left plugged in all the time, and I suspect have caused more than one house fire, along with other "on all the time" consumer junk.

Those AC line caps do have a resistor in series with the cap.  I forgot what value.

Fred

May I suggest the CDE  CD16 (500V) or CDV16 (1000V) family of silvered mica caps for snubber and RF applications. These puppies are designed for line bypass application use. The recommend values under 1800pF. They will cost you over $3 apiece new, but well worth it.

In most sets it is very easy to install the line to ground and line to neutral caps just after the fuse in the set.  I would much rather have it take out the typical 1 or 2 amp fuse quickly than wait for the 20 amp circuit breaker to open on a partially failed cap. 

The newer caps may fail differently but some of the older caps used tended to fail at a little higher resistance than dead short and they cooked for a bit before a breaker would open.  I like to minimize the smoke inside.

Thanks for posting this. I was not quite aware of the 'safety' class of capacitors and have used discs.