Filter Caps on Mains?

[The New Steve H]

Fun new question here.

I dug out my old HP 3300A function generator to get it working, and I found there was a dual capacitor (0.01 uF per side) across the AC in wires. The internal caps had a common ground, and their other legs went to neutral and hot.

The reason I learned about the cap is that it had fried. The generator blew fuses when I turned it on, and I discovered that the reason was that one side of the cap had shorted. I had to get two new caps to replace it.

Here is the question: is there any point in putting similar caps across the mains on guitar amps? The purpose is to kill high frequency interference.

Like a guitar amp, the HP has big reservoir caps after the transformer, but they still used filter caps farther upstream.

I doubt anyone would hear a difference, but what the heck.

I suppose the down side is that if a cap shorts, the amp won't run until you fix it.

[The New Steve H]

I'm wondering why anyone would bother filtering AC in, when there are big ol' caps to filter the DC down the line. Could it be to prevent the exposed AC lines from "broadcasting" RF inside the cabinet, to other components that might pick it up?

[The New Steve H]

I found an interesting page referring to these things as "Y capacitors." Evidently, the risk of shorting is well known, and ceramics are most likely to do it. There are special "safety caps" for this purpose, and the affordable ones are designated "Y2." They are supposed to resist shorting from power surges and so on.

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

[Colossal]

Look up metal oxide varistor

[Phil_S]

I think you'll find a MOV on Trainwreck amps.

[The New Steve H]

I think we're on different pages here. Y caps are used to kill EMI, not spikes.

[vibratoking]

IME, X and Y caps are used for EMI purposes concerning compatibility to standards, such as UL, CE, and others. The purpose isn't to stop EMI from entering the device. The purpose is to stop it from exiting the device and interfering with other devices in the vicinity. They will provide filtering to the input of the device, but I very rarely see them used for any other reason than standards compatibility.

[The New Steve H]

It seems like you never get the same answer from two people when you talk about electronics.

I found a page about old radios, and it said Y caps were used to keep interference OUT. I suppose a person's view of a component's purpose depends on what he's used to using it for.

[Stevem]

These days you can get IEC panel mount sockets with all the EMI / noise suppression stuff built right in!

[rp]

double post

[rp]

You want to avoid possibility of fires so if you use you should install these, you want the ones that fail open. I think these are the right X/Y types but please check or have others confirm:

http://www.mouser.com/ds/2/427/440lseries-239711.pdf

[R.G.]

Technical notes:
- yes, the primary purpose of across-the-line and line-to-ground caps on the AC mains is to keep the thing from broadcasting hash back out. These caps can sometimes quiet transient hash from the line in that will fly through the PT's coil-to-coil capacitance and not be sucked down by the filter caps because of the filter caps' ESL. This is less of a problem with today's improved electrolytics.
- The X- and Y-rated safety caps are metalized foil with the metalization done so that punctures between the two plates actually eat away a circle of the metalization right at the puncture and so they stop punctures by making a larger insulating area around the puncture. Neat stuff. They fail safe. Mostly.
- yes, MOVs are for a different purpose, although a MOV can protect the X and Y caps by eating high energy transients - even though the X and Y caps don't need as much protection as disk ceramics to. Putting MOVs on all amps is a smart thing to do. The AC mains are much trashier today than they were in the 50s and 60s.

[The New Steve H]

I got some input from a guy who claims he was around back in the glory days of analog:

You are on the right track. The HP product line was heavily used when I was in the HF/VHF radio design labs in the 70’s pre digital era.

The only digital item in the plant used to crank formulas and run chart data points was a big honking HP10 electronic calculator the size of a breadbox. The TI 50 hand held slide ruler had just appeared around then but I could not afford it.

By definition there is stray RF from god knows how many pieces of equipment floating about, so the power cord between your device and the wall is basically a really good antenna. The caps at the power cord entry to the box were to ground any stray RF from outside going in and conversely, anything inside from going out. Usually there is a large capacity, maybe .33 Mf or larger if space allows, and also a .0001 ceramic disk across it, from each wire to ground. The .33 should be NP (non polarized) but I have seen folks using electrolytics or tantalum both of which are polarized (the latter known to catch fire when they fail). If there is a coil in the can, it is inline and has a ferrite core, that would buck RF as well, I think it is only caps. Make your own filter, there is no reall critical issue.