An absolute noob's modding question

[Nick Tam]

You can always use an AC rated cap on DC but need to know the AC rating for DC caps used in the first stage of a power supply.  The second stage is already filtered to DC.  In the power supply or at the output cap use the same DC voltage rating as Bottlehead. 
 

I'm not quite sure what you meant there, do you mean the output cap DC ratings have to be matched to the last capacitor used in the power supply?

[Grainger49]

You are right, the output cap needs the same or higher voltage rating; those two points are connected.  Only go higher if you can't find the value you want with a matching voltage rating.  Otherwise you are wasting money and the cap will be bigger. 

When the amplifier turns on the power supply passes the output of the rectifiers right to the plate of the tubes.  This is because the tubes are not conducting.  It is a higher voltage than when the tube is conducting and the power supply is filtering. 

But... in what you quoted I was meaning to say that you can use AC rated caps in power supplies.  Many of us here use motor start/run caps for power supply caps.  They are better sounding than electrolytics but very large.  I mean really large for their uF value.

Also, I meant that Bottlehead will use a capacitor that will not explode on surges.  Paul Joppa likes a conservative design.  There is also a consideration of temperature with all capacitors "under the hood."  PJ likes 80 degree C rated caps.  That is what you will find in Bottlehead kits.

Electrolytic caps are DC rated with the possibility of an AC rating.  After the rectifiers you have very lumpy DC, no AC at all. There is no AC at the first cap of the power supply, just awfully lumpy DC.

Electrolytics can be used as a cathode bypass cap because there is much more DC there than AC.  Electrolytics are intended for DC rather than AC.

[Paul Joppa]

To clarify, the last cap in the power supply carries exactly the same AC signal current as the output capacitor - in a cathode follower circuit. Neither cap has any significant ac between its terminals, but both have plenty of AC current.

I normal amplifier stages, the same thing is true - the AC signal current (not voltage) goes through the power supply. However, in parallel feed circuits the plate choke resists that current and the current loop is closed through the parafeed capacitor instead of the power supply.

[Noskipallwd]

I contacted Mundorf support a couple of years ago wih question about their different product lines, and general use. The tech that replied stated their general rule for bypass cap values are 1% of the larger caps value. 220uf bypassed by a 2.2uf, and so on. The size of the output caps limits the choice of caps you can install, no teflons, no Mundorf S/Os, and so forth.There is a good choice of affordable Metallized Poly. caps. though, from Dayton Audio, Solen, Mundorf, Axon true caps at 91uf, which folks that have used them state that 91uf was large enough. I replaced the last cap in the PS with a 220uf Mundorf Mkp, but that was darned expensive. Almost as much as the kit itself, not worth the expense I think. You can have a lot of fun rolling caps in the Crack! Hope this helps.

Cheers,
Shawn

[Nick Tam]

This M-Cap?

http://www.royalkabel.com/mcap-250-mkp250-vdc-220-uf-p-330.html

I was somewhat confused before when you guys were saying that the output caps are actually power caps, probably because the ones that came with my kit are those "audio grade" FW caps from nichicon so I was never sure.

I'm planning on replacing those with either the M-Cap MKPs or Obbligato Black Film 100uFs because those are the only 2 caps that are readily available in local stores. There's also Solen as well but I'm not going to bother with those. The power range for those are 250VDC and 400VDC so they should be fine.

As for the power cap, I read somewhere that it is generally unsafe to use a non AC rated capacitor in an AC circuit, which I assumed the power circuit was completely AC, but apparently the circuit at the last power cap is already filtered to DC. You guys were also saying that the power cap should not have a higher VDC rating than the output caps? So I guess I could cross off the TubeCap unless I upgrade all of the output caps to the same voltage?

[Laudanum]

Replacing the output caps with film caps, you are safe with the same voltage rating as the electrolytics that came with the kit.  Actually, I believe it was stated that 160 volt cap should be plenty which I think is the stock caps rating.  But, you are likely to find the film caps rated at 250, 400 or 630 volts (dc).   Any one will do fine. Higher VDC rated versions of the same caps are larger.  Some claim the higher voltage rated versions of the same cap sound slightly better.  Personally, I'll take the installation space savings and cost savings of the lower voltage rated cap because my ears wont hear the subtle difference even if there really is one.
All the affordable, large value film caps mentioned ... Solen, Mundorf mcap, Dayton, Axon, Erse are all of the same basic type (MKP).   I went with the Mundorfs myself, but I suspect that any would provide some upgrade in sonics to the stock electrolytics and I suspect they would all be similiar although not the same.  These basic MKP types are generally considered the entry level of "audiophile" film capacitors for whatever that's worth to you.