Upgrade Capacitors

[rockpassion]

Ok, For 2016 I am looking to do some minor mods to my BeePre.  I am looking to upgrade the 10uf 250 volt capacitors (the capacitors near the on/off switch, what are these capacitors called).  I think I am going to use the Mundorf  MCap Supreme Silver Oil  or the Mundorf MCap Supreme Silver Gold Oil.  I believe these are rather big so I will need to figure out how to mount them.  My question is do I need to stay at 10uf or should I go lower or higher, both are rated at 1000 V.  I am outputting to Quicksilver V4's if that helps. 

These are pretty costly and I have read great reviews but wonder what kind of performance enhancement I might expect.  Is this overkill or should I look at getting some Russian PIO capacitors.  Any input would be appreciated.

Thanks

[howardnair]

not having a beepree--so not knowing what these caps do--my 2 cents is speculative-- the mundorf silver oil or  I myself would consider the Jupiter 300v round case  -I use the Jupiter caps a lot--don't rule out obbligato caps either--russian pio's are always very good but 10uf might be excessively large--and of course the capacitor thing is selective and always good for a discussion-
Howie

[mcandmar]

That is a though one, personally i'm in love with the Jupiter flat stacked HT capacitors, i much prefer them over the Mundorfs.  Unfortunately capacitors of those sizes get very expensive very quickly.

The only Russian capacitors i know of in those size/voltage are the box cans, either the silver MBGCH, or the grey K73P-4.  Personally i think the K73 are the better sounding.  Good news is they are cheap as chips, and certainly worth experimenting with to see if changing the capacitors does anything for you before spending big bucks.

http://www.ebay.com/itm/K73P-4-Paper-PIO-capacitor-10uF-250V-tol-1-Lot-of-4-/251312649836?hash=item3a8366b66c:g:x9YAAOxyXTRR95qW

[Paul Birkeland]

Spending the money on a 1000V cap is not necessarily the best plan, as the price of the capacitor and its size will go up to meet the specifications for the high voltage rating.  When the size gets to be that large, the leads coming out of the cap become unusually large, and you really must find a way to secure the cap against the chassis so that those leads don't snap off under the weight of the cap.

(I personally just put the 10uF Russian caps in mine)

[rockpassion]

Are the caps I am talking about coupling caps if not what do they do in the circuit.

Thanks

[Paul Birkeland]

Are the caps I am talking about coupling caps

Yes, the 10uF caps are coupling caps.

[rockpassion]

Paul, thanks. 

Another question.  I have been reviewing Johnsonad modifications to the output capacitors.  He used a teflon bypass capacitor.  My thought is to do the same type of mod only use Russian KBG 10uf or KF75-10 10uf capacitors, then bypass with some FT3 teflon capacitors.   My first question is how to determine the value of the bypass capacitors.  The FT3 seem to only be available in .1, .47, and .22 uf values.  Also what would be the minimal voltage value. 

thanks again. 

[rockpassion]

Paul,

What type of Russian caps did you install and which caps did you modify.  Have you had time to evaluate their performance.

While I really like my BeePre's I have found the feedback here at BH forums is so good that I could not let this go.  It seems in this case that an inexpensive mod can make a fairly nice change so it makes sense to me to do it.  Also, as a hobby I lie to tinker where and when I can. 

Thanks

[Doc B.]

To be clear, there is no real science to determining these film bypass cap values. The rule of thumb is typically use one around 0.1 to 0.1uF. My experience has not been that the positive outweighs the negative with this method. Other folks really like it.

[rockpassion]

On bypass capacitors how important is the Voltage rating.  I can get some k40-9y 0.1uf at 1000v.  Is it ok if I am bypassing a 400v - 650v capacitor? 

I am always confused about voltage on capacitors.  Is it ok to go higher and what affect does it have if any?

Thanks

[Paul Birkeland]

Higher voltage is fine.  A 0.1uF at 1000V cap may be physically larger than a 10uF at 250V cap.

I agree with Doc B. that bypassing coupling caps almost always does too many things that I don't like.

[rockpassion]

thanks guys, I respect your opinions and you have always been on the money in the past so, I will forgo the bypass and just  experiment with replacing the output and coupling capacitors. 

I have ordered a couple of different types of Russian capacitors and will try an A/B comparison.  I do not (at this time) plan on buying more expensive capacitors.  I will post my results.

Thanks for all the help.

[Adrian]

On bypass capacitors how important is the Voltage rating.  I can get some k40-9y 0.1uf at 1000v.  Is it ok if I am bypassing a 400v - 650v capacitor? 

I am always confused about voltage on capacitors.  Is it ok to go higher and what affect does it have if any?

Thanks

The only thing you need to consider when deciding on the voltage of a cap is the voltage of the circuit where it is placed.
For example, the output cap supplied for the S.E.X. is rated at 630V.  This cap is in part of the circuit that could see a maximum potential of 375V.

The voltage of the cap must always be NO LESS THAN the maximum voltage it will encounter at any time.  A good rule of thumb is to go twice the circuit voltage, but this is probably "belt and suspenders" conservatism.
Any higher voltage than necessary to protect the cap from excessive potential is a waste.  Higher voltage does not mean higher performance, it just means that the cap will be physically larger to account for the additional diaelectric required for that higher voltage.

Lesson learned:  I recently replaced a cap in another amp I was building and did not even think about the voltage rating.  I placed a 16V rated cap in a 25V circuit.  It didn't last 30 seconds before it overheated, popped, and released its magic white smoke.  (Like the Direct TV commercial,"don't be that guy!").

edited to correct the output capacitor rating to 630V

[mcandmar]

It is important to factor in the power supply voltages when the circuit is not under any load. For example on power up there is delay before for tubes heat up and start to conduct. Where there might be a nominal voltage of 375v in normal use, it could be floating over 400v until the tubes have warmed up.  In which case a 400v rated cap would be on the ragged edge of safety during startup, hence the use of higher rated 600v caps.

[Paul Joppa]

Voltage ratings are a bit of an over-simplification. Here's a 4-page white paper from CDE:

http://www.cde.com/resources/catalogs/filmAPPguide.pdf

On page 3 at the bottom right there is a plot of lifetime vs. temperature and voltage, which gives a more nuanced picture.