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I am considering changing out some capacitors in my preamp. There are four larger caps, two rated at 2.2uF - 400V and two at 6.8uF - 250V. These four are all bypassed by polystyrene caps at 0.033 uF - 1000V.
For the larger four caps, I am considering using the same values of Mundorf Supreme Silver/Gold, which are all rated at 1200VDC, so I should be ok there. However, for the bypass caps I would like to consider using Sonicap Platinum caps but those are only rated at 400VDC. My other consideration would be to use the new V-Cap CuTF (Copper Foil Teflon Film) series that are rated at 600V. Since the larger caps are rated at either 400 or 250 VDC, does it make sense that the bypass cap has to be rated at 1000V, or am I safe simply matching or exceeding the voltage rating of the larger cap that is being bypassed?
If I actually need 1000V or greater, I could use the Mundorf Supreme Silver/Oil that are rated at 1200VDC, as the bypass caps.
Follow Ups:
I agree with what you have already been told. The B+ supply in any real world commercial tube preamp is going to be way below 630V, let alone 1000V. The rule of thumb is to use a coupling cap that can tolerate the entire B+ voltage with some headroom. That's a safety consideration. Then, as Al suggests, the designer may have found that 1000V caps of the brand and type he selected sounded better than lower voltage ones.
polystyrene caps do not handle voltage overload very well, at least in my experience.
Stu
Since the highest voltage rating of the main signal caps being bypassed is 400V, I guess I will feel safe if I can get to at least 600V with the bypass caps. I appreciate the help.
If you are replacing electrolytic caps, you didn't say in your post, I would try the Mundorf without a bypass. There was a school of thought, therefore a reason for bypassing, back in the day, but with modern caps it hardly seems worthwhile.
Edits: 04/18/10
These are signal caps, Vishay-Roederstein Metallized Polypropylene Film Capacitors 2 ea. of MKP 1841 @2.2-400V and 2 ea.of MKP 1840 @6.8-250V, with all four bypassed by Electrocube 0.033-1000V polystyrene caps.
If I were to follow the manufacturer's path, the four larger caps would all be the same brand & similar model, and the four bypass caps would all be exactly the same. However, of the better caps available, only the Mundorf line carries the two larger sizes in the same model. The teflons and many of the other better reviewed caps such as AN, Jensen, Dueland, Sonicap, etc. do not come in the larger 6.8 size or, in some cases, the larger sizes are for speakers and not electronics so those larger sizes are not available with the necessary voltage rating.
For bypass caps, I can either use Mundorf and stay over the 1000V rating, or use something like the new V-Cap CuTF if I am comfortable at 600V (which, based on the voltage ratings of the larger caps should be fine, according to Al's post below). I understand that, compared to teflon, the oil caps are supposed to sound warmer and richer, which I like, but I am somewhat concerned with using oil caps based on reports of leakage and resulting equipment damage, especially with the Jensen caps. I have also read the AN caps have had some problems.
I have been conversing with one individual who modified this specific preamp using Dueland VSF caps bypassed with Eurocaps, but he had to have the larger 6.8-250V cap specially made by Dueland. I am exploring potentially easier and less expensive paths using off-the-shelf caps that I might purchase on sale from somewhere like Partsconnexion. However, using the Dueland VSF (CAST is just too expensive) bypassed by the new V-Cap CuTF could be interesting and worth a try.
In the past, bypass caps were deemed necessary in order to obtain the desired upper frequency response. The smaller the cap, the greater the upper frequencies passed. New material science can produce larger caps with more than adequate upper frequencies, so the bypass caps may not be necessary: a lot depends on your taste and how the components you have is voiced.
A general rule of thumb, if your power supplies are well built and or regulated, don't be afraid of using larger value of coupling caps in order to get adequate bass. The size, while making a theoretical difference, is not as important as the voicing and I find that a larger coupling cap has never hurt performance (if it does, it will create motor boating where the bass transients will cause the unit to sound like a motor putting along). Its been a while, but IIRC the Mundorfs didn't need a bypass. Don't forget you can also parallel the caps to get a larger microfarad rating.
I prefer a single coupling cap. All too often adding a bypass makes the unit sound like a badly designed two way speaker where the highs do not have the same quality of sound as the bass. That is why it is important when bypassing caps to have the caps of similar sound quality
Stu
A quick search with Google shows no ratings higher than 630 volts for polystyrene caps. Since the polystyrene film has not been manufactured for years, it is not likely these could be reproduced.
The reason for selecting the high voltage rating is to reduce the peak electric field across the dielectric film. The original equipment designer may have determined by testing that doing this improved the overall sonic performance.
You are safe as long as the bypass caps are not rated LOWER than the main caps.
polystyrene production has resumed in China. This was several years ago. Polystyrene production was discontinued in the US and Europe because dioxin is a byproduct. I guess environmental laws in China are much more lax and they have picked up the manufacture of styrene.
Stu
"You are safe as long as the bypass caps are not rated LOWER than the main caps."
Thank you Al for confirming. I know enough to get into trouble, so I appreciate the help.
BTW, the caps are Electrocube, which apparently are still available from 50 to 1600 VDC. They are used in several currently manufactured preamps from this designer.
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