Bottlehead Forum
Bottlehead Kits => S.E.X. Kit => Topic started by: wwmhf on April 15, 2010, 05:58:52 PM
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Hi,
I am trying to improve my Bottlehead S.E.X amp by replacing the coupling capacitors. Now, the amp has a 2u 630V. I am wondering if I can replace it by a 2u 500v capacitor. Please help.
Thanks
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Just to be sure, you have the iron/C4S upgrade, correct?
According to PJ, the coupling caps should be at least 630v rated, so what you choose will really depend on what your wallet can handle. If I didn't have to foot the bill, I would probably use Jupiters.
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Just be sure to get the HT (high temperature) versions if you go for Jupiters. The original beeswax ones sounded great, but several have failed in SEX amps - shorted out, blown fuses, etc. I have not heard the new ones, but the old ones sounded extremely good as long as they did not get too hot. Doc B has had them in his over-the-top 6C45 headphone amp (which is very well cooled) for years.
Back to the original subject, the parafeed capacitor is subject to very low-frequency transient voltages that can be higher than the power supply voltage. That's why I recommend at least a 600v rating. Usually I admit that I'd prefer to see 800 or even 1000 volts, but the fact is we've never seen a problem with 630v rated parts.
Most all of our experience is with polypropylene dielectrics; mylar for example is significantly different. If you do some research - CDE (Cornell-Dubilier) has a lot of good information on their site - you will see that the relation between voltage and lifetime is a complicated issue with a lot of uncertainty.
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First, remember that new capacitors take a week to sound as they will. The better, more expensive designs take longer.
I have always said changing your output caps is like spicing your food. Each person has a different taste in food and caps.
That said, I have suggested the Obbligato 2.2uF@630V Polypropylene (PP) in Oil caps from DIY HiFi Supply:
http://www.diyhifisupply.com/catalog/35
At $5.50 each plus a $7 shipping fee (that is kind of hard to find but they don't get shipped unless you also click on that key) the Obbligatos are the most reasonably priced PP cap out there. Then there is the sound. They are smooth, rich in the midrange but not too much so. They will make your amp sound a little more clean and tubey. Again not too much so. At their price, you should have a pair to play with regardless of whether you like the sound initially or not. It takes a week to 10 days for most orders to reach me. I have bought often.
But we can only recommend capacitors that you might like. It depends on what you are wishing to enhance or impart into your system.
Enjoy the trip.
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My amp does have the C4S upgrades, but not the irons.
To summarize my understanding, I understand that I should use capacitors rated at least 600V. The cap suggested by Grainger49 is attractive. I will start my playing with this new toy from buying the capacitors soon.
Thanks a lot to you all.
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One more question: How do I choose the value of the capacitance? I vaguely remember reading a post by Paul saying that the value of this capacitor is between 1u to 4.7u. What is the sound character of using a smaller cap and what is the character of using a larger cap?
Thanks
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PJ has advised a number of times that the exact value is within a wide range for different iron. The MQ upgrade probably wants to have higher values. Like my Paramour, when I upgraded to MQ iron I went from 3.3uF to 10uF. By the way, I have an Obbligato there.
The range for stock iron, like you have will most likely include the 2.2uF that I suggest. Obbligato does not make a smaller value now. They used to, but don't now.
The difference you hear with changing values is mostly, but not completely, in bass extension. You might get 2 or 3 notes lower with larger values than with smaller. Not that most of us would really notice. I don't think I would.
The MQ iron upgrade is not cheap, but it was worth it to me.
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For the stock 30-henry plate choke, I advise 0.5uF to 2uF, with 1uF being the nominal recommendation and that's what is supplied with the kit.
For the Magnequest choke, which is 50 henries, the nominal value is 1.7uF, range is 0.85uF to 3.4uF, and the standard stock value is 2uF.
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What about that .1uF cap? Any benefit to swapping that one out? Does that one have to be 600v?
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What about that .1uF cap? Any benefit to swapping that one out? Does that one have to be 600v?
Yes and yes.
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My experience with interstage coupling caps in my Seduction and Paramours was that the interstage caps don't make as big a difference as an output. It seems odd to me. I am just saying not to invest as much in the interstage as the output.
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Those interstage coupling caps are interesting to me too.
A related questions is how its value effects the sound character. I would like to know what to expect if its value is smaller than 0.1uf or larger than 0.1 uf.
Please advice
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Those interstage coupling caps are interesting to me too.
A related questions is how its value effects the sound character. I would like to know what to expect if its value is smaller than 0.1uf or larger than 0.1 uf.
Please advice
The coupling capacitor and the resistor that follow create a high pass filter. Shrinking the coupling cap will raise the frequency of that filter (not usually a good thing). Going way bigger will lower the frequency of the high pass filter (not usually a bad thing). On the Seduction output, for instance, .47uf to 2uf is an acceptable cap size, as the load presented to the output of the Seduction can vary a bit.
Changing the .1uf to anything bigger or smaller is pretty worthless, as the load on that stage is fixed.
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Shameless Bump.
I was just going to start a new topic on this question, then I thought "hmm, might as well have a look around, I can't be the only curious soul."
So here's my synopsis from reading this thread.
1] Upping the output cap to roughly 2uF from 1uF will give smoother mids and a lovely sound.
2] The interstage coupling cap can also benefit from an upgrade, but the output is the first priority when it's shopping time.
I've been looking at Clarity Caps from Madisound. A pair of 2.2uF of caps cost about $11, with $10 shipping (those crooks). At this price I can't really justify NOT getting them, especially after payday :D
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I tried a little experiment tonight.
I had been using ERSE caps as the interstage(0.1uF) and output caps(1uF) in my sex amp. I also recently bypassed the ouput caps with a giant pair of .47uF teflon V-Caps which made a slight improvement to the soundstage depth and width.
Tonight I took the vcaps off the output caps and swapped them in for the interstage caps. I know it's not the stock value but I just figured it's close enough.
Suffice to say it increased the air and space between the performers and seem to give kick drums more life and dynamics. I swear I heard on a few songs the decay of the kick drum!
Now these aren't cheap caps. I'll admit I took a chance with spending that much but feel they are a worthwhile investment and make a noticeable improvement in the sex amp's performance. I originally bought them for another amp but decided to try them on the sex amp for fun. I have to disagree with CB on the interstage cap not being important. Being that it's upstream I would think it's more important despite the small value. And my experiment with the v-caps seem to confirm that.
I do like the ERSE caps in the output stage although I still want to try a Sonicap in that position.
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Ok, now I'm getting confused with all this terminology. Which ones are the interstage caps, the output caps, the coupling caps....? There's like 5 names for two caps, and I get confused with which one is which and what they do.
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As far as caps directly in the signal path on the SEX, I count Cc which is the cap between the driver and output stages, and Cp, the parafeed cap. Either of them could be called a number of things, I suppose: Cc, for instance, could be referred to as "inter-stage" and Cp as "output". But, then, Cp could also be referred to as a "coupling cap" since it couples the output of the tube to the output transformer. And, either one of them could be referred to as DC blocking caps, since that's what they do.
Gets confusing damned quick, IMO.
And, don't get me started on "de-coupling caps"...
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Ok, now I'm getting confused with all this terminology. Which ones are the interstage caps, the output caps, the coupling caps....? There's like 5 names for two caps, and I get confused with which one is which and what they do.
Sorry for the confusion. I fixed my post to hopefully make it more understandable.
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Thank you guys so much. And Corndog, now everything makes sense. I now have the confidence to start messing around without fear of killing my baby.
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I thought I'd drop an update.
I finally got new caps for my SEX after all this talking. I got a good deal on some Clarity Cap ESA range in .47uF and 2uF. I can say after about 50 hours on them so far I am very happy with this purchase. Even with my cheapo Sony bookshelf speakers I immediately noticed an "opening up" in the music, and an ability to hear each instrument in much more detail. But it still hasn't lost its smooth musical nature. This is just with a stock SEX plus the C4S boards. The hardest thing about this whole deal was trying to bend and trim the 20AWG solid core leads to my will and make it all fit nicely.
Thanks for all the information and patience in dealing with newbs to the tube world.
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Spaghetti, it is that insulation that you have stripped off wires. When I strip a long piece it is set aside so I can insulate leads on capacitors. That way I don't worry if they will ever touch anything else. A more expensive option is shrink tubing.
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There was already a jacket on it. Kind of thoughtful of them.
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I thought I'd drop an update.
I finally got new caps for my SEX after all this talking. I got a good deal on some Clarity Cap ESA range in .47uF and 2uF. I can say after about 50 hours on them so far I am very happy with this purchase. Even with my cheapo Sony bookshelf speakers I immediately noticed an "opening up" in the music, and an ability to hear each instrument in much more detail. But it still hasn't lost its smooth musical nature. This is just with a stock SEX plus the C4S boards. The hardest thing about this whole deal was trying to bend and trim the 20AWG solid core leads to my will and make it all fit nicely.
Thanks for all the information and patience in dealing with newbs to the tube world.
Great to hear! I've been meaning to check out those Clarity caps too. I've read some great things about their ESA and MR caps.
I gotta say I LOVE my sex amp now with the v-caps and erse output caps. It really is the best sounding componant I have now and I haven't even upgraded the iron or added c4s boards yet! Even through my cheap $20 philips earbuds it sounds amazing.
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Wow, that is nice. I may be looking at the Iron down the road, but first is speakers and a Seduction. I looked at the MR caps too, but I decided that I may need more than $20 leftover for food for the rest of the month.
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Wow.....up till now I've been running obligatto premiums in both the interstage and final coupling positions with stock iron and thought things were pretty good.....that is until I picked up some of the Ampohm FE-AXL-AL caps for both positions. I've only put the interstage caps in right now as i didn't realize how big both were but the leap in musicality is extraordinary. Listening to some live Leonard Cohen almost out of the box and I'm almost transported back to a year and a half ago when my wife and i saw him live. I would offer these up as an option when you look at the price compared to others as well. Would consider them similar to mundorf silver_in_oil with a little more voluptousness in the midrange with no detail being given up at all. Can hardly wait to see how these things break in. Now to figure out how to shoehorn those 1.0uF's underneath the hood!?
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Is the output cap value solely reliant on the inductance of the OPT and plate choke in a parafeed arrangement, or do other factors come into play as well? I ask because I'm running the S.E.X. in parfaeed, but using cathode feedback and Edcor XSE10 (8k-8 ohm) wired as standard transformers. Plate chokes are 30H, and the transformers themselves between 10 and 20H (per reports online; can't verify this for myself). I'm therefore assuming that the 2.2uF caps I have in there are more than sufficient? If CFB makes the reflected load on the amp a bit more stable and easier to drive, would it stand to reason that the value actually could be smaller than usual?
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In parafeed, the modeling that I've done shows the plate choke inductance to be the most important. The object is to provide the tube with a load that does not fall too far below nominal, so that the tube distortion is not increased. This effect is independent of feedback, which reduces the intrinsic distortion but does not affect the causes of that distortion.
Here's my take on the reasoning. Parafeed with a really huge capacitor means the output transformer inductance in parallel with the plate choke's inductance dominates. That inductance will have a reactance that falls with frequency, and will present a low impedance to the tube at very low frequencies, leading to tube distortion. This is mostly a problem at high signal levels, where an ungapped parafeed output transformer will have large inductance, so the plate choke with its airgap will dominate.
Parafeed with a really small capacitor will present the tube with a capacitive reactance that increases as frequency decreases, so the tube is lightly loaded and has low distortion. The price you pay is reduced frequency extension. (This can actually be useful in the tweeter amp of a biamp setup, where the parafeed capacitor can be part of the crossover network.)
When you have the best balance, the tube sees a relatively constant load impedance, which is also largely resistive (not capacitive or inductive, i.e. a small phase angle) to the lowest possible frequency. Done right, the response and load impedance are better, to a lower frequency, than series feed with the same inductance. That's one of the technical advantages of parafeed.
This analysis is what has led me to recommend a capacitance of twice the plate choke inductance, divided by the load resistance - about 1uF for a 30H plate choke and 8K load. In the modeling, capacitors between half that value and twice that value are pretty good - the model is quite tolerant of modest variations in the parameters.