Bottlehead Forum
Bottlehead Kits => S.E.X. Kit => Topic started by: arsun on December 26, 2009, 08:29:33 AM
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As the title says, I am thinking of upgrading both the film and electrolytic and film capacitors in the SEX amp. Having MQs and C4S boards on order, the film ones should be 0.1uF and 2uF each with at least 600V rating. I was thinking of Mundorf Silver/Gold in Oil capacitors, but they have a rating of 2.2uF instead of 2uF. Does this matter? Another candidate is 0.1uF TFTF Vcap and 2uF OIMP. I would love to go both with teflons but aura teflons are only ones with 2uF@600V rating and when I checked their prices, I gave up on that idea :) What do you think about those combinations? Can you recommend one over the other? I also would appreciate any other recommendations, preferably around the same cost, maybe a bit more, maybe a bit less...
For the electrolytic ones, I am thinking Elna Silmic II 1000uF 50V capacitors to replace 1000uF 35V ones. I read just a while ago that Grainger recommended 70uF@630V Obbligato Polypropylene Film in Oil caps instead of 47uF electrolytic ones, so I might as well get them, any other suggestions?
I appreciate your valuable comments. Thank you...
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BTW, will 22uF 630V Obbligato caps will be a good replacement for 22uF 450V electrolytics, or is this a bad choice?
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I was just goofing around and came across Mundorf TubeCaps. It seems like these can be used instead of Obbligato caps too. They have the values of 47uf @ 600V , 20uF and 30uF. There is also one 100uF @ 550V. 47uF seems to be a match, will there be a benefit to get 100uF? Also, is it better to get 20uF or 30uF to replace 22uF?
Thanks.
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For the parafeed cap (2.0uF) the useful range is 1uF to 4uF - it does not need to be precision, except the two caps should be within say 10% of each other. Different values will likely sound a bit different in the deep bass from each other, but it also depends on the particular speaker so I wouldn't try to predict which value would sound best in your room, your system, and with your speakers.
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Thank you for the reply, but I think there has been a misunderstanding. I am not trying mix and match different capacitors for the parafeed cap. I will either use Mundorf 2.2uF or Vcap OIMP 2uF. In case I will choose Mundorf, I will order Mundorf 0.1uF capacitors. If I use OIMP from VCAP, I will order VCAP TFTF 0.1uF ones. I hope that clarifies things a bit.
Also, do you recommend bypassing any of these capacitors with a lower valued one? I am still a bit vague about this bypassing issue. The total capacitance will the main one + the bypass cap right? So I am guessing the small-valued one might only affect by the order of magnitude it brings to the table. Am I thinking wrong here? I am sorry if this sounds funny or stupid, but I am really a newbie when it comes to these things.
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Here are my thoughts about bypassing capacitors.
Many capacitors, especially large value capacitors, have some kind of poor performance at high frequencies. This is especially true of traditional electrolytic capacitors which can become resistive or even inductive as low as 10kHz. In addition to not handling the high treble correctly, this can seriously degrade a circuit's immunity to RF noise, for example. A high-quality capacitor with a low-loss dielectric that maintains performance to much higher frequencies can "take over" the capacitor function by maintaining a more capacitive, lower impedance at these higher frequencies - effectively shorting out the badly behaving cap.
The technically-identifiable issue is that the combination now includes an inductor (the large cap which becomes inductive at high frequencies) plus the bypass capacitor, and they can create a resonance in the impedance function which, in severe cases could easily do more harm than good for the sound. I believe this is the reason some people do not like the sound of bypasses, while others do like it. A simple solution is to put a small resistor in series with the large capacitor so that the resonance is well damped, though almost nobody actually does that.
My own inclination is to use capacitors which have acceptable high frequency performance, and avoid using bypasses at all. But then, I design stuff and don't have to deal with getting the most out of a capacitor I already have - I can go get the one I want. For electrolytics, there are now many on the market that are rated for use in switching power supplies and remain capacitive well above 100kHz. Mylar has some problems at higher frequencies but polypropylene is nearly as cheap and maintains performance much higher.
There are several more subtle ways that capacitors can degrade sonics; microphonics and dielectric absorption are two well-known examples. Bypassing to improve those problems is not so well understood and capacitors are rarely specified for those issues, making it nearly impossible to analyze the situation. Experimenting is the only practical answer, and it requires substantial reserves of patience (and often money!). Experimenting with bypass capacitors is less likely to make a big difference here, compared with swapping the whole capacitor. But it's also less expensive, so the two may be equally cost effective.
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My experience with my Paramours says that the Parafeed cap, the 2.2uF you are looking at, is the most important cap sonically. PJ mentions that the value is not critical. The 0.1uF is the interstage coupling cap. I have swapped this cap in two pieces but I'm not sure it has the bang for the buck that improving the last stage power supply cap has.
In power supplies I put Polypropylene caps in my Paramours. I have use PIO caps in my Foreplay and Seduction power supplies. In your SEX amp that translates as the 47uF@450V caps. That is the value that I used for my Paramours, IIRC.
Just my suggestions.
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Thank you Paul and Grainger for your valuable comments. Greatly appreciated.
As far as I could get from the discussion, bypassing requires great luck to hit the best combination values at the first try, and I am not lucky when it comes to these things :). Also, it seems like the parafeed capacitor is the most important one to make a difference/upgrade in the sound, so that needs to be chosen carefully. I think I will pass this bypassing thing :). The best candidate seems to be the Mundorfs then. I would love to be able to stretch to teflons for this value, but they are really expensive...
Very small question about the resistors: I saw someone recommended to replace grid stoppers with 120 Ohm ones, can I ask the reason of such a decrease in value? I will keep most of the stock resistors as they are, maybe just upgrade the ones in the headphone section and get 120 Ohm tantalum ones. While at it, I might also order additional resistors, maybe one in place of 499 Ohm one in C4S board (found 500 Ohm ones, probably will get that) and the grid stoppers as recommended somewhere else around the forum. What do you think about this? I don't think resistors would matter that much, but I will place an order anyway, so adding a few more items, if they will indeed have any effect, won't matter for me.
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Grid stopper resistors are not especially critical about value. The grid does not draw current normally except at very high frequency where the grid impedance is dominated by the electrode capacitances. So as long as they are smaller than the driver's plate resistance they will not affect the high frequency response. For the output tube they can limit the grid current on overload, which may soften the overload distortion character - but you should not be overloading the tube anyhow.
In the original Paramour we used 499 ohm metal film resistors which worked fine. When the Seduction came out I switched to 220 ohms because we used them as plate stoppers as well on the 6DJ8, which has a very low plate impedance, and I wanted the stopper to be insignificant with respect to that resistance. People have commonly used anything from 50 ohms to 10K ohms.
The gold standard is carbon composition for grid stoppers because they aer bulk carbon and have virtually no inductance. But in modest values (say, less than 1000 ohms) I am inclined to take the advice of VoltSecond. He has extensive professional experience which I respect, and says the metal film works equally well - the inductance is so small as to be inconsequential. I would expect carbon film to also work well.
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Thank you very much for your valuable input. Is there any recommended type for the resistors in C4S boards (499 Ohm ones) ? Should I go with film, tantalum or carbon ones? I am probably getting tantalum for headphone 120R resistors. But I am wondering about the one in C4S boards... I promise, this will conclude my resistor questions :)
I will update here after completing the amp too. It won't be before the end of Jan or mid of Feb though. I still need to raise some funds...
Thanks again. I wish everyone here a happy new year...
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For the current set resistors in the C4S, go for the best you can afford - i.e. tantalum bulk foil if you can. 5% is enough precision, it's all about low noise and linearity in that application.
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Paul,
In the non-SEX applications, which resistor is the current set resistor? I'm guessing the one attached to the base of the two transistors.
Of course I could be all wet and it could be the one in the emitter circuit.
You know I don't yet understand Current Sources or Shunt/Series Regulators.
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For the current set resistors in the C4S, go for the best you can afford - i.e. tantalum bulk foil if you can. 5% is enough precision, it's all about low noise and linearity in that application.
Thank you for your reply. What is the watt rating for the 499R resistors on C4S boards? I cannot seem to find such an information. Also, is the value of 499-500 Ohm a must, or can you provide a range for it? I located some shinkoh tantalums at 500 Ohm rating, but there are two versions of it, one is rated at 1/2W, the other one is 1W. Audio Note tantalums are 475Ohm at 1W and 560Ohm at 1/2W. Then I also found some Caddock USF340 (0.33 watt, 1%, - radial lead) - Ultra Precision Tetrinox Film Resistors. All those three options are from the same web site and prices are close, so I can get any of them, I am more inclined to Shinkoh resistors according to the information you provided. However, just to be sure, among all those options, which one would you pick and with what watt :) rating?
Thanks again... I am decided more or less on every part for my upgrade path. I will let you know of the results. I really appreciate all the valuable input you provided...
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The current is set by R1, from the emitter of Q1 to the B+ supply - hope that answers Grainger's question! Current is approximately 0.855v divided by the resistance, so about 1.7mA. This matches the stock SEX amp. If you are using 6DN7s you can raise that current by using a smaller R1 resistor and it might even improve the sound. I would not go below 237ohms (about 3.8mA - this is the value used for Foreplay and the Paramount driver).
You can calculate the resistor dissipation, it's 0.855 volts times the current, or 0.0015 watts. Obviously pretty much any resistor will have plenty of power margin!
"Which would I pick?" - sorry, you know me, I don't do much experimenting with components. Here's what John "Buddha" Camille said in the original C4S manual: "The value of R1 sets the limit current of the circuit and is critical to proper circuit operation. For best operation this resistor must be a low noise metal film or bulk foil precision resistor. ... "
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Paul,
Yes, thanks. That is exactly what I wondered about. And the calculations will go some distance toward helping me understand these.
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Paul, thank you very much for your informative reply. I guess I am all set. Numerical values also helped with it. Is there any resistors, which might benefit from an upgrade? Maybe 1.27K Ohm ones? Are the value and the type of those important? They are referred to as metal film in the parts list in the manual, but carbon composite in the instructions. That made me curious...
I am only upgrading 120 Ohm ones in the headphone section and the grid stoppers along with current set resistor in C4S board as of now.
Thanks.
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I wish you everyone here a happy new year and thank you for your help so far.
Apart from the question regarding the 1.27K Ohm resistors (if it is worth to upgrade and/or is there a range for that value), I have another question about the 22uF 450V electrolytic capacitor: Can I replace it with a higher capacitance value like 30uF while ordering (like the recommended 70uF value from Obbligato instead of 47uF 450V)?
Thanks again.
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Arsum,
Happy New Year!
The resistor you asked about is on the cathode of the driver stage. I have upgraded a couple of plate resistors but never a cathode resistor. I haven't upgraded a plate resistor for quite a few years. There are more places to affect the sound that give greater bang for the buck than the plate or cathode resistors. Stick with Doc's value on the cathode resistor.
After a quick look at the schematic I think you know all the components that give you improvements in the sound.
Increasing the 22uF to 30uF shouldn't cause any trouble. The amp will turn on a tiny bit slower as larger capacitors are charging, but you may not even notice that.
If you want to spend some cash that will make a big step in sound quality upgrade to the MQ Iron.
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Thank you Grainger for your reply. I decided on the following then:
Resistor upgrades: 120 Ohm ones in the headphone jack and current set 499 Ohm ones in C4S boards, also 220 Ohm grid stoppers.
Electrolytics: 1000uF, 22uF and 47uF ones. 1000uF will be replaced with Elna Silmic IIs having 50V rating. 22uF will be replaced with 30uF Mundorf M-Tube Caps, 47uF will be replaced with again 47uF M-Tube caps. (The reason I switched from Obbligato caps is their huge size- I saw a picture of 22uF cap and it is really big, I think I will have difficulties with installing that considering I want to replace both 22uF and 47uF ones- and I want to order only from one place).
Interstage coupling caps: Vcap teflon 0.1uF
Parafeed caps: 2uF OIMP
I already have MQ irons.
That covers it, but I am open to further suggestions. I want to cover all the important sections in the amp. I am really looking forward to this. I think I have been bitten by the DIY bug :).
The part cost is really huge, but I am buying each month something, so this is a long time project :)
For the caps that don't have wire leads (M-Tube caps), which wire/cable do you recommend to make the connection to the capacitor lugs? Will the teflon cable work that I am using in the amp?
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Sounds like some fun upgrading there! It would be great if you would do 1 upgrade at a time (i personally have a hard time doing this as once i get under the hood i wanna do everything at once!) and let everyone know which ones were the most worthwhile to you.
As far as wire to connect the mcaps i would be tempted to use something stiff to help you hold the caps in place better (so they don't flop around as much)
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Thank you for the comment. I thought also that it would be nice to hear one change at a time, but then I decided against it because of the burn-in times of the caps especially.
If the teflon wire is ok, then I will probably use it, cause I have some spare now. I am thinking of using some blu-tack to hold the caps in place.
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you can build a simple rig to burn in caps. That way you can burn in a bunch at once and then install them at your leisure. I forget where the link is but it is someplace in the community section. Came across it looking at Foreplay II stuff linked there.
I normally use velcro to hold large caps to the sides of the case until I decide if I like them or not then I if I am going to keep them in I'll make a simple bracket out of an old cotton T-Shirt. Kind of like a little cap sock and screw it to the inner wall of the case. Blue tack is great for vibration dampening but if you have a decent size cap it will only keep it in place for so long (in my experience). But I also shoehorn some huge PIOs into my gear. My Foreplay has about 2lbs of caps in it.
As a slight aside this post inspired me to get some resistors to test out in my gear. I let you know the results I get. Got a combination of Takman metal films and Shinkoh Tantalums for the signal paths and C4S in my paramours.
Be well.
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Found the link for that cap burn in rig, click on "jsn's foreplay page" on the community page.
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. . . I thought also that it would be nice to hear one change at a time, but then I decided against it because of the burn-in times of the caps especially. . . .
You can burn in a pair of caps for a week then start on the next pair. That way you listen to each improvement for a week while preparing the next tweak.
I have used this successfully many times. My fast burn in method doesn't do a whole string of caps at a time but it puts a higher voltage across them while burning in. In addition to the higher voltage amplifier output, the more caps you parallel the lower the current becomes through each one. That is why I do a pair at a time.
Click Here For My Post On A Fast Break In Method (http://www.bottlehead.com/smf/index.php/topic,80.msg286.html#msg286)
I'm not saying JSN's method is bad. I haven't tried it, I don't know.
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I know it's a bit late, but thank you very much for the information about burning in. I appreciate it...
Some recent news, due to budget constraints, I still did not order anything, but will do so in 2 weeks. I made up my mind about pretty much everything, however the size of TubeCaps are making me consider about them. I can shave a bit size and some money, if I switch from TubeCap to the MKP line of capacitors again from Mundorf. I can also pickup the correct value of 22uF with MKP. The problem is that they are rated 400V, not 450V as the manual recommends. How tight is the recommendation for these electrolytic capacitors? Also, does anyone here have any experience with that MKP line?
Before I forget, I got the top metal plate and the top part of power transformer powder coated by some local company to a very nice green. If you are in Chicago area, I recommend uvmetalarts for that. No affiliation, just a very satisfied customer. Their prices are also very fair. I will post some pictures in the gallery section as soon as I finish it. I also have a different headphone jack (which makes me think how to install the 120R resistors, but still :)) and teflon tube sockets. I polished the top sections of MQ irons, they are nice and shiny. It is looking really good. I am looking forward to finishing this project...
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I recommend that you NOT use 400v rated capacitors in the SEX amp; stick with at least 450v rating for the electrolytic capacitors.
There are two ratings, the working voltage and the surge voltage. The surge rating is the maximum short term voltage, valid for only a few seconds. This should be compared to the peak voltage in the power supply, which occurs shortly after the amp is turned on, before the tubes start to conduct. In the SEX amp, with the design value of 120v from the line power, that is 470v. Allowing for up to 10% extra (132v line) which is within some specifications for power lines, the peak can be 515 volts. Surge voltage ratings for 400v capacitors are 450v; for 450v caps they are 500v.
In normal use the power supply produces a nominal 380v, which becomes 420v at +10% line voltage. However, capacitor lifetime depends on temperature, with the best lifetime occurring when operated at 50% to 80% of the rated voltage. So ideally I'd like to use 500v or 550v capacitors - unfortunately those ratings are very rare.
Polypropylene capacitors have an expected lifetime that is dependent on temperature and voltage. At 85C and rated voltage, they are expected to last only 5000 hours, but at 90% of rated voltage they are expected to last a 80,000 hours. (This comes from some CDE data sheets; there's a wealth of information available there for those who are interested.)
I can't find the data right now, but mylar is more sensitive and I would not operate a mylar capacitor at more than 70% of its rated voltage.
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Thank you for the information you provided. Seems like Tubecaps are the best option then, they are rated 600V anyway... Here is another question then, do you see any benefit of upgrading 220uF capacitors? The tubecaps only go to 200uF at 550V and MKP goes to 220uF at 250V. If there is a benefit of upgrading these, then I might add them to the order too, in that case, which option will be better specs wise? They are the same price with this value of cap... I really appreciate your valuable comments. Thanks again.