choosing a coupling capacitor

[wwmhf]

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

[Paul Birkeland]

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.

[Paul Joppa]

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.

[Grainger49]

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.

[wwmhf]

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.

[wwmhf]

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   

[Grainger49]

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.

[Paul Joppa]

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.

[corndog71]

What about that .1uF cap?  Any benefit to swapping that one out?  Does that one have to be 600v?

[Paul Birkeland]

What about that .1uF cap?  Any benefit to swapping that one out?  Does that one have to be 600v?

Yes and yes.

[Grainger49]

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.

[wwmhf]

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

[Paul Birkeland]

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.

[Maxwell_E]

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

[corndog71]

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.