choosing a coupling capacitor

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Offline Maxwell_E

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Reply #15 on: July 14, 2010, 07:42:36 PM
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.

Max Tomlinson
SEX amp, Tode guitar amp


Offline JC

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Reply #16 on: July 14, 2010, 08:58:02 PM
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"...

Jim C.


Offline corndog71

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Reply #17 on: July 15, 2010, 03:59:17 AM
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.

The world was made for those not cursed with self-awareness.

Rob


Offline Maxwell_E

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Reply #18 on: July 15, 2010, 04:04:38 AM
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.

Max Tomlinson
SEX amp, Tode guitar amp


Offline Maxwell_E

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Reply #19 on: July 29, 2010, 05:26:53 PM
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.

Max Tomlinson
SEX amp, Tode guitar amp


Offline Grainger49

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Reply #20 on: July 30, 2010, 12:57:34 AM
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. 



Offline Maxwell_E

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Reply #21 on: July 30, 2010, 04:05:24 AM
There was already a jacket on it. Kind of thoughtful of them.

Max Tomlinson
SEX amp, Tode guitar amp


Offline corndog71

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Reply #22 on: July 30, 2010, 05:59:01 AM
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.

The world was made for those not cursed with self-awareness.

Rob


Offline Maxwell_E

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Reply #23 on: July 30, 2010, 08:13:22 AM
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.

Max Tomlinson
SEX amp, Tode guitar amp


Offline Japhy

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Reply #24 on: August 12, 2010, 05:12:09 PM
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!?
« Last Edit: August 12, 2010, 05:14:28 PM by Japhy »



Offline Dr. Toobz

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Reply #25 on: October 20, 2010, 04:35:54 PM
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?



Offline Paul Joppa

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Reply #26 on: October 20, 2010, 07:47:55 PM
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.

Paul Joppa