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In Reply to: RE: Question... posted by Lew on February 16, 2017 at 11:26:14
If you are concerned about steel shell and leads of K72s, use FT or FCh types. One must thing with all Russian teflon caps is to electrically connect outer shell with one of the leads, and insulate the shell with polyester tape.
Follow Ups:
"If you are concerned about steel shell and leads of K72s, use FT or FCh types."
Don't forget the FT series caps have steel shells as well.
"One must thing with all Russian teflon caps is to electrically connect outer shell with one of the leads, and insulate the shell with polyester tape."
Why? If the cap is used as a coupling cap then at one lead you'll typically find high voltage DC with the audio signal riding on top of it. On the other side you'll find the next stage tube grid (most often) with a grid stopper resistor possibly in the way.
Those two connections provide COMPLETELY different electrical scenarios to expose to the shell. So what is the rationale for doing so? It seems to me that connecting the tube grid to the cap shell is asking for trouble from stray capacitance and possibly noise pickup as well.
So I am curious to find out what are you accomplishing making one of those connections to the shell. Can you share that? Thanks.
"Don't forget the FT series caps have steel shells as well."
The FT-2s I have are aluminum.
Some say the steel case of the K72s cause audible problems but I haven't heard that about the aluminum case on the FT series caps.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I should have said "metal" not steel. My bad.
Edits: 02/21/17
For coupling capacitors, the outer foil should go to the grid. This way it is statically grounded through grid leak resistor, and provides electrostatic shield for the whole capacitor. On many old caps there is a marking indicating the outer foil, so that a cap could be connected the right way.
I tried connecting caps both ways, and there wasn't much difference.
Ummm, I don't agree...
The proper way to connect the outside foil is to the low impedance side of the circuit. That is (in the case of coupling caps) usually the plate of the prior stage. The grid is a high to very high impedance point in the circuit - which will cause a noise voltage to be generated.
In many circuits the orientation is a non-issue but if it is go to the lowest impedance.
If you'd like to hear from a real expert follow the link below to Randall Aiken's site page. He explains WHY this is the best choice. It's excellent reading and he knows his stuff BIG TIME!!
nt
Jim - this is about noise pickup by an unconnected piece of metal. These capacitors are big, so the shell, if not grounded one way or another, presents a good antenna for picking up stray electromagnetic fields. Same reason why transformer cores in low level audio transformers are electrically grounded.
One way to ground the shell is to strap it directly to ground. This is not good because this may add quite significant shunt capacitance to ground. The better way is to connect the shell to one of the leads, and use that lead as outer foil connector.
Okay - so you are assuming one lead goes to ground then, yes?
In your previous post, I don't think you made that clear, but it makes sense among the various alternatives.When the Russian Teflon capacitors (of all kinds) were "discovered" by TubeDIY aficionados, there were several posts about totally removing the metal casing, de-soldering the typical low quality leads in favor of better wire, and some even encased the capacitor in PVC tubing or coated them with epoxy. That's what I was trying to do when I killed an 0.47uF FT series (I think it's FT, anyway). The idea of doing this mod seems to have died on this forum, but I am willing to believe it would make an improvement to the sound, which is why I tried it once. In this thread, no one has mentioned the efficacy of doing that.
It occurs to me that if you are going to ground the case to the "outer foil", by which I think you mean the capacitor lead that is attached in the circuit AS IF it is the outer foil, do you also have to identify which lead actually IS connected to the outer foil? Because if you do, I found that to be impossible, possibly because of the metal case getting in the way. Catch-22.
Edits: 02/17/17
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