DC voltage across a cap vs. Signal transfer - diyAudio
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Old 8th October 2012, 10:52 PM   #1
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Default DC voltage across a cap vs. Signal transfer

I was asked why I used cathode bias vs. Fixed bias. Well, started thinking. In cathode biased tube DC voltage across a coupling cap between a driver an a power tube is lower than in fixed bias (grid biased with negative DC voltage related to cathode). I wonder if there are any measurements of caps "behaviour" which give data to compare two topologies?
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Old 8th October 2012, 11:09 PM   #2
Enzo is offline Enzo  United States
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I think you are over-thinking this. You have not yet even put it in a real context. The grids on a power tube will be further negative with fixed bias, yes, but by how much? Your circuit could be anything from a 6AQ5 single ended to a pair of 8417s in push pull. In one example, the difference might be less that 30 volts, in another it might be over 100 volts.

The two topologies have enough performance differences to study on their own, without wondering too hard about the coupling caps at various voltages. Much more to worry about the cathode bypass cap, at least in cathode bias.
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Old 8th October 2012, 11:17 PM   #3
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6c4c needs about -45V Vg-k. A driver (C3g) operates at Vp=200V.
With fixed bias DC across coupling cap is 245V, with cathode bias -> 200V.
However, does it make a difference if the DC voltage across the cap is smaller?

PS. I try to avoid bypass caps.
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Old 8th October 2012, 11:41 PM   #4
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Well, actually, it is about caps, not bias topology. I presented the starting point of my mindmap. That is why I put it "Parts", not Tubes/Valves
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Old 9th October 2012, 10:25 AM   #5
DF96 is offline DF96  England
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If a coupling cap uses a significantly non-linear dielectric or has mechanical stability problems then increasing the DC voltage across it could increase distortion. The solution is to buy a better cap, which could easily be a cheaper cap if the original was a peculiar 'audio' cap. Alternatively, buy a bigger cap so that there is less signal voltage across it (provided this won't upset LF stability).

In any case, this will be a small issue.
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Old 19th October 2012, 01:40 PM   #6
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From J. Broskie
The Tube CAD Journal,Design Idea: Polarized Connections -pg 1
[...]
But, on the other hand, certain audiophiles, whose ears I trust, tell me that in fact this device does make a very noticeable improvement. What could be going on here? I have read where a manufacturer of short-wave radios found that by applying at least a small polarizing voltage on all the capacitors used in the unit brought about a very measurable increase in high frequency response (RF).
"[...]
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Old 19th October 2012, 02:48 PM   #7
DF96 is offline DF96  England
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OK, you choose:
1) John Broskie's unnamed audiophile friends' ears, which claims an improvement,
2) Circuit theory and component properties, which suggests virtually no change.
I know which I would rather believe.

I have never heard of the story about SW radios. Most radios are designed using circuit theory. I suggest that either JB or the person who told him that story has got the wrong end of the stick.

Last edited by DF96; 19th October 2012 at 02:49 PM. Reason: correction
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Old 19th October 2012, 04:09 PM   #8
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I treat the blog entry as a hypothesis not truth. Moreover, belief or blind choice is not the way I do things. I am not going deep into physics, either, as I finished my technical university some time ago. However, I would rather see numbers and will be looking for proof or denial.
DF96, nothing personal.
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Old 19th October 2012, 04:11 PM   #9
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"... when I'm 64"
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Old 27th October 2012, 06:29 AM   #10
jitter is offline jitter  Netherlands
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I guess the only way to find out is to measure the frequency response of an AC signal sweep through the cap repeated by the same measurement a couple of times of an AC signal sweep superimposed upon several different DC levels and compare the measurements.

Anyone here who has the equipment to do this?

Last edited by jitter; 27th October 2012 at 06:32 AM.
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