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Old 6th February 2004, 04:26 PM   #21
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Hi, fdegrove -

Actually, in my later projects, I have (had) been becoming very concerned with microphonics, to the extent where I actually fabricated a couple chassis where I shock mounted small signal tubes encased in a solid block of black anodized aluminum with heat sink fins attached!

I once fabricated a transformerless 3 tube microphone preamp with an unusually low noise factor (something like 4db) and 90db common mode rejection - I've got the schematic somewhere. It was very instructive to check the relative microphonics of different tubes at the input stage. With a 'good' 12ax7 & listening to the output through a headphone amp, I could tap the side of the tube with a pencil and hear maybe a small thunk if I whacked it hard enuf. With a 12at7, it sounded like there was a little spring reverb in there. With a 6dj8, it sounded like a glass chandelier landing on a concrete sidewalk. The related factor appeared to be transconductance/filament power.
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Old 6th February 2004, 04:38 PM   #22
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Hi,

Quote:
It was very instructive to check the relative microphonics of different tubes at the input stage. With a 'good' 12ax7 & listening to the output through a headphone amp, I could tap the side of the tube with a pencil and hear maybe a small thunk if I whacked it hard enuf. With a 12at7, it sounded like there was a little spring reverb in there. With a 6dj8, it sounded like a glass chandelier landing on a concrete sidewalk. The related factor appeared to be transconductance/filament power.
I absolutely believe you...
As you may know I designed a MC headamp with 6DJ8s, they can tend towards microphony.
What I always do is mount sensitive tubes horizontally on very good quality sockets with a springloaded chimney.

There are more tricks but we digress...

Cheers,
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Old 6th February 2004, 05:13 PM   #23
marz.tv is offline marz.tv  Sweden
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thoriated: Holy XXX! The LEADS of my caps are magnetic! How extremely high end, iron leads. Ummm... Why did you ask that?
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Old 6th February 2004, 05:18 PM   #24
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Hi,

Quote:
Ummm... Why did you ask that?
Copperweld is magnetic as it contains iron(?) particles....

Cheers,
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Old 6th February 2004, 05:32 PM   #25
marz.tv is offline marz.tv  Sweden
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What is the chemical designation for copperweld? Or the Swedish term if there is one.
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Old 6th February 2004, 06:05 PM   #26
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Hi,

Quote:
What is the chemical designation for copperweld? Or the Swedish term if there is one.
Copperweld is a company, it's name is so widespread throughout the industry as Coca Cola.

Basically it's about clopperclad steelwire.

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Old 6th February 2004, 08:50 PM   #27
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Default Re: Re: Re: Thorsten, what film capacitors do you use?

Konnichiwa,

Quote:
Originally posted by arnoldc

hi kyw, are capacitor microphonics similar to tube microphonics?
Not quiet, the issues are more complex. With enough AC voltage modulation applied a Capacitor will "sing" like a canary, indicating mechanic resonances excited by the signal (similar to electrostatic speakers). It will also act as pure "receiver".

As other mentioned, materials also play their big roles. In case you are stuck for answers, look for plastic encased cap's that contain no magnetic parts. The dielectric and foil/film vs. metalised construction is less important than that the capacitor may be wound tightly under tension and then enclosed and that no magetic parts be used. Also look for the capacitors slew rate, the more the better....

Sayonara
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Old 6th February 2004, 10:59 PM   #28
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Hi, Marztv -

If your leads are magnetic, I personally would keep them as short as possible. My experiences with steel leads have not been positive from a SQ viewpoint.

KYW brings up a good point wrt even caps with the best dielectrics using magnetic components. I see those Russian teflons on Ebay, but I force myself to turn away. However I have a stash of 0.22uF 600VDC teflon film and foils custom made by Reliable Capacitors for that extra special project.

I have nothing against oil filled capacitors - in fact I use a number of AC rated 'oil'-filled polypropylenes as power supply filters. Maybe I'm missing something, but my understanding is that the oil is mostly there to prevent corona discharge, not to significantly affect the dielectic properties.

My phono preamp has some air variable capacitors that are used to adjust input capacitive loading, HF phase compensation and crosstalk compensation. My approach with this design was that if I could add a circuit to compensate for something, I would do it as long as I could do so without adding any active stages for the bulk of the signal. This preamp has three stages from phono input to amp output. I actually considered building a stacked plate air capacitor for the RIAA shaping but figured I had to draw the line somewhere.
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Old 7th February 2004, 01:30 AM   #29
jlsem is offline jlsem  United States
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Quote:
I have a bunch of these, that I've used as coupling caps in many great sounding projects. Appr. 3cm length, with grey epoxy or glass ends.What are they? Some kind of paper in oil, right? How old are they?
Sprague 118P's are metalized polyester film, yours in a hermetically sealed brass can, probably from the 60's or 70's.

John
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Old 7th February 2004, 02:17 AM   #30
marz.tv is offline marz.tv  Sweden
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Thanks all who have replied to my posting.

Frank: Sorry for my misunderstanding, I got the impression it was a term for a kind of metal or alloy, like 'mylar' is polyester... But I guess these leads are just steel, because they appear homogenic.

thoriated: I had no idea the leads were magnetic until I just checked it. It never crossed my mind before. I'm not usually worried about wire quality for short runs inside an amp, but I think I'll cut them tighter in future projects, especially after reading this thread.
I have a large collection of old surplus components, so that's what I usually dig out when I'm about to build something. Some of it is really nice, like big oil caps and resistors with silver leads, but other things are... well, old junk. I have maybe 100 of the 0.33uF spragues, so I'll definitely keep using them though

John: Thanks for making things clear. Where did you find that information? Is there a source on the net somewhere? I also have bigger 1uF/600v kinds of Spragues that are also marked with 118P. I take it that they are also metallized polyester then? I usually put those as bypasses for PSU electrolytics, or as a small bank of uF between the rectifier tube and choke, with good sonic results.
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