220 Mega Ohm Resistor - Really

[DrummerChick]

I am looking for a 220 Mega Ohm resistor so I can build a replica of a Neumann UM 57. The schematic says 220 Mega Ohms with 20 % but I would prefer 1%. I can't find anyone who will sell me less than 1,000!!! I'd also prefer not to put several in series if I don't have too...
J

[PH104]

careful who you buy it from. Fingerprints can ruin it.

[kaos]

Mouser sells 200 meg and 250 meg, Digi-key used to as well (probably still do). Is it so critical that it needs to bee 220 meg @ 1%??

http://www.mouser.com/catalog/637/656.pdf

[HollowState]

High meg resistors like this are made by Victoreen and some others. They often come encapsulated in glass but not always because 200 meg is not all that high of a value as far as high megs go. Keithley instruments uses some in their electrometers and picoamp sources. And there're often much higher up in the gigaohm range.

If you have a way of measuring high megohms, you might be able to make one with India ink painted on a short insulated rod. You'll need to experiment with length. The more ink, the lower the value. But India ink is pretty high resistance wise. I did this as a kid for a Geiger counter project in an old magazine artical. This works better for higher resistance around 100K meg. BTW, for your microphone project a tolorance of 1% is totally unnecessary and a waste of money. It's just not critical and you won't hear any difference. 200 megs will work just as well.

Victor

[Speedskater]

"R&D Electronics" also known as Electronic Surplus was down the street from the Victoreen and Keithley factories in Cleveland. In the distant past we purchased high Meg value resistors from them. They recently moved the store to an Eastern suburb. Don't know what they have in stock, these days.

http://www.electronicsurplus.com/

[HollowState]

Originally posted by Speedskater
"R&D Electronics" also known as Electronic Surplus was down the street from the Victoreen and Keithley factories in Cleveland.

I wonder if that was the place I once visited about 25 years ago while on a trip to the Dayton Hamvention. The owner (?) had dozens of the old Tektronix 500 series plug-ins on a shelf behind the counter. He wouldn't sell any of them because he foolishly thought they would be real valuable some day and was hording them.

Victor

[Michael Koster]

The Ohmite Mini-Mox recommended by kaos are the ones often
used in microphones. Those are the blue ones.

The UM-57 has 2 of these resistors, one to supply the fixed
polarization voltage to the capsule center plate, and the other
as grid leak for the EC92. I'd go with 250M. The value should
not be critical but could influence the LF roll-off slightly.

I'm looking inside a M-582 I have but can't see any high ohm
resistors to see what type they use.

The important thing is to keep the grid circuit away from anything
with high leakage. FR-4 PC boards are too leaky. Polycarbonate
is OK; any plastic that will hold a static charge. I think teflon is
OK also. The grid circuit should also be physically as short as
possible and right under the capsule mount.

Does the UM-57 use an M7 capsule? What capsule are you using?

Sounds like a fun project. They have a lot of DIY mic discussion on
the Prodigy Pro board.

Cheers,

Michael

[Wavebourn]

You don't need 1% really. If I remember right M7 has something like 80 pF capacitance, what impedance that means on 40 Hz frequency?

[Michael Koster]

The K49 is about 80pF/side and I think the M7 must be about the
same.

That's about 50Mohms at 40 Hz, and there are 2 220M resistors in
parallel which results in a 110M/160M voltage divider.

That's a little over -3db at 40 Hz. That sounds about right. The U47
has 100M, the M49 has 2x150M in parallel with about the same
capacitance per capsule side. The C12, which has a similar
polarization scheme to the UM-57, has 2x250M in parallel.

[Speedskater]

Originally posted by HollowState

I wonder if that was the place I once visited about 25 years ago while on a trip to the Dayton Hamvention. The owner (?) had dozens of the old Tektronix 500 series plug-ins on a shelf behind the counter. He wouldn't sell any of them because he foolishly thought they would be real valuable some day and was hording them.
Victor

Probably not, back then there were other surplus shops. This shop had a warehouse for of real nice old test equipment. They have been in business for 50 or 60 years and know the value of all their stock. Cleveland is a 5 hour drive North of Dayton, did you stop in Columbus or Lima?