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Old 24th February 2004, 02:48 PM   #1
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Default 300Meg resistor?

I was looking to clone a Nakamichi mic preamp in order to use some extra electret elements that I have (the CP-3 is a great measurement head BTW). It uses a 2SK118 FET, 2.2k source resistor, and what looks like a 300Meg 1/16th Watt (carbon?) resistor. Any leads on finding these?
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Old 24th February 2004, 02:49 PM   #2
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300 MOhm = practically no resistor ( infinite resistance ). I guess it is either kaputt or you are reading the colour bands wrong. Did you measure its value ?
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Old 24th February 2004, 02:53 PM   #3
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hahaha, 300mohm. I think you could achieve that maybe by putting two leads in a glass tube full of air!

Sorry about that. However, I doubt a 300mohm resistor, you could change the resistance probably by blowing on it. And really put it out by barely touching the leads

lol This is a bit of sarcasm, however, I think Jean-Paul is probably right
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Old 24th February 2004, 02:56 PM   #4
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300 MOhm = practically no resistance
would not no resistance be more like 0 Ohm? and 300MOhm more towards infinite resistance? So better practically no transconductance?

A friend of mine found a 0,5 GOhm resistor in an old measurement instrument from university lab.... its cylindrical about 50*10mm
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Old 24th February 2004, 03:08 PM   #5
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Default Big resistors...

Hi Scott

In my Philips components and materials the max. values for metal glazed resistors are 68 M Ohms...

In my junk box i have some of 33 M Ohms...

Not very far from your needs...
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Old 24th February 2004, 03:08 PM   #6
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Hello Till, of course you're right. I edited my post quickly with the words I actually meant.

Tube_dude, I have a box full of brandnew 66 MOhm resistors. Want some ?
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Old 24th February 2004, 03:09 PM   #7
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Lol, yes, there's a common misconception; that little resistance, for some reason, is a very high value.

I'll just settle with almost no conductance, or transconductance if you prefer. Yes, very very high resistance.

But either way, this great thing in the brain is so nice to automatically fix grammar before it is noticed consciousely.
It can be troubling sometimes when one can read over and over a statement that really makes sense, but not grammatically. Darnit
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Old 24th February 2004, 03:12 PM   #8
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Nope, it's the bias resistor directly on the electret capsule and it wants to be as big as possible, it overranges all my handhelds (time for the Keithley). I've used 10,000Meg resistors to measure the bias current on 20 fAmp opamps. Dick Burwen used 1000Meg in his mics and I think John Curl advised B&K to put even bigger ones in theirs. Those little Panasonic capsules actually let the gate float until the drain-souce leakage just balances the gate-source forward bias (just barely in enhancement), clever trick but lousy for dynamic range. A great interview question is to take one of these appart and ask someone how it works, since it appears to be only a FET and nothing else (I stumped Bob Pease with this one).
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Old 24th February 2004, 03:14 PM   #9
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Scott:

In the US, Caddock <http://www.caddock.com/> makes a high-voltage "MG" resistor that goes all the way up to 10,000Mohms. OTOH, I think that the physical package could be fairly large. Don't know for sure, as I haven't used the high-value MG's myself. In a smaller physical package, Caddock's TF series goes up to 125Mohm, and their MK632 and MK620 go up to 100Mohm.

As an aside, when I was at one of my favorite component retailers today, I happened to notice some 500Mohm and 1Gohm(!) resistors in stock, in what I think were 1/4W packages. I asked the proprieter about them, and he said that every now and then someone would buy a few.

I didn't look at the lower values, but I'd be surprised if there weren't some 300Mohm resistors in stock, too. I'm in Tokyo, but if you can't find the resistors locally, I'll pick up some the next time I visit that retailer and post them to you.

hth, jonathan carr
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Old 24th February 2004, 03:17 PM   #10
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Yes it's really Orange/Black/Purple/Nothing. 300Meg +-20%. I've found lots of high value chip resistors but no tiny (cheap) leaded ones. BTW the little pc board seems to be coated with a special dope too.
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