Making sure the 1 G resistors are really 1G

[david-p]

I am making a U47 clone with variable polar patterns, using the EQU47 body.  This involves several large value resistors in the polarizing circuitry, which I have mounted on a separate piece of perf board above the EF14 tube with point to point wiring.

I am wondering whether I need to take precautions to avoid leakage around the high impedance end of the circuit.  Specifically,

-- Should I use glass tubes to feed the capsule connections down through the capsule baseplate?

-- Should I worry about the resistance of the perf board itself?

-- A lot of the Chinese mics use PTFE standoffs on the PCB at the amplifier input.  Are these really necessary?

Any thoughts or experiences on this?

Thanks in advance,

David

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[pyjaman]

Some could say it's not necessary to be so cautious, but such high impedance  circuits are REALLY very sensitive. And I've personaly experienced several problems with badly insulated/not clean enough hi Z parts in a tube mic: noise, "shy"and "bass-less" sound, cracks and pops...
From my very small experience, you could get good results without beeing THIS cautious, but I'd say be creative and do the best you can. Of course you don't need to provide ultra high insulation to both ends of the hi res components, just the hi z connections. For the wires through the baseplate, maybe your capsule wires are already teflon insulated. some of them are (Peluso, Tim Campbell...). if not, an idea I had once was to roll some plumbing teflon tape around the wire and make it hold in place with a shorter piece of heatshrink tubing. Resistance of the perfboard could be a problem - more or less important depending on its material. That's why teflon standoff are used. I didn't have some, so I made a little board with a small piece off teflon and kingston turrets.

Just 2 cents I have laying around I can put in it...

Laurent.

 

[0dbfs]

Yes it's important.

Stick a meter into your fingers (not all the way) on each hand/thumb and measure the resistance through your body... Now consider how much higher 1G is than what you read through your body. I read about 2-4M... Sometimes as low as 400k!

Teflon standoff's are pretty cheap and you usually only really need three of them for a mic depending on the ckt.

Cheers,
jb