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Old 11th March 2007, 08:31 AM   #1
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Lightbulb Microphone With A Low Impedance Tube

Hi
I am building a high output tube mic and would like suggestions about tubes to use. I am leaning towards pentodes mostly but triodes I'l consider also.

I'm looking for tubes that have as low grid leak as possible, are low impedance (hopefully 2k or less in pentode but a little more might be ok), low capacitance, decent amplification (hopefully about 25 or more) and sound great.

I am really looking forward to everyones suggestions. So far I have used 12a-7's, 6au6 and ef86. I know they are fine tubes but what will be even better for low grid leak, low ra and sound.

Thank you! Have a great day,

Jor
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Old 11th March 2007, 09:43 AM   #2
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Hi,

What microphone element will you be using for your tube mic?

Of those you listed, EF86 would be the best choice.

Cheers!
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Old 11th March 2007, 12:39 PM   #3
EC8010 is offline EC8010  United Kingdom
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6BS7.
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Old 11th March 2007, 02:36 PM   #4
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D3A!! Very high transconductance, and when triode connected internal noise generator impedance equivalent to about 60 ohms. (johnson noise equivalent) You probably won't find a quieter tube.. Mu = 77 so plently of gain as well.. RP well b below 2K
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Old 11th March 2007, 03:26 PM   #5
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Quote:
Originally posted by kevinkr
D3A!!
Or the E810F/7788 triode-connected. However, the trouble with these high-gm hot rods is that grid current will be a problem if the grid is directly connected to a high-Z capacitor mike element. I think this is why EC8010 suggested an electrometer tube with grid current in the picoamp range. "YoungFred" hasn't told us enough about his application to know for sure what he needs.
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Old 11th March 2007, 03:47 PM   #6
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Yes, I'm assuming from the "low grid leak" requirement that this is to be the head amplifier for a condenser capsule. A valve that has a little of the E180F's properties and some of the 6BS7 is the EC1000.
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Old 11th March 2007, 07:08 PM   #7
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Default 6688

You haven't said what type of capsule or transducer element you're gona use. There are a bunch of tubes you could use but why re-invent the wheel. The 6688/E180F is a pentode you could use. A 6au6wa/7543 is not sloppy either. The 6DL4/8255 is a triode. The 6J4/8532 is a good triode.
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Old 11th March 2007, 09:43 PM   #8
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Default Thank you for all the advice so far!

Hi

I'm going to use an m7 capsule. I've also used vf14's and ef14's in u47's that I have restored or built. Those are large tubes. I was looking for something that is even better because lots of times people use the best tubes they know of and might not know about great tubes that have been made. Does the 6ej7 have low grid leak? I would guess not.

The EC1000 sounds like it might fit the bill. What is the plate resistance?

When I look for at tube specs how does one know if it has low grid leak? I would like this mic to be fairly high gain and use a tube that has low plate resistance, they sound more open to me. All input welcome! Thank you so much! Have a great day! Talk with you soon,

Jor
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Old 11th March 2007, 10:06 PM   #9
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EC1000 are fantstic tubes, but not many were made, so thy rare and very expensive if you get a good one. i have had an eye out on e bay for a long time for one.
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Old 11th March 2007, 10:22 PM   #10
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ra is likely to be around 2k in use.
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