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Old 7th February 2013, 01:33 PM   #21
rmb is offline rmb  United States
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Btown, I prefer the 7586 nuvistor for use in a microphone. The 7586 has the same heater voltage as the 5703 but the input impedance is much higher and the self noise is much lower. I used the 7586 to replace the hard to find (and very expensive) AC701. You want to run the heater voltage on the 7586 at 5.6 volts since this lowers the "shot noise". All the resistors should be metal film, and the impedance coupling capacitor should be high quality mylar. The 180 meg ohm grid resistor is fine. I recall the plate resistor is 100 k (assuming a 150 volt dc supply.) I will attempt to post a schematic. The 6111 subminature may also work, but I haven't tried one of those.

If you are fortunate to own a Neumann U 47, my tube of choice to keep that running (if the VF14 is actually bad or the heater is open) is the 407A. The VF14 is extremely rare and expensive, and my understanding is that Neumann and Gotham Audio had the pick of the last run of the Telefunken plant on those.
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Old 7th February 2013, 05:19 PM   #22
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How would you replace the tube with a 6111? Because I have one, if i remember the heater voltage is alright, but it would need alot of changes to resistors.
-Chris
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Old 11th February 2013, 11:18 PM   #23
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I would like to be able to output line level and have been working on some mods but have been thinking about nuvistors to keep noise down. Basically they are just tubes and should sound as good as tubes, i just want to keep the creeping digital solid state out of the mix.
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Old 12th February 2013, 02:28 AM   #24
rmb is offline rmb  United States
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btown2009: Getting line level off a typical condenser mic capsule will require two or maybe three gain stages. Inside the body of a small mic (i.e. AKG C-60 or Telefunken M221 style), this may be difficult to do even with nuvistors, allow room for the components, and keep undesireable interstage coupling down. The C-60 has a rather high output (compared to a dynamic mic) so you don't need a lot of outside gain. I normally pad a C-60 26 db before the preamp. You may be able to build a line level output in a larger tube mic where there is more room for tubes and the components. A tubed line level output stage will dissipate more heat than a simple follower circuit for a mic capsule since more undistorted voltage swing is required at +4 dbm than at -50 dbm. Check out the schematic for the Ampex 60 db plug in preamp used in the 351 and later recorders and mixers of theirs. The circuit is not suitable to interface a condenser mic capsule directly but uses two nuvistors for gain.

The nuvistor is a good low level tube. The 7586 in particular has a rugged internal structure to minimize microphonics and a special cathode structure for industrial service. A 6DS4 is useable but does not have the enhanced characteristics of the 7586. Neumann used the 7586 in the U-64 mic while its counterpart KM54 used the AC701. Run the regulated DC heater voltage on the 7586 around 5.6 volts to keep shot noise down.
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Old 12th February 2013, 03:32 AM   #25
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Well I am experimenting in larger mics and was thinking maybe a high quality nuvistor, coupled directly to a high gain tube such as a 12##7 or something similar to that.
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Old 12th February 2013, 04:33 AM   #26
rmb is offline rmb  United States
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If you have space inside the mic for a 7586 nuvistor mounted so the pins face the capsule, and the 12A*7 type can be optimally placed, this may work. The metal can of the 7586 should be well grounded through a wire clip or other arrangement. For curiosity, what type of mic are you building this preamp for?
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Old 12th February 2013, 04:37 AM   #27
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I have moved onto building my own microphones and have been doing alot of experiments and needed a good "push" in the right direction. I haven't experimented alot with nuvistors, so it is a newer field for me.

I thought about something like a 12ax7, but I have seen alot of 12bh7's in audio circuits for mic pres, and am still scratching my head at was best to use.
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Old 12th February 2013, 05:24 AM   #28
rmb is offline rmb  United States
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OK on your building your own mics...You may be better off using a 12AT7 or a 12AX7/7025 following the nuvistor input stage. The 12BH7 is a good audio tube but it may be noisy at low level. Are you taking the capsule polarizing voltage off the cathode of the nuvistor input tube as done with the C-60, or is the capsule DC from an external source? If you direct couple the plate of the nuvistor into the grid of the 12A*7 series tube, the plate resistor for the nuvistor must be carefully determined to apply proper B plus to the nuvistor, the sum of the plate and cathode resistors must be sufficiently high to prevent excessive plate current. The cathode and plate resistors for the next stage must bias the stage following the nuvistor correctly. With direct coupling, the plate voltage of the first stage is also the grid voltage of the following stage, and the B supply voltage must be somewhat higher to compensate.

It shouldn't hurt to run the regulated DC heater voltage low (5.6 v) on all the mic preamp tubes. Is the output transformer in the mic or are you sending unbalanced audio output to a transformer in the power supply as done with the AKG C-60?

I built a 7586 preamp into a dead Syncron Au7A but the capsule needs to be replaced...any leads on decent dual membrane capsules?
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Old 12th February 2013, 05:37 AM   #29
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I've considered powering the capsule with an external source, however that would require more wires than what i have to work with in the cable.

Actually the 12at7 sounds like it may do the job very nicely, i've never thought about that, but i think 7025 would be even less noisy and could do the job better perhaps?

The capsules i am using are from thiersch i think is the brand, also neumann's are easy to find aswell and you should be able to find a decent replacement from them. Thiersch elektroakustik

If i perhaps asked you to help me do some calculations (and i must admit here i am not the best) could you help me?

I do highly appreciate your help with this project as you have helped me move further on this project than what i could have possibly done on my own.

Also what is wrong with the capsule you have? I have a friend who may be able to repair it, this guy is really good.

-Chris
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Old 18th February 2013, 05:18 AM   #30
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How would i carefully select the resistors? or how would i go about doing this also?
Thank you,
-Chris
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