Tube condenser microphone help! - Page 5 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 23rd February 2013, 07:35 PM   #41
rmb is offline rmb  United States
diyAudio Member
 
Join Date: Nov 2012
Location: western PA
Good morning Chris...

Your circuit may work, but you should consider increasing the value of the 50 meg resistors. If you consider the equivalent grid circuit with respect to the high impedance of the capsule (typically 40-50 pF) and audio frequencies, you have the 50 meg grid resistor in parallel with 100 meg polarizing circuit which results in an equivalent 33 M impedance (with respect to audio) across the capsule. The 2 meg resistor and 0.1 uf capacitor decouples the polarizing supply so this is not considered. The leakage resistance of a 7586 tube (approx. 1000 megohms) is negligible. You should increase the grid and charging resistors to make the equivalent audio circuit resistance (with respect to the capsule) on the order of 150 megohms.

Last but not least, the 30 k and 100 k series divider for the capsule polarizing voltage puts 88 volts across the capsule as shown. The "target" voltage for the mic capsule should be 60 volts, so change the 30 k resistor to 100 k and this will put 58 volts on the capsule, close enough. The 100 k plate resistor and 5 k cathode resistor should work; the 7586 plate current should be on the order of 0.5 milliampere and the plate voltage (across the tube) should be around 60 volts with those values. The 5 k cathode resistor effectively sets the operating point of the tube so be careful when changing that value. A lower resistance will lower the grid bias and the tube will draw more current and vice versa. The resistors and capacitors should be good quality since any noise generated by noisy resistors or capacitor leakage will be amplified in the circuit as shown.

Measure voltages with a vtvm/fet vm to avoid loading high impedance circuits. A 20,000 ohm per volt meter loads high impedance circuits.
  Reply With Quote
Old 3rd March 2013, 12:00 AM   #42
diyAudio Member
 
Join Date: May 2009
So whats the news on using the 407a tube in the u47, what changes need to be made?
  Reply With Quote
Old 3rd March 2013, 12:08 AM   #43
diyAudio Member
 
Join Date: May 2009
When replacing the nuvistor for the tube in the c60, should anything else be changed out?

I mean when using the 7586 in place of the ac701k in the c60, besides adding the 100k plate resistor, what else needs to be added?

Last edited by btown2009; 3rd March 2013 at 12:15 AM.
  Reply With Quote
Old 3rd March 2013, 12:15 AM   #44
rmb is offline rmb  United States
diyAudio Member
 
Join Date: Nov 2012
Location: western PA
Chris: One other item worth testing: The cable between the power supply and the microphone. If you have a AKG C60 or one of those minature tube mics (i.e. KM54, M221A,B, etc), check the cable carefully, especially at the mic end. The conductors are very thin and I found a break in my M221 cable on two of the conductors. The cable can be tested for continuity and shorted conductors using an ohmmeter. The conductors are probably 22 gauge stranded, and have thin insulation. Probably going to have to call Neumann and see if they still sell bulk multiconductor cable for the KM54...old cable with the extremely thin insulation with 120 volts of HT on it may be deteriorating...I cut back the mic end and reinstalled the Tuchel connector and the mic came back to life but that old cable....

The point I am making is that you can spend a lot of time working on a mic but if the interconnect cable is bad, anything you do with the mic is wasted time. The guy I got the mic from several years ago told me the AC701k was bad and didn't want to spend the money to replace it. Turned out the AC701 was perfect....

Good luck...
  Reply With Quote
Old 3rd March 2013, 03:36 AM   #45
rmb is offline rmb  United States
diyAudio Member
 
Join Date: Nov 2012
Location: western PA
Chris, are you modifying a real C60, or is this a new build of a C60? I recall reading that you built a C60 using a 5703 subminature triode. I just finished working on a M221A Telefunken/Scheops mic. This mic is similar to Neumann's KM54 and both use the AC701k.

The AC701k is run very conservatively and should not go bad...even though a mic I have had for over 30 years had two bad AC701's in it when I got it. My mic was from a studio where it probably was left on whether or not it was being used, and the AC701's were noisy when I tested them on a test fixture. Power supply problems or noisy resistors/leaking coupling cap can create noise. The last thing you want to do is "condemn" a AC701, since you are either into a very expensive replacement or a modification to keep the mic running. I modified the circuitry in my mic for 7586's but the capsule bias was NOT derived from the cathode in my mic..

Since the capsule bias IS derived off the tube cathode in the C60, you have to add/change some resistors to use the 7586 nuvistor. Use 0.5 watt metal film resistors to minimize noise. You need to install a nuvistor socket in the mic and face it so the grid pin can be directly wired to the capsule with a short piece of solid wire. Ground the shell of the nuvistor using a piece of #20 solid copper closewound 3 or 4 turns around the nuvistor shell, the other end connected to a stable ground point. Use good quality (teflon) #22 wire to connect the heater, plate and cathode circuits of the nuvistor into the mic circuitry. Route this wire bundle away from the grid. Here are the rest of the mic modifications.

1. Add a 4.7 k resistor in series with the nuvistor plate and HT.
2. Make a series divider with a 560 ohm and a 27 k ohm resistor. Open end of 560 ohm resistor to 7586 cathode. Open end of 27 k ohm resistor to ground.
3. Connect 180 meg ohm resistor to capsule to junction of 27 k and 560 ohm resistors.
4. Connect output coupling cap to the 7586 cathode, watch polarity, plus side to cathode. (Make sure voltage rating of this cap is at least 75 volts; preferably 150 volts)

In the power supply, you will need to remove the 4 volt zener diode and replace it with a 6 volt zener diode....you will have to reset the heater voltage to 5.6 volts with the supply loaded with a 45 ohm resistor (5 watts) across the heater pins, or with the mic connected. If you use a resistor for the initial setting, the final adjustment should be with the mic connected. You must have the zener diode in place to crowbar the supply in case something happens and the heater voltage increases beyond safe limits. The 7586 isn't necessarily pocket change so the heater voltage must be protected. With the mic connected and heater voltage stabilized, check the voltage at the junction of the 560 ohm and 27k ohm resistors with a vtvm. The voltage should be around 60 volts. If this voltage is excessive, you can collapse a mic capsule. If the voltages are OK, shut off power and reassemble the mic. Connect the mic to a good headphone amplifier and listen for artifacts while increasing the gain. Hopefully, you will hear background sounds with a low electrical noise floor.

Be careful when working on this mic or the power supply; the voltages in the equipment are hazardous and the HT filter capacitors will hold a charge for a while when the power supply is turned off.
  Reply With Quote
Old 3rd March 2013, 04:24 AM   #46
diyAudio Member
 
Join Date: May 2009
Everything is new build.
What about putting the 407A in the u47 mic, what changes are needed there?

And I do not have .5 watt resistors, just 1/4 watt, Which have to work, should i double up on them?
  Reply With Quote
Old 3rd March 2013, 10:29 AM   #47
rmb is offline rmb  United States
diyAudio Member
 
Join Date: Nov 2012
Location: western PA
Chris: 0.25 watt resistors will work fine. If you build the self bias circuit with the 4.7k in series with HT, you may need a 9.1 k in parallel with 10 k on that one. Try a 4.7 k first (tack it in temporarily) and measure the voltage drop across it with a vtvm. If you are seeing more than 25 volt drop across the 4.7 k resistor, you need the parallel combination. (I am leaving some power headroom since these components are working inside a closed space.)

Be careful because the 4.7 k resistor is at HT potential off the supply.
  Reply With Quote
Old 3rd March 2013, 10:48 AM   #48
rmb is offline rmb  United States
diyAudio Member
 
Join Date: Nov 2012
Location: western PA
Chris: Do you have a U 47 with the VF14 in it? Or is this a new build? If you have a U47, the conversion is built as a plug in, so you have to make a template of the VF14 pins, transfer that to a suitable piece of 1/8 or 3/16 in thick plexiglass, and use 0.7 mm brass screws to simulate the VF14 pins. Countersink screw heads into the plexiglass base to avoid shorts. Micrometer measure the pin diameter to be sure 0.7 mm is a good number for the plug in build.

New build can be done on plexiglass base and use a 9 pin ceramic socket that can take the small metal shield. The socket must be mounted with the pins toward the capsule, so the capsule connection can go to the grid pins via a short piece of solid wire. (The VF14 is a metal shell tube so the 407A should be shielded also.) The heater of the 407A is connected in series for 40 volt operation. (36 volts from the supply will work.) Countersink screw heads on the plexiglass base to avoid shorts. I will pull my notes and look up the rest of the values and connections.
  Reply With Quote
Old 3rd March 2013, 02:59 PM   #49
diyAudio Member
 
Join Date: May 2009
Thanks for the tips, Now here comes the problem, the circuit for the c-60 becomes somewhat complicated, enough so that i need to use a breadboard, but i am concerned about the noise a board will inject, are my concerns valid?

And in this case, the VF 14 replacement is also a new build. Is it an exact replacement drop in, in this case?

Problem is too, I cannot find a good supply of the high value resistors, or the high quality capacitors.

Also, The capsule in the u47 has 3 legs, one is the backplate, one is the output, and one is shield/ground.

Can i connect the backplate to ground and polarize the grid side, and use a coupling capacitor to grid to rid it of the dc voltage?

Last edited by btown2009; 3rd March 2013 at 03:24 PM.
  Reply With Quote
Old 3rd March 2013, 03:32 PM   #50
diyAudio Member
 
Join Date: May 2009
U47 moded

My last question on the subject of 407A is the vf14 looks like it uses 34 volts plate, will the 407 be ok with that?
So its a drop in replacement then?
Attached Images
File Type: jpg moded u47.jpg (115.6 KB, 52 views)

Last edited by btown2009; 3rd March 2013 at 03:46 PM.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
The $5 DIY Tube Condenser Microphone!!! Minion Instruments and Amps 8 1st July 2011 11:07 AM
condenser microphone question jonslaten Analog Line Level 9 13th November 2010 07:24 AM
Condenser microphone repair argonrepublic Instruments and Amps 2 13th January 2010 12:15 AM


New To Site? Need Help?

All times are GMT. The time now is 11:54 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2