Tube condenser microphone help! - Page 7 - 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 18th March 2013, 05:11 AM   #61
rmb is offline rmb  United States
diyAudio Member
 
Join Date: Nov 2012
Location: western PA
Chris....the 407A is a dual triode. Your schematic shows this tube as a pentode, which is incorrect. I connected both sections of the 407A in parallel to get the impedance down for the output transformer. The heater is run at 36 volts like the U47. (Btw, a little nomenclature, you should remove references to Georg Neumann in the title block for schematics for modified equipment of theirs.)

I note that Frank Capps used the 5703 tube in their CM2250 condenser mic. I have one here. Don't know if Capps is still in business. They made cutting stylii besides this mike. The main reason I used the 7586 nuvistor to replace the AC701k is that Neumann used the 7586 nuvistor in the U64 mics sold in the USA so that tube had Neumann's "blessing"..The 5703 may also work but you may have a "noise penalty"...run the heater at 5.6 volts and hopefully the incidental noise will be adequately low...
  Reply With Quote
Old 19th March 2013, 03:45 AM   #62
rmb is offline rmb  United States
diyAudio Member
 
Join Date: Nov 2012
Location: western PA
Ewlewl: Is the AC701k definitely bad in your C60 mic? I finished working on a Schoeps M221A mic using a AC701, and the problem turned out to be the cable on the mic end connector. Fixed it and the mic sounds great with low noise floor with the AC701k.

The C60 develops bias for the capsule off the AC701k cathode current. The plate of the AC701k goes directly to HT so any noise on the HT leg will show up in the mic. Same deal with the two cathode resistors. There should be a1500 ohm and a 82000 ohm resistor in series in the cathode leg. Grid bias (approx. -1 volt with respect to cathode) is developed across the 1500 ohm resistor, and the drop across the 82000 ohm resistor generates the capsule bias (approximately 60 volts). The plate current through the AC701k is on the order of 0.7 mA. Since the grid derives bias from the cathode resistor string, it may be worthwhile to look at the curves for the 5703 at -1 volt grid bias, or wire the 5703 in place of the AC701k and measure the cathode voltage using a vtvm or other high impedance voltmeter. The plate current can be calculated from this measurement. The absolute value of grid bias can be measured across the 1500 ohm resistors, and the capsule voltage can be measured across the 82000 ohm resistor. Check that this voltage does not exceed 65 volts or so with the 5703 because you don't want capsule collapse. I recall the load line for the 5703 is somewhat different from the AC701k. Also compare the noise floor between the AC701k and the 5703.

I've had good luck using the 7586 nuvistor in place of the AC701k...

Just my $0.02 worth....
  Reply With Quote
Old 20th March 2013, 12:53 PM   #63
rmb is offline rmb  United States
diyAudio Member
 
Join Date: Nov 2012
Location: western PA
Ewlewl: When you installed the 5703 in place of the AC701 in the C60, did you take a VTVM or other high impedance DC voltmeter and measure the cathode voltage or the voltage at the junction of the 1500 and 82000 ohm resistors? If this is higher than 65 volts or so, you may run the risk of damaging the capsule.

The load line for the 5703 says this tube will draw 1 ma plate current with 40 volts on the plate (measured plate to cathode) and -1 volt of grid bias. If this is the case with your mic, Ohms Law (V = IR) says the voltage drop across the 82k resistor with 1 ma of plate current will develop 82 volts which is excessive for the capsule. This makes sense when the HT supply is on the order of 120 volts applied directly to the plate. (82 v + 40 v = 122 v)...I suggest measuring voltages while the mic is open to minimize risk of capsule damage.

The plate current of a typical AC701k follower circuit is on the order of 0.5 to 0.7 ma. which will generate 41 to 57 volts across the 82k resistor, which is OK for the capsule.

Just my $0.02 worth...
  Reply With Quote
Old 21st March 2013, 12:00 PM   #64
diyAudio Member
 
Join Date: Mar 2013
Default bias

thanks for the info i'll check this out some more
  Reply With Quote
Old 19th April 2013, 07:13 PM   #65
rmb is offline rmb  United States
diyAudio Member
 
Join Date: Nov 2012
Location: western PA
Chris...if you have a high input impedance preamp connected to a headphone amplifier or speaker, connect the resistor across the input and listen (or measure) for added noise (not 60 Hz hum). Some resistors will be quiet (especially metal-film) and others may be noisy (i.e. carbon)...

The old Syncron capsule has damaged membrane.
  Reply With Quote
Old 19th April 2013, 10:48 PM   #66
diyAudio Member
 
Join Date: Mar 2003
Location: Haarlem, the Netherlands
Quote:
Originally Posted by rmb View Post
Chris...if you have a high input impedance preamp connected to a headphone amplifier or speaker, connect the resistor across the input and listen (or measure) for added noise (not 60 Hz hum). Some resistors will be quiet (especially metal-film) and others may be noisy (i.e. carbon)...

The old Syncron capsule has damaged membrane.
Shouldn't you DC bias the resistor under test somehow? Without bias or signal, all resistors produce only thermal noise (4kTR). (If you should find a resistor that produces more noise than just thermal noise without bias while being in thermal equilibrium, connect it to a normal resistor and you have a perpetuum mobile.)
  Reply With Quote
Old 20th April 2013, 12:08 AM   #67
rmb is offline rmb  United States
diyAudio Member
 
Join Date: Nov 2012
Location: western PA
MarcelvdG: You can DC bias the resistor to make it carry current; you can make it the "grid leak" resistor, etc. Depending on the preamp, a DC biased resistor may need to be capacitively coupled into the preamp, thus involving another component in the noise path. You're right about the 4kTR noise...Boltzmann's constant is everywhere....
  Reply With Quote
Old 12th November 2013, 03:54 AM   #68
rmb is offline rmb  United States
diyAudio Member
 
Join Date: Nov 2012
Location: western PA
ewlewl: I spoke to John Peluso on the phone last week. He does capsule reskinning in his shop. I'm planning to send him my two Capps CM2250 mics and see how they turn out.
  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 04:01 PM.


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