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Tube condenser microphone help!

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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...
 
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....
 
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...
 
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.
 
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.)
 
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....
 
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