"Just change the tube"
NOT!
Even a beautiful tube like this Cryogenically treated EH6072 won't correct the circuit weirdos in an APEX 460 and other Chinese mics.
"Just change the tube" all the do-it-your-selfers say.
"I bought a NOS 6072 from like... 1867 and dude it sounds amazing!"
I'm sorry, that is just not so. We have done the tests. Using original Chinese 12AX7 tubes, MESA Boogie 12AX7 tubes and a Russian made Electro Harmonix 6072/12AY7.
Our lab opens the mic and places it in a Faraday Cage to prevent electrical interference. We inject the test signals directly into the mic. preamp so we can measure the electrical characteristics of the preamp without acoustic interaction.
In the graph below you see the electrical response of the mic preamp of a stock APEX:
*controls were not adjusted between tube swaps
By "just changing the tube" you get a little better response using the MESA tube. But the 12AX7 is no better that the original in terms of noise, gain and distortion.
But SURPRISE!
You get a totally ruined response by putting in the preferred choice 6072 tube. We have tried this same test with NOS RCA 6072s and see the same result. Ever wonder why your APEX 460 sounds overly bright?
For the techies out there this is the result of the much lower output impedance of the 6072 (1/2 the impedance of a 12AX7) driving the transformer and changing the resonance "Q" of the output transformer and the RF "protection" capacitors that APEX puts on the output lines. (BTW I think that those capacitors are an error. Their value is much to high for RF decoupling)
But when you fix the circuit up you get something that is more appropriate for a large diagphragm condenser microphone. Did you know that the Neumann u47 internal preamp has a flat response out to 40,000 Herz (cycle per second)?
We Make it Smoooooth
Look below at the smooth output of the BF circuit. It is flat to 20,000, then .5 dB down at 30,000 and only 1 dB down at 40,000 Hz. This is using the stock transformer!
Now this is more like what you want inside your studio microphone.
Don't be fooled. You can't just change the tube and expect to get the results you want. To get the inside scoope how to improve your Chinese tube microphone without spending too much, see our paper called:
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