Tube Mic schematics and modifying a APEX460?

It was suggested that I purchase a APEX460 to have an audition. So I searched out the APEX site to get some info and found they are cool enough to publish schematics.

I can follow the sound through this one:



But, I having trouble with this one. It leaves me with questions like... what's the audio transformer doing in that circuit? And, why are people pulling the existing *12AX7* tube and placing a 12AT7 in it? (when the service manual suggests it was supposed to arrive from the factory with a *12AT7*?):





Anyone have any thoughts about it?

thanks,
mike

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Just a slight bit of speculation: one thing I've learned in life is not to trust electrical schematics 100%. Part of the problem is the engineering department themselves. The guys doing the schematics are working from BOM's and various other things. Once a product reaches the manufacturing stage, any number of things can affect what actually goes out the door. Parts shortages are the most likely culprit. When a supplier runs out of a particular part, substitutions are needed. Until the correct part comes in.
 
That T-X swap in the tube - I had that done recently to a cheapish Marshall combo amp. The tech suggested it as a solution to a noisy hi-gain channel. It did reduce the noise. But it didn't sound as warm as before. So it was a trade-off. What's worse, a higher noise floor or a slight lack of warmth? I'm assuming perhaps that's partly what we might be talking about here with the tube swap but I don't follow all this closely enough to really care much.
 
Interesting stuff though. I had a Studio Projects C1 modded last year and was impressed with the results. The brittleness of the original was gone and the mic had a smoother overall freq response. And I've always liked Apex mics. They were one of the very first crop of Chinese mics I remember using. Have fun. Did you try to contact Mike Joly? He might be able to shed some light on this.

what's the audio transformer doing in that circuit?

Which diagram are you referring to? The AKG or the Apex? In both cases, the transformer's primary function is impedance matching, and possibly phase inversion to compensate for the inverting amplifier. The AKG's is fed off the plate, the Apex's is a cathode follower, presumably to match a low-impedance primary on the output transformer.

The 12AX7, IIRC, has a significantly higher gain than a 12AT7. (I cleaned out my office last year and threw out, among many other things, my old dog-eared RCA tube manual -- why would I ever need this again, I asked myself.) Using the higher-gain tube would introduce a fair amount of harmonic distortion, but that's exactly why people put up with the hassles of tube microphones - for the distortion. (Ah, excuse me, "color".)

AFA the Apex schematic the xformer is in a cathode follower configuration pin 8 of the tube schematic .However there are some errors I see in the schematic regarding connection points between the power supply schematic (lower schematic) and the mic schematic, the upper one.
for example, pins 2 and 3 on the power supply box show the audio output (to an XLR) however pin 2 on the mic schematic is the FIlament!! IT looks like connector CZ3 has the pins numbers backwards Ie: 1 should be 7 and vice-versa.. the tube pin 1 is plate , usually high voltage WHICH shows as pin 7 on the power supply....

You're right, a cathode follower has unity gain and is used for impedance matching. The gain comes from the other half of the twin triode.

I love the fact that so many SONAR users are closet engineers.

Thanks folks!!!

best regards,
mike