mshilarious said:
I'll be publishing the schematic very soon. It might be useful for mic modders who want to gut something like an MXL63 to make their own tube mic
And here it is! I'm 95% certain this is the final design, or very close to it. For those into tube mics, you'll see that some of the design is (I think
) rather clever, and some of it is counterintuitive. For example I left the grid biased above the cathode, because it works better that way
The 6418 tube is rated at 15V, so it isn't starved plate, but the filament is a little hungry--I could only manage 7mA, it is supposed to get 10mA. However, if I made some changes to get the current up a bit, the rest of the circuit is adversely affected. It is a very interdependent design, as the plate voltage sets filament current via the transistor.
Please be aware that the overriding goal of my designs is to use as few parts as possible, so I can fit them in an XLR connector. So if you look at something and think, "hey if he added three parts here . . .", then that is either because a) they wouldn't fit, or b) I'm an idiot
Obviously this is a very different design than I suppose AT used on the 3060, no transformers or DC-DC converters. Transformers are expensive, and DC-DC converters come with their own particular set of problems from a commercial production point of view, so I avoided them. As a result, the circuit wastes power, but hey, it's still milliamps, so I don't think the electric bill will be too bad
The funny thing here is that the tube actually serves no electrical purpose in the circuit--it provides less current than the WM61A capsule--so from that point of view, it is completely superfluous, and Q1 is very necessary to be able to drive a preamp. However, it is in there doing its tube thing, increasing second order harmonic distortion by 10dB over MSH-1A, and actually reducing harmonic distortion above that. So far I think it sounds smoooove, but I need to assemble a prototype in its case, because I'm getting tired of singing into a breadboard
As far as the applicability of the circuit to another application, any electret capsule should be relatively easy to drop in, and if it doesn't have an internal FET like WM61A, then the tube would really serve as a first-stage buffer
I'm sorry but I don't have experience with true condensers, so I don't know if there is enough voltage. It seems to me you'd want to take a separate tap off pins 2 & 3, which hang out at about 19V, and convert up if necessary.
Oh yes, Q1 doesn't have a part, actually I am using 2N5457 right now 'cause I've got 'em laying around, but I plan on trying a BJT instead, your favorite 100mA NPN should do the job.
