My first tube mic schematic

[0dbfs]

Triode curves:  http://scottbecker.net/tube/sheets/137/5/5840.pdf

I'd eyeball about for 100K plate @140V B+: -2.5V bias @0.75mA idle.

So, that works out about 3K for your cathode resistor. You can select6 this value with a pot if you prefer. Or you may prefer the sound with it biased a little hotter.

Hi,

I believe that I see in this example how the 0.75mA @ idle correlates to 75V drop across the specified Rp and referencing the triode curve sheet, 65V @ the plate w/.75mA is in the -2.5V bias range which calculates to the ~3.3k cathode resistor.

That leads me to the question I ask myself of where did we get the 0.75mA (-2.5V) idle spec/requirement from? That question in turn leads me to calculate through some different idle current/bias points, Rp values, etc..

Eventually, I get to the point of seeing the potentially large voltage swing at the plate (dependent on the grid signal) and start thinking about step down trafo's that would yield a satisfactory mic-level output signal. For the moment, I will keep that in mind but shelve it for later consideration.

Hopefully, the answer to my own question about how we came up with the .75mA is that it depends... Uhh on the typical input signal level, and the desired swing at the plate, which depends on the specified output trafo among other things (of course)... So, to consider the source and move forward I lean towards focusing on better understanding the signal from the capsule.

To keep the questions simple, I am wondering how one would come up with 0.75mA idle (or how would one justify that value), and what the typical voltage swing is at the grid from a "typical-capsule"?

On another note/comment, It appears to me that in this range (.75mA) the transfer curves are less linear. This must affect the dynamics of the signal (asymmetrically?) if that is accurate.

Thanks for your input!

Best,
jonathan

EDIT:
PRR hints in another thread how one might "eyeball" a particular operating point/bias/mA@idle:

Small-signal audio stages are often biased so the device output pin (plate, collector, etc) is "about" half-way between the supply rails. A Fender preamp has zeroV and +300V rails, and the 1.5K and 100K resistors around the tube set the plate to 200V.

 

[pasarski]

The above question kind of puzzles me too. Is it just that we don't want linearity from our tube mics? Is there examples of tube mic circuits that operate more in the linear part of the transfer curves?

(BTW my mic is coming together slowly but surely)

 

[PRR]

> It appears to me that in this range (.75mA) the transfer curves are less linear.

Probably not.

Don't look at the lines, look at the spacing.

The linearity doesn't change a whole lot at low currents; may improve.

Linearity is between Mu and Child's Law; both change very slowly.

Over a wide range, distortion may be estimated by the ratio of signal voltage to supply voltage. For a happy stage, THD approaches 5% when peak signal is 20% of supply voltage. Taking 100V typical supply for a mike, we could run 20V peak signal cleanly. Assuming 7:1 step-down, that's 3V peak into the mixer; a VERY hot signal. At more likely levels, THD is more like 1% pure 2nd.

> we don't want linearity from our tube mics?

If you want linearity, plagiarize the AKG 414. A dozen transistors will pass mike signals dead-clean. The 414 is a great mike, but does not add anything to the performance. Sometimes we want to add some flavor. Aside from simplicity and brag-factor, I bet the slight distortion of a tube mike is part of the attraction.