tube mic' grid cap

[Guitarist]

I've looked a few tube mics and noted that some have very small 1000pF to .01 uF caps and others have seemingly none as with Neumans (if I remember correctly). 45 Volts of DC on the grid can't be good, no? I'm just wondering why this is cap needed and so dang narrow. On a Apex460 I widened this cap and got "pumping" so I reverted to the tiny one which stopped it. I'm baffled how lower Hz can get through this as evident on the output.

It is amazing how knowing and modding tube amps for years makes it so easy to mod a tube mic. I was reading a forum thread elsewhere about modding this mic and they seemed all over the map, confused and frustrated by circuit changes. Especially replacing the capsule and transformer before optimizing the cheap and easy stuff.
Yep, with the usual 12ay7 resistor changes this mic was WAY better !!

[Steve Conner]

The coupling capacitor can be so small because the resistor associated with it, that gives it its RC time constant, is huge. 100 megohms, 200M, even 1 gig.

This huge resistance is needed to preserve the capsule's own low-end response, for the same reasons as piezos need a high load resistance, only more so.

This is why the guts of condenser mics need to be kept dry and free from grubby fingerprints. It also means that tubes in tube mics need to be selected for exceptionally low grid leakage.

Mics that don't have a blocking capacitor apply the DC bias to the other plate of the capsule. Essentially using the capsule itself as the DC block cap. On some designs this is impossible, because the diaphragm is connected to the grounded body of the mic, leaving only one electrode for DC bias and signal.

[Guitarist]

Ah...ok thanks, that makes sense. Thank you again!