The AKG SolidTube is a large-diaphragm vacuum tube condenser with fixed Cardioid polar pattern. It was one of the first ‘affordable’ tube condensers for the project studio market, with an MSRP of $1500.

The mic uses a commonly available tube, the 12AX7/ECC83, selected for being “readily available from musical instrument dealers carrying spare parts for guitar amps,” according to AKG. 

The mic’s frequency response is nominally flat from 20 - 20KHz with a presence peak at 4kHz and 10kHz. Its self-noise is 20dBA, moderate for a large-diaphragm tube mic. 

Surprisingly, the SolidTube’s edge-terminated capsule is a prepolarized or electret type, rather than a true (externally-polarized) condenser capsule. Given its ~9V polarization voltage, a capsule swap to a true CK12-style capsule is not possible without more-extensive circuit modifications.