Stock Ionovac
Early on I used the Ionovacs in stock form, restored with with new capacitors and rectifiers. I needed to drive them with a dedicated amplifier since they were paired with 114db/w horns. I found that in stock form the sound output was limited and that distortion rose quickly at higher output levels. However, in spite of the flaws the sound was remarkable. I replaced the step up transformers with units from an Eico amplifier using a lower step up ratio. The result was less sensitivity but the output increased substantially with lower distortion.Direct Drive Amplifier
The lower step up ratio transformers resulted in a substantially better performance suggesting that there were more gains to be had with stiffer drive on the modulator tube screen. In addition using tubes to drive a step down transformer and then using another transformer to step the signal back up seemed like bad idea. For years I contemplated building a dedicated tube amplifier that would directly drive the cell without the coupling transformer. I finally got around to building the amplifiers and the results are simply stunning. Much higher output and noticeably improved clarity.
The tweeter amplifer consists of two 6SN7 gain stages feeding a 6BL7 cathode follower. This design provides plenty of voltage swing and current to provide stiff drive to the Ionovac's 6DQ6 screen grid. The amplifier incorporates a 3rd order low Q high pass filter. Two poles are implemented using the coupling capacitors (1.8nf and 270pf). The 1.5mh plate choke is sized to provide the third high pass pole.
