7788 tube mic preamp design

[rafaro]

As some of you may know the 7788 tube is a high transconductance pentode stockpiled by the military but finally released about 8 years ago. The high transconductance should translate into being able to reveal finer nuances of sound in preamp designs. I have seen it being used as a first tube in phono amps circuits and also as a driver in SET 845 amps. But have not yet seen it used in a tube mic pre design where it should in theory give superior results compared to 12AX7, 6072 etc designs. Anybody come up with a circuit for it yet? On search here and elsewhere I have not come up with anything. Would appreciate help ifrom the tube Gurus here n designing a tube mic pre with this tube...do have some UTC tube input transformers available. Any advice suggestions appreciated.

[Brian Beck]

The 7788 is a superb tube. It should do well for a low-noise mike front end. I suggest you do a search here and elsewhere for one of the 7788-based RIAA stages that have been posted. Remove the RIAA equalization, adjust total gain for your application and there you go. Like all high figure-of-merit tubes, be careful about excessive grid current. Keep Va as high as Pa will allow to reduce this problem.