I just got my InnerTube mod kit and thought I'd document my comparison. The following is a down and dirty comparison of my 1971 U87
recorded as stock, with the InnerTube mod, and with the mod and a NOS Telefunken E88CC tube replacing the NOS CCCP tube that came with the kit.
This is a small piece of voiceover copy used for example only. I read it 3 times as closely as possible with each setup. The segment is short enough so that you can clearly hear the difference between the 3. I ran through a Shadow Hills GAMA preamp (Iron tranny), straight into an Avid MBox Pro.
The mic had the -10dB pad engaged on all 3 takes and the preamp was padded as well so I could crank up the pre, because that's how I like to use it. I tried to match the levels by ear, didn't really spend more than 10 seconds on it.
I found the results quite illuminating.
1) 1971 Neumann U87
2) Same U87
- InnerTube mod installed - NOS CCCP 6H23N (supplied w kit)
3) Same U87
- InnerTube mod installed - NOS Telefunken E88CC