[Central-Electronics] 100V Audio Filter Rebuild

[Jon Kannegaard]

Has anyone rebuilt the Audio Filter plug-in as suggested on Nick Tusa’s site? These are supposed to be a unity gain stage. Mine is -3 dB even with the suggested 6BL8 or any of its cousins substituting for the 6U8. Voltages at the the tube pins are not way off. Plate resistors are just a tad high, 10%. I tested a few accessible resistors. None were way off. Caps are disc ceramics, famous for drifting off value but not for becoming leaky.  I didn’t test those.

I have a feeling I could rebuild it and nothing would change.

Also, the surrounding circuitry on this serial #386 doesn’t match either of the two REVs of schematic I have. The mic pre-amp is E), none of the above. The resistive divider on the output of audio filter doesn’t exist.  There are probably other changes I haven’t noticed. Does anyone know anything about the really early units or have a first REV schematic?

And finally, there is a quite a bit of noisy 8 MHz signal showing up on the output of the Audio Filter plug-in, coming in from the downstream stage and wiring. I’m surprised by this. Anyone else notice it? Removing the tube in that next stage, the phase inverter, makes no difference. It might just be the wiring picking up signal from the nearby oscillator. Maybe it doesn’t matter. It makes for an ugly scope trace, though.

Jon, K6JEK

PS: I’ve been using it with the Limiter and Filter bypassed, with those functions provided by the external audio chain, so yes I know you can do this. But it just ain’t right.