Good enough CFs

[mashaffer]

Had another thought that might work. The main issue for me is to maximize the use of current inventory due to severe budget constraints and the difficulty of getting expenditures authorized through the domestic authorities if you get my drift. It is bad enough that I am going to have to justify a PS filter choke let alone more tubes when I have a bunch in the bins already.

I have two 6N1P and a hand full of 12AX7 along with a smattering of RF (but no two identical) tubes. As I was drifting off to sleep last night it struck me that the need for high current and gm is mostly necessary on the output side of the equation.

The application is a BP filter for woofers (50-200Hz) and LP or BP for the subwoofer (10Hz to 50Hz). I intend to use Steve Bench style 3rd order HP and LP sections to implement. So there will be either a LP or HP section first followed by the complimentary section.

Since the Zin for the second stage will be rather high and of known quantity, why not use 12AX7 running about 1mA for the first section and the 6N1P running about 12mA or so for the second section. That way the high performance tube is used on the output where it is most important and the AX7 can be used in the less demanding input section. At a latter date I can always modify for a better input tube.

Is this a reasonable way of doing it?

Then I have to decide whether LP or HP should be used as the first stage based on the fact that three three inputs (the two filters plus the VAS of the treble amp) will be driven off of the volume control.

Of course there may be some difference in output performance of the LP v.s. HP to consider. I will have to run some sims to get an idea.