What's up with this 6SL7 input stage?

[GodOfChunder]

Howdy... first time poster, long time lurker.

This is the mic input on a circa '53 Califone record player. Any thoughts on what the additional 68k resistor and 0.02uf cap are doing there??? I suppose in series with the plate resistor it's dropping the current a bit, and with the input referenced to the bottom of the 6.8k cathode resistor a reasonable bias is still maintained. Maybe it's adding another high-pass pole (around 120Hz), and acting as a kind of mic rumble/wind/thump filter? I don't think I've ever seen this before.



I haven't played around with any measurements yet, but wanted to wrap my head around the design theory first. Thanks for any and all thoughts on this!

Cheers,
KK

[PRR]

Welcome.

The audio gain is somewhat less than 220K/68K or about 3, maybe 2 in practice.

The input impedance is boot-strapped FAR higher than the 1Meg resistor.

This is in line with the application: cheap phonograph. Ceramic cartridge, high output but high impedance.

The 0.02u(??) cap is odd. It suggests response rising above (as you say) 120Hz, maybe to ~~1KHz before leveling out. I do not know what model you are scavenging, but many Califones had the speaker in with the turntable so maybe they had to accept low bass to avoid feedback howl (rumble).

I'll be danged. Califone is still in business.

[GodOfChunder]

Oh yeah, I guess I wasn't looking at it that way... Neglecting the capacitors, to the input it looks just like a cathode follower. While this is actually the mic input, the phono input does indeed have the same scheme (albeit with an additional equalization network).

Anyway, since I'll be doing a guitar amp conversion on this (because of course I am), and I'm just gonna cascade the phono tube stage for more gain, I'll probably just re-do the whole input into an early Fender-esque deal. So most of this discussion will be moot, but just wanted to understand the design.

By the way, this is Califone model #12V, in case you're interested. The one I have is sans speaker. (Also, I have some special plans for the turntable... opto-mechanical tremolo)

I worked for a portable P.A. manufacturer in a past life, and Califone was one of our competitors. At one point we hired away their chief engineer.