All 6sn7 preamp for 6v6? First schematic design!

[6267]

I am designing an all 6sn7 preamp (because I have two 6sn7s unused) for a 6v6 p-p design. I calculate gain (using RC charts from RCA handbook) at around 14^3, which gives me 27v from a .01 v input. Peak to peak is this 27x2, or 27/2? I want to drive a 6v6 p-p running at around 300v in class A. Will this preamp work? is the B+ sufficiently decoupled (I don't mind out of phase coupling to add a bit of compression)? Any ideas or redesigns are welcome indeed! (note: I don't want to buy any new tubes unless necessary--also, the cathode bypass caps are the wrong value).

[Steve Conner]

Why the 12k cathode resistors? It looks strange, but I guess the 6SN7 is a lower-mu tube and that's what it needs. Might be worth trying a 100k plate resistor and a smaller cathode resistor, to let the tubes have more current, especially in the PI.

Also, why the strange PI arrangement? It doesn't guarantee balance by any means, unless the gain of the tube exactly matches the loss in the voltage divider, and even then, it will mess up as the gain falls off with frequency.

It looks like you were thinking of the see-saw PI:
http://www.freewebs.com/valvewizard/...paraphase.html

The answer to your signal question depends on whether the .01v input is RMS, peak, peak-peak or whatever. If I assume it's a sine wave of .01v RMS, then the output is 27V RMS, which is 38V peak at each plate of the PI. That is 76V peak-peak or a total of 152V p-p between plates. Whichever way you look at it, should be enough to dropkick a pair of 6V6s into the middle of next week.

[6267]

I accidentally doubled the cathode resistors when combining them instead of halving...oops..glad that was a schematic and not soldered! The PI is a paraphrase http://www.freewebs.com/valvewizard/paraphase.html, because I want it to sound as grumpy as a hacked up 40s radio. It's a harp amp, so I want that old time, low-fi, nasty, possibly evil type of sound.

[deividmonnerat]

I accidentally doubled the cathode resistors when combining them instead of halving...oops..glad that was a schematic and not soldered! The PI is a paraphrase ..., because I want it to sound as grumpy as a hacked up 40s radio. It's a harp amp, so I want that old time, low-fi, nasty, possibly evil type of sound.

Can you post the corrected schematic?
Thank you,
Deivid Monnerat

[6267]

I never actually used that schematic, but I do know it's riddled with mistakes...it was a quick one off...I think I just cribbed the RC coupled data out of the 1949 RCA tube book. If I were to do it again, I'd probably do something like change one 6SN7 to a cathode follower or put some local feedback around some of the triodes to reduce gain. I was sort of hypothesizing a layout when I drew that, and so I was mostly focused on power supply, not dc bias etc. All the resistor values should be assumed wrong, and I seem to remember noticing some missing ones too. Better off finding a better schemo, I think .