Cathode Biased 12AX7 input stage, my first...suggestions, criticisms?

[2ndserve]

I have this Newcomb PA amp I've been using with my Les Paul. The mic inputs were wired Grid-leak bias. Nothing wrong with how that sounded, but as there are two "Mike" inputs, (Yes, Newcomb spells it MIKE) I decided to rewire one for Cathode bias. I did it, without doing any serious listening/playing...it sounds fine. But, being new at this I would appreciate any feedback on it.

The grid: I removed the cap and replaced it with a 68K resistor. Seems pretty standard as that resistor combined with the grid capacitance make up a low pass filter for radio frequencies. The 10M resistor to ground was left in circuit.

The plate: I replaced the 560K plate load resistor with a 100k one, that brought the plate voltage up to 156V. B+ here was a little lower than the schematic, at around 240V depending on what day it is.

The cathode: I initially used a 1.07K (fixed typing error that had it at 107K!) resistor for bias but after checking it, the cathode voltage was .85V. 12AX7 Plate curves and the very useful AX84 theory document, suggested I should be looking to bias it at around -1.1V so I instead dropped in a 1.5K resistor and that got me exactly there.  Nothing sounded wrong when it was biased at .-85V. I calculated gain running a 1Khz test tone into it. It had a tad more gain biased at .85V which seems counterintuitive to me, but either way the gain was around 40 whereas the gain on the grid-leak input is more like 60. I used a .1uf bypass cap and perhaps this value could be improved upon.

the signal heading out the plate hits a .05uf coupling cap before running into volume control (a 330k pot) and an odd tone stack before it gets to the grid of the next tube. I've read, and my calculations seem to back this up, that the following grid resistance is usually high enough to basically ignore.

Like I said, nothing seems wrong sonically and from what I know the voltage and amperage calculations make sense in seem in line with schematics I've viewed, but I'm new at this and won't be shocked if there are suggestions here that might improve things in the circuit or my understanding of it.

Thanks for any comments in advance!

[PRR]

> I initially used a 107K resistor for bias

107k ? ? ?

> cathode voltage was .85V.

0.85V/107k is 0.008mA which is insanely low.

"1.07k" (1k) would be a reasonable value.

> I instead dropped in a 1.5K resistor

You will never go wrong stealing Fender's values.

Differences 1k-2k may only come out when played HARD with hot pickups.

I do think, with cathode bias, the 10Meg gridleak can be lowered to near 1Meg so grid leakage won't significantly change the bias. There is much variation of Ig between different 12AX7.

0.1uFd cathode "bypass" is hardly significant. Only lifts the top overtones of guitar. Stick in a 10uFd, your main-band gain will come up. OTOH Marshall made a name using 0.68uFd for a brighter shaved-bass voice.

[2ndserve]

oops, it was indeed a 1.07K resistor.

And thanks on the other comments as well! The whole grid-leak thing is confusing to me...as was the preferred value of the cathode cap.

[ALBATROS1234]

i completely agree with prr and i was going to post you should change the 10meg grid lead to around 1meg or else such a large value will affect you bias on that stage. 470k to 1 meg is pretty good and standard value for cathode bias preamp stages.

[2ndserve]

Well, rather than just popping in the values suggested, I did a tad more homework on grid leak which I understand a bit better and (sorta cheated) with an online calculator looked at differing frequency values for the cathode bypass cap.

So, with further review and a bit of soldering, I went with the exact values PRR (1M and 10uf respectively) suggested and then turned the bass pot down. :)