grid leak bias in 5f1 - NFB question

[dinkotom]

Hello,
I would like to try grid leak biasing (.022 caps go to grids, grid leaks are 5M and cathodes are grounded) on both preamp stages in my homemade 5f1 Champ just to hear how it sounds.
As cathodes are grounded and if NFB stays in place (on the second cathode), it's grounded too. Do I leave it as it is or should it be removed? (I asked about this twice over at tdpri where I've read about it, didn't get the answer, maybe I should ask once more...)

[bluesfendermanblues]

So you want to connect the V1 cathodes to ground - instead of using 1.5K cathode resistors - why?

The stock setup with anode (100K) and cathode (1.5k) is the 'golden ratio", 100/1.5 = 66.7.

However, if you want more gain, you could try reducing the V1a cathode to 820 ohms (e.g. paralling the 1.5K with a 1.8K) in order to simulate the 'Pull V1 to get a hotter signal in the vibratochannel" often used on Blackface and silverface amps in order to get a hotter signal.

http://fenderguru.com/amps/super-reverb/

[dinkotom]

I would just like to hear how does it sound.

Here's that post from tdpri:

"Here's How: Ground both 12AX7 cathodes (pins 3 and 8 ). After the pair of 68K resistors, connect to where they intersect and are joined - a .022uF capacitor between them and the input grid at pin 2. Then - also from pin 2 - install a 5M (5 million ohms) resistor to ground.
Now, you could just do the one side (V1a - Valve one - section a. Do not ground the cathode of V1B - leave everything as is), but if you really want to maximum early break up, do the other side also.
Here's how to do V1B:
Where the amplified first stage signal comes off the volume pot's center lug (wiper), put it through another .022uF cap to pin 7 of V1B. Again, as in the first case, install a 5M resistor from pin 7 to ground."

I understand all of this, except not sure what to do with NFB.

[martin manning]

By grounding the second V1 cathode you would be disabling the NFB, so you might as well disconnect it. That may be a large part of the "maximum early break-up" the tdpri poster talks about. Another option would be to leave the 1k5 and NFB connection as-is, add the cap from the volume pot wiper, but connect the 5M from grid to cathode. You will then have grid-leak bias and NFB.

[dinkotom]

All clear, thanks Martin!