cold bias gain stage?

[12AX7]

I've experimented with them but i have some Q's. First, with a 100k plate R,  what cathode resistor value is considered large enough to be cold biased? The JCM800 used a 10k, the SLO and 39k. Mine is a jcm800 style preamp that is pretty much exact except the cold biased 2nd stage is a regular gain stage. I made it switchable and i like the tone with a 10k cathode R, but there's not enough gain so i am considering one of 2 possibilities. 1, a smaller resistor in hopes of the tone it generated before but with enough gain to be more usable, hence the question about at what value does it stop being a cold bias stage in it's ability to generate the same harmonics they are known to? Or is it a matter of degree, IE:10k=more asymmetrical clipping than 5k and 39k even more than 10k? Also, and this one i'm really curious about, if you bypass a 10k cathode what happens? Do the frequencies that the cap passes cover up any benefits of the cold clipping in that range so that you only get the cold clipping generated harmonics is the ranges the capo doesn't pass? Are there any examples of cold biased stages that are bypass in any amps you know of or is this just a ridiculous idea? Thanks.

[BetterOffShred]

I'm pretty curious about this too after going and doing some looking around!  Dual Rectifier also uses a 39k on V2B, I did find a little study a guy did with a pot and harmonic saturation, kind of interesting, but as far as the math and voltage and everything, AX84 has a couple posts by Merlin about it, but it was above my head currently 

https://warpedmusician.wordpress.com/2016/01/17/dual-rectifier-cold-clipping-mods/

That's all in a Dual Rectifier mind you. 

-Brett

[12AX7]

Interesting read, thanks. Still many questions but interesting none the less because i rarely see more than a few words on the subject and they're always the same. I tried 39k with 4 stages and 10k with 3 stages and my preamp configured exactly like a 2204. There is a nice bit of extra complexity but so far my preamp configured in modded 800 form with a regular gain stage in place of the cold stage still wins out in overall tone and usability. i think with 4 stages, one being cold,  I'd likely be able to end up with something better than my modded jcm form. But i'm not sure i wanna go thru god knows how many more hours of tedious experiments.  But i WOULD like to see if i could get enough gain out of 3 stages and a "slightly" cold bias stage and maybe bypassing it. But that was one of my questions. I think i'll try 10k on V1B again but bypass it with a largish cap to get some gain back and see what happens. But the theory is over my head so it's just a shot in the dark.

[PRR]

[12AX7]

yeah, i know. But i don't have one, i just have several short clip leads i use for that. Not as quick but does the job.

[2deaf]

With a decent signal across a large cathode resistor, the negative portion will clip while the positive portion will continue on its merry way.  It you put a bypass capacitor across the cathode resistor, the asymmetric signal will cause the capacitor to charge positive which moves the tube closer to cut-off.  Also, this positive charge on the bypass capacitor will take longer to discharge with a larger cathode resistor when the signal is decreased.  This undesirable effect is probably the main reason you rarely see bypass capacitors on cold-biased rigs.

[VMS]

Here is some info about the 39k stage in slo preamp:


https://ampbooks.com/mobile/classic-circuits/soldano-slo-preamp-3/

[12AX7]

With a decent signal across a large cathode resistor, the negative portion will clip while the positive portion will continue on its merry way.  It you put a bypass capacitor across the cathode resistor, the asymmetric signal will cause the capacitor to charge positive which moves the tube closer to cut-off.  Also, this positive charge on the bypass capacitor will take longer to discharge with a larger cathode resistor when the signal is decreased.  This undesirable effect is probably the main reason you rarely see bypass capacitors on cold-biased rigs.

Thanks, thats what i needed. So much for that idea. I wonder what happens if you bypass the cathode resistor and gradually jackit;s value up. Maybe theres a point before that happens where it will generate at least some asymmetrical clipping while allowing for decent gain. Probably not, but thats the way i look at things being a non techie. Gotta try it tho. what the heck.