bias of preamp tubes with a pot

[ALBATROS1234]

simple question, i have not seen this done but in my experimentations i find bias of triode preamp tubes usually uses between 820ohm to 4.7kohm , has anyone ever used a 5k pot and used it as a gain control so to speak? it seems mid way is usually around 1.5k but sometimes one might bias a preamp triode a bit colder or hotter. is there any real sense in doing something like this? perhaps use a 500ohm resistor in conjunction so you never dip below 500ohm resistance and you can theorhetically get up to 5.5k. or perhaps to experiment with a cold clipper stage use a 20k pot. has anyone done this type thing or am ijust overthinking crap as usual?

[sluckey]

Been done lots of times. I did it on my final amp...

     http://sluckeyamps.com/smoky/smoky.pdf

[ALBATROS1234]

theres nothing new under the sun i guess. it just kinda dawned on me today so i wondered if that would work out well. thanks steve.

i really like 6AQ5 tubes they sound better than 6v6 to me but how did the 6AK5 pre tube sound, i tryed kathode bias on pentode V1 tubes and could never get much ummph out of them, i typicaly grid bias pentodes, which i really tend to prefer, close second is the trusty 6SL7 though, to me much more character than the common 12ax7 albeit i dont have and of the high dollar ones mine are all pulls or sub $30 12ax7. have been tempted to get a telefunken ,amperex, or mullard but kinda think i am going to spend $60 or more and come to the realization that they are not that much better to justify quadrupal prices but i could be wrong.

[nandrewjackson]

I built an experiment of a preamp a few years back, I had pots on all the cathodes of 12ax7s.


500 ohm resistor at the tail so as not to completely short to ground, as you said. And a 50k pot for the 3rd triode in a high gain cascade.


It was a neat experiment,  but I ended up always landing on the common values for good sounds.


I.e. 1k , 2k, 10k, 39k,

[nandrewjackson]

Oh, I recommend trying new production mullard 12ax7 and new production  genelex gold lion.


Put them to the test, I did last year, with a bunch of new production tubes.


Those two have some very unique, musical,  sounds

[dude]

I’ve been using pulled 12ax7’s from old organs, etc. good results but every time I put a new JJ 12ax7 , seems the amp has a tighter tone and punch. Are my pulls have way beat...? Tone is so subjective that after half an hour of trying to find the best first stage preamp tube, I can’t tell what’s what.  . Results: hype can play a lot into what’s good tone, 

[ALBATROS1234]

thanks guys, i will try a new mullard reissue, i have read good things. i only have a handful of new tubes but havent been really impressed. perhaps i am lucky, but so far i tend to much prefer old nos or slightly used tubes. like rca 6v6 from the 50s smoke every other 6v6 i have ever tryed, the only ones close were a national union and one particular sylvania. got 4 telefunken 7189s from a late 50s stereo amp of unknown origin that smoke any el84s i have, although a funkwerke el84 that i bought nos which has mucho character. rca tubes seem to be the best american tubes but i do like sylvanias a great deal. i do need to try more new issue tubes though i just have been disapointed with what i have tryed. thanks again gents.