Proper 12AX7 tube plate voltage?

[Rhythm5]

I have a designer working on a bass preamp for me with 1 12AX7 that comes after the EQ section as a "tube drive" control for distortion. Right now we have 100v to the tube plates. With a 12AU7 in there I can't get enough breakup and with a 12AX7, there is way too much breakup.

Does anyone know what would be the best voltage to run to the tube plates so I can use a 12AX7 and get a lively, rich tube tone that will do very smooth, controlled tube breakup?

Excuse the lack of tech speak as I am new to design. Thanks

[Dave Curtis, dB AudioTech]

Have you tried a 12AT7?

[GibsonLover]

Try some simple software

This is not a 'dig', but I'm surprised that a lot of folks on this page don't use a simple software tool like TubeCad. It's not a true CAD program, but basic circuits with common variations that allow to to pick a general topography, pick a tube, voltages, etc. and arrive at surprisingly accurate responses. It's only $40 at AES, and is a huge learning experience that gives you a better seat-of-the-pants estimates for What-If questions. Other programs by the same guy at glasswareaudio.com are just as helpful, namely SE AmpCad.
Food for thought. If this is your passion, as for a lot of us, a few tools go a long way. Remember, they are only $40, so if you claim the results are not accurate enough, you would be missing the point, or you've entered something wrong...:-)

[Bruce / Mission Amps]

This is not a 'dig', but I'm surprised that a lot of folks on this page don't use a simple software tool like TubeCad. It's not a true CAD program, but basic circuits with common variations that allow to to pick a general topography, pick a tube, voltages, etc. and arrive at surprisingly accurate responses. It's only $40 at AES, and is a huge learning experience that gives you a better seat-of-the-pants estimates for What-If questions. Other programs by the same guy at glasswareaudio.com are just as helpful, namely SE AmpCad.
Food for thought. If this is your passion, as for a lot of us, a few tools go a long way. Remember, they are only $40, so if you claim the results are not accurate enough, you would be missing the point, or you've entered something wrong...:-)

Not discounting your opinion or experiences but the problem with all that computer generated, test variations and responses is....
my (your/our) computers, computer models, programs, monitors, O'scopes, meters and all the rest of that test gear makes or reproduces absolutely NO MUSIC.
It is a quantum leap from Spice, Tubecad or any other computer program to actually playing the amp with an instrument and letting the real thing (your brain and heart) tell what's good and that is the true aspects of seat of the pants.
The harmonic relationships of all this, modulation and other distortion products, (etc.) alter the amp tone and response so much that I find it more fascinating that novice and intermediate builders are still trying to use these computer programs.
Could just be a sign of the times vs my age (getting old)... but personally, I wouldn't waste my time or money on computer simulation programs, that are going to tell me nothing about how a circuit mod changes the actual sound of anything.
There is nothing like real world, hands on, trial and error when it comes to squeezing out the good stuff in music... which least we forget, is the whole idea of this industry or hobby in the first place.

[GibsonLover]

I can't disagree with any of that, but it's kind of a different thread. I hate computers too, but it's a pretty simple brain exercise to operate any computer application. The idea is that if you are unfamiliar, as your are not, with the voltage relationships in a simple tube stage - there's learnin' to be done. We all gotta start somewhere. No computer will tell you what it's gonna sound like, but at least you can feel good that it's biased correctly before you start with the ear training part of the exercise. That's not why you use a computer.
Just like learning an instrument, you have to start with the mechanics. As an advanced player, musical formulas are more second nature, and you do not dwell on the mechanics any more.
Maybe not so much a sign of the times as breaking down a problem with different solutions. A computer is only one step beyond having a calculator next to your bench if you are tired of doing math long hand.

[martin manning]

I believe the two approaches are really complementary.

A question such as the one asked by the OP can easily be answered by plotting a load line on the plate curves and finding the idle point. Seeing where you are asking the tube to operate on its characteristic reveals a great deal of information, like how much headroom you have, and how much distortion is to be expected. A little more work will get you frequency response information, and quantitative predictions of distortion levels.

With a lot of experience these things could become "known," but the number of trials required to get there can be dramatically reduced by using computer tools, and in the end I think a more complete understanding is possible. The analytical work used to be tedious, but a computer will happily try dozens of combinations of operating voltages and component values. True, you can't hear the results each time, but by testing selected cases you can quickly learn the types of sounds produced in the various neighborhoods as well as the routes that can be taken to get to them.

I'll offer up the Excel tool I created to learn my way around as a no-cost way to get some automated analytical capability. It's posted here:

http://ampgarage.com/forum/viewtopic.php?t=7463

The thread can be viewed by "guests," but I believe you have to register to see and download the file. Sorry, but this forum doesn't seem to be able to accept .xls attachments. I'm happy to e-mail copies too.

MPM

[defaced]

You can upload the Excel spreadsheet to a place like yousendit.com/rapidshare.com/megaupload.com and post the linK.

Valve Wizard also has an Excel spreadsheet that kicks out load lines, response curves and such (way down at the bottom): http://www.freewebs.com/valvewizard/links.html

I like simulations for things that don't involve my ears. Like power supply design or 555 timers.

[martin manning]

The Valve Wizard site is indeed a great resource. I took inspiration and knowledge gained from there and, with the help and encouragement of the Wiz and some others, went a bit further.

MPM

[nbtone]

Check out this one: www.nitewalkerpreamp.com
Plate voltage is 40vdc, just like on a Bogen PA from the 60's.