Correect 12AX7 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

[loudthud]

You really need to provide more information to get an informed answer. In most preamp circuits, 100V is far too low for a 12AX7 plate voltage. But, you didn't say if that was the plate voltage or the plate Supply voltage. And I'm not saying that applies in every case because there is always more than one way to do things. Can you post some type of schematic or suggest a circuit that is similar?

[capnjuan]

... With a 12AU7 in there I can't get enough breakup and with a 12AX7, there is way too much breakup.

Hi R5: before yanking out resistors to manipulate voltage, try subbing in other 12A_7 tubes.

12AU7, Amplification Factor = 17
12AV7, Amplification Factor = 41
12AT7, Amplification Factor = 60
12AX7, Amplification Factor = 100

If you have or can borrow one, try a 12AV7 or a 12AT7 before messing with plate voltages. Good luck! CJ

[Direwolf]

Max plate voltage on a 12AX7 is 300v. Here is a link to the data sheet that has the information for the 12AX7: http://tdsl.duncanamps.com/show.php?des=12AX7

That said, it really depends on the circuit. I think I'd definitely try some of the other tubes like capnjuan suggested before I lit up the soldering iron.

[Sonny ReVerb]

Higher plate voltage would allow more headroom before clipping. The 12AX7, with 100V on the plates, will clip much sooner than if there were 200V or so on the plates. The higher plate voltage will allow you to swing a bigger signal before breakup.

[tedmich]

plate voltage <-> breakup is not a first order relationship, the old real McTube and the PAiA Stack-In-A-Box tube preamp ran at way low plate voltages and broke up like all hell...

best to try the tube switch suggested, once you find a gain that works you could mod a stage to make a std. 12ax7 work if you need to...but maybe you'll just stick with the sub!

[GibsonLover]

You really do need more info. A 12AX7 may be distorted because the biasing is off (as you suggest changing the plate voltage) but it may be overdriven in the first place and biased perfectly. Post a schematic or tell us which preamp you're building. A tube with lower gain is still going to distort if the input is overdriven (that is, regardless of gain).