Proper 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

[tubeswell]

100V on the plates is on the low side for a 12AX7. You won't be able to bias them very well. They are not a high-current tube, so if you are running lower plate voltages and need higher currents, you probably want an AU, AY, or AT7.

In general terms 12AX7s generally run okay anywhere between 100V and 250V (or somewhere in the middle of that). If you run each triode at (say) 280V B+ with a separate 1k5 cathode resistor for each cathode (or 820R - 1k for higher gain), and something between 47k (less gain/more compression) - 220k (high gain) plate resistor (100k is more typical), with a 1nF - .1uF coupling cap to the next stage and a 470k - 1M grid load resistor in front of it, that is more typical.

The Valve Wizard site has a good article on the science of the triode.

[Old Tele man]

...here's a very generic rule-of-thumb: the lower the plate voltage, the more complex and muddier the tones (think blues and warmth); while the higher the plate voltage, the more sterile and bright the tones (think Hi-Fi and strident).

[Merlinb]

Absolute voltages are largely irrelevant. It sounds like what you need is a way to make the 12AX7 a bit less sensitive so it doesn't break up so early. Adding a pot between the bottom of the cathode resistor to ground (grid leak to the top of this pot) will allow you to control this. This will put DC on the grid, so coupling caps are necessary. Effectively you are 'jacking up' the whole gain stage with a resistance.

[Robert M. Martinelli]

Hi,
Being that a bass amp I find the ECC82/12AU7/5814 and the like to be more appropriate ( higher mutual conductance and better output current capability ).

Remember that, even if their mu il rather low ( 20 ), those tubes are double triodes so you can cascade the two triodes, multiplying the two stages' individual gains, thus managing to shape the gain/breakup to your liking.

Hope this helps

Best regards

Bob

[guitician]

Try 5751, 12at7 or 12ay7. They have gains between 12ax7 and 12au7.