How come 12ax7 can't be used as power tubes?

[tildeslash]

Can an amp be made using them?
I am sure it will be low powered but heck - aren't we all looking for a nice low powered amp - well most of us are!?!?!

[rooster]

Plate resistance is pretty darned high, and it's hard to find an OT that will work for one. A 12AU7, OTOH, can work pretty well. Many have done it, it's called the "firefly."

rooster.

[Doug H]

A 12ax7 can work, IIRC there was a commercial product I heard about a few yrs ago that used one for power amplification. But I think they custom-wound their own output xformers for it. The impedance load required is untypically high. As mentioned, a 12au7 makes a much better candidate. It has lower Zout and therefore can drive more current (which is good for power amplification), etc...

BTW: Here's a link for the Firefly- http://ax84.com/index.php?pg=legacyprojects&project_id=firefly

[John Phillips]

A 12AT7 works well too - that's basically what the reverb driver in a BF/SF Fender is... a little output section. If you connect a speaker to the reverb send, it works :) (allowing for the deliberate bass roll-off in the Fender circuit). The ZVex Nano uses a sub-miniature military tube with very similar characteristics.

[billyguitar]

I have seen pictures of a high end stereo amp that used all 12AX7s but I can't remember the name of it.

[fakeox]

Valvetronix

[Wakarusa]

Not to mention that the maximum wattage rating on 12AU7 is 2.75 watts per triode, 12AT7 2.5 watts and 12AX7 a lowly 1.2 watts.

For added fun (and probably a unique amplifier) you can run several 12AX7 in parallel. This will reduce the required OT primary impedance and increase the power output. Finally, a reasonable use for those matched triode 12AX7s :)

[leofenderbender]

Valvetronix

Yep, Vox Valvetronix - the 12AX7 output makes it passable as a modeling amp...

[aleclee]

ValvetronixThe 12ax7 isn't used as a power tube, it just simulates one. It actually has a SS power amp.