OT impedance for 12ax7

[fossilshark]

I found this circuit and i want to build it, except the only audio tranformers i have are 500 ohm primary instead of 1k, would this hurt the tubes? Could i put another 2 triodes in parrallel to match the impedance/give a bit more power?

[sluckey]

No danger. I'd use 500Ω if I had it. No need for more tubes. BTW, Radio Shack has a suitable 1k/8 transformer for $1.56. Or you can get them on eBay for $55.00. Free shipping.

[PRR]

That plan is so lame in so many ways.

So lame that 12AX7 damage from "mis-matching" just can't happen. (It is already pretty mismatched; the OT "should" be 10K-40K primary for good power output.)

Build it with whatever parts. It may work fine as a modest headphone amp.

A JRC386 will whup its butt for power and cleanliness.

[fossilshark]

Im just a hack man  thanks

[Tony Bones]

the OT "should" be 10K-40K primary for good power output.)

You're better than me. I have no freaking clue how a 12AX7 will behave with both the plate and grid at +9 volts!

[PRR]

> no freaking clue how a 12AX7 will behave with both the plate and grid at +9 volts!

"Poorly".

Note the grid is not AT +9V. Forward-bias grid is very-roughly a 1K impedance. Say 500r for two. So 10K to +9V is maybe +0.4V on grid.

The poor JFET has to drive this ~~500r impedance also. Since it works with a ~5K DC feed, it won't make a lot of swing. However the dirt-scratch guess of drive needed is 9V/Mu or less than 0.1V. That won't be right since we are SO far away from what 12AX7 was sold for, and the data used to sell it. At this point it is quick/cheap to Just Try It.

However "damage" is quite unlikely at such low voltages, no matter what you throw-in with the tube.