tubedriver rejecting 12at7..

[dschwartz]

hi guys!
i just build a chandler tube driver that sounds pretty good with a 12ax7, but with a 12at7 sounds like crap, gatey as hell, miss biased or something..

anyone knows something about it?

[DWBH]

Well, I'm no expert at all, but I can't tell you this: that tubedriver was design to work at 100% with a 12ax7. Not, a 12at7.
The same with amps. Most amps are designed to work with 12ax7's in the preamp. However, most of the people put 12at7 in there and say that it doesn't sound good. Well, it doesn't have to. If it was designed to work with 12ax7's, than it should work with 12ax7's.
With you replace a 12ax7 with a 12at7, nor you're using the total pedal/amp capabilities, nor the tube is working at 100%.

Now, there are cases in which the design sounds good with both tubes.
I guess if you want to put a 12at7 in that tubedriver, you'll have to make some changes to the circuit.

[alfafalfa]

Daniel,

What is your supply voltage for the tube ?

I think it will do better with a different plate resistor as well as a different cathode resitor.
But it depends on the supply voltage.

Alf 

[dschwartz]

well i have 12v for heaters, 14V+ anode and -15V cathode..

[Mick Bailey]

You may need to try a couple of 12AT7 tubes. My recent Shaka Tube is very responsive to different types and works well with AX AU and AT types, but some sound duff and these would probably sound duff in my tube amps too. I don't alter the bias usually and keep it at the most negative setting - BUT, I have a 12AT7 that has to be backed off a quite bit to get a decent sound. I would question the tube before looking elsewhere.

[alfafalfa]

Oh I didn't realize it was a low voltage project otherweise you could have consulted the datasheet.

For 250 volts it says:  11 to 12 K plate res. and  200 ohms cath res. but that's for a clean sound.

[dschwartz]

FYI..
i added a bias pot (like the shaka tube) and 12at7 worked perfectly by trimming the pot