12au7 Bias ?

[bigaudioscotto]

Hey guys

I am using 12au7 as drivers in a SE amp
I am getting 167v on plates and 5.8v on cathodes
does that look decent or should I bring down
the voltage on the cathodes a bit maybe go
from 1k to 1.5k on that resistor.

[DeathRex]

A schematic would be nice.

[DeathRex]

I guess with 5.8 volts across a 1K cathode resistor would give you 5.8ma, about half of a 12AU7's max. And 161 X 0.0058 is almost a watt, also in spec. But what's the load line? You can get Morgan Jones' book, or search the internet to find out how to find the load line.

For a SE amp you might be able to get away with making it simple and using only 1 driver tube. With only 1 driver tube you'll need more gain than a 12AU7, like a 6DJ8 or a 12AT7.

[bigaudioscotto]

F01-KT88-Amplifier.png (15.61 KiB) Viewed 4623 times

This is the Schematic using 12au7 instead of 6N1P
also I am running EL34 in place of KT88

[DeathRex]

I made this schematic, but used a 6L6 instead of KT88. The 6N1P has just enough gain, 35. In triode mode you'll need a 12AT7 to get an input sensitivity of 2VPP. With a 12AU7 you'll probably get a sensitivity of 4-5VPP. Best thing about the schematic, you can try a 12AU7 if it doesn't work you can easily pop in a 12AT7.

[bigaudioscotto]

The 12au7 seem to sound great driving EL34 and seem to have plenty of gain
as well.also I am running Triode,
I may throw in some 12at7 and see what happens I guess it can't hurt.

[DeathRex]

If you rewire the socket to put 6v filament on pins 4 and 5, you can try a 6CG7, 6DJ8, 6BZ8, 6BQ7, 6GU7 or a 6N1P.
They say a 6N1P sounds alot like a 6SN7/6CG7 with more gain.

[Gingertube]

Its fine as it is.

5.8mA is probably a little less current than the best idle point for a 12AU7 (12AU7 has a bit of a bad rep in DIY HiFi mostly cause nearly everyone tries to run them at about 2 mA and then complains that they sound bad, a case of abusing the tube and then blaming it for the less than stellar result, but run them at 8 to 8.5mA and they sound very fine indeed.

You don't have any real lee-way to increase the current, it would bring the anode voltage down too much, if you want to "mess around" anymore I would consider a current source anode load for the 12AU7, otherwise how you have it right now is a good compromise between idle current and anode voltage. Bring the anode voltage down too low and grod current goes up as does noise.

Cheers,
Ian

[bigaudioscotto]

So I put a 1.5k cathode resistor in for the 1k on 12au7
and the numbers went from 167v plate 5.8v cathode to 194v plate and 8v cathode
does that seem more optimal.

Thanks
Scott

Its fine as it is.

5.8mA is probably a little less current than the best idle point for a 12AU7 (12AU7 has a bit of a bad rep in DIY HiFi mostly cause nearly everyone tries to run them at about 2 mA and then complains that they sound bad, a case of abusing the tube and then blaming it for the less than stellar result, but run them at 8 to 8.5mA and they sound very fine indeed.

You don't have any real lee-way to increase the current, it would bring the anode voltage down too much, if you want to "mess around" anymore I would consider a current source anode load for the 12AU7, otherwise how you have it right now is a good compromise between idle current and anode voltage. Bring the anode voltage down too low and grod current goes up as does noise.

Cheers,
Ian

[azazello]

I think, U cathode must be about 2 - 2.5 V and U plate about 140-160 v. You can change 1.5 kohm to 500-600 ohm and plate resistor to about 20-25 kohm.