simple 12AU7 cathode follower

[Dave Cigna]

Are you driving a typical Fender/Marshall style tone stack? You might try adding a coupling cap after the cathode follower just to make sure that the tone stack isn't dragging the cathode followers output down to a low DC voltage. It shouldn't be, but if it is for some reason (and severe enough) then it could result in a very heavy load on the gain stage.

If I wanted to try a 12AU7 in that position I would replace the 100k plate and cathode loads with something around 15k and replace the 1.5k cathode bias resistor in the first stage with 500 ohms or so. You won't get anywhere near the gain of a 12AX7, but it will get the 12AU7 running at an operating point where it should be happier. (That's assuming your power supply is in the 200 to 300 volt neighborhood.) If that works, then you can experiment by varying things a little at a time in either direction to see how you like the sound.

-- Dave

[ReidK]

As another point of reference, Acrosound used the 6SN7 (almost identical to the 12au7 except for basing and heater) as a CF in this hi-fi amp.

Reid

[SY]

almost identical to the 12au7 except for basing and heater

...and 10 times lower distortion.

[RenegadeAmps]

6sn7 is a far better tube for Musical Instrument than the 12AU7 as well... 6SN7 can make for a terrific longtail PI. And it's good for a low distortion preamp tube as well... I've got a couple of dozen NOS 12au7s that I keeptrying to find a good use for but alas... every time I try to use them in Guitar amps (even ones that I've tried to desgin to make good use of the 12au7) I'm less than impressed...

[ray_moth]

12AU7 is good for airgun practice but not much else.

[cerrem]

There is nothing wrong with the 12AU7...
It's all in how you use it....
I have used them in Hi-End designs with great results, provided you design the circuit correctly...
SOme folks just stick tubes randomly in circuits to hear what it sounds like...that won't make do....
WIth a direct coupled follower....The cathode resistor used in the follower only dials in the current but has no effect on the bias...
The follower is biased from the voltage difference between the plate and grid.... that is also the input dynamic range....
You can vary the cathode resistor all you want, you will still see the same voltage betwene the grid and cathode, since this only moves the current....
The plate resistor from stage one actually controls the biasing for the second stage direct coupled follower...
The correct way to design this is with simulataneous equations...and you will see all the numbers wiork out perfectly...

Chris

[Southside Ron]

Okay gang...I checked all the wiring and it's okay. So I separated each section and put a signal to each stage one at a time and scoped the results. Everything is working fine except the 3rd amplifier stage is clipping on the negative side of the cycle. A few voltage measurements and I think I found the problem. The plate resistance is too low for my circuit. The circuit is somewhat based on design standards for 12AX7's that has about 10 times the plate resistance. For the same signal level output (I need 18 to 20 volts P.P.) I cannot get this level with a 200 volt supply. The current required by the 12AU7 running through my cathode and plate resistors is consuming all my plate supply voltage, nothing or not enough left for signal swing. I'm pretty sure this is my problem. Up next is to boost the plate supply to 250 volts or lower my plate and cathode resistor values and see what happens. I'll keep you posted.

[Southside Ron]

Well...I thought I would get back to you all. First off thanks to Cerrem for the positive input on proper circuit design and 12AU7's.

I did manage to get my circuit working rather well. I find the sound quite sparkling! As far as 12AU7's and audio, a properly designed circuit and consideration for layout can make a good quality 12AU7 tube sing.

I have seen this tube used in quite a few hi end stereo schematics as followers or phase splitters. As any design with amplifiers there is always trade off's to achieve a desired result. High gain devices with negative feed back to reduce distortion, low gain high current circuits to get stablity without feedback, which would allow the unique characteristics of the device to come through. I like to think that the lower capacitance of the 12AU7 helps to give me that extra sparkle.

Thank you all for your input!

[kmtang]

I am using 12AU7 as the buffer for my clone marantz 7 line amp. I like it better than the one with 12AX7 buffer. It has more flesh and blood.


Johnny