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simple 12AU7 cathode follower

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I am a guitar player of sorts and decided to build one of those boutique style amplifiers. I studied electronics in the early 70's so I figured why not.

The simple basic amplifier design that boutique builders favour uses 12AX7's in their preamps. I would like to use 12AU7's as I have several of them.

The 2 tube preamp design with a cathode follower driving a tone stack is what I settled on driving a EL34 S.E. output stage. I dug through all my notes, read several manuals on tubes, searched the web, rattled the old brain and came up with everything I needed for my design. Unfortunately when I connect the cathode follower into the circuit it kills the previous tubes gain, as if it is loading down the previous stage plate's output. I wanted to use a direct coupled plate to grid configuration.

what am I doing wrong? I do not want to use a White follower as it would require another tube. I know that the direct coupled follower is popular in guitar amp design but, with 12AX7's. Although the sound quality may not be the greatest in this design it seems to be the rage with the guitar players.

I look forward to your help
 
Could you draw a schematic for us?

However, I can guess. If you are Direct coupling the Cathode follower to the output grid, the bias is set at the grid of the Cathode follower, and you "need" a negative supply in order to bias correctly and have a maximum voltage swing.

Doug
 
Pulse cathode follower

1/2 a 12au7 doesn't make a very good cathode follower. You need to double the transconductance and half the plate resistance or pick a high transconductance tube like a 7308. I've got a Navships manual and in it are a bunch of what the US Navy called pulse cathode followers. You could take a 12au7/5814A or better yet a 6GU7 and tie both cathodes together, then tie both plates together. Put a 470 ohm to 1000 ohm resistor from grid to grid. Put a 10-20K,10 watt resistor Rk from cathode to ground. Lots of heat here in this resistor Ought to work fine without any grid degenration but you could try 1m and 2.7K resistor in series to ground from the 1st grid. If you have a handy -15o volts you could apply it to the junction of the 1m and 4.7K resistors. Use 2-5 ufd capacitor (big hummer) on the output from cathode to outpit. Should drive a 100 phm load.:D Another trick if you insist on using just half a 12au7/5814a is to take a 22K 5 watt resistor Rk and conect it to the cathode from a -150 volt power supply. A lot of heat will be dissipated in this resistor Don't ground it. Put a 680K resistor from grid to ground and a .047ufd input coupling cap and use the same size big hummer cap on the output. :hot:
 
Thanks for the input!
My understanding that with a direct coupled follower that the cathode resister was used to set the idle current in the tube. The higher the cathode resister the greater the voltage swing avalible with a trade off of reduced cathode current. A good follower should have 10mA current or more.

http://www.glass-ware.com/tubecircuits/Easy_Cathode_Followers_3.html

Although this circuit seems simple enough it is not direct coupled to the plate of the previous stage. A similar circuit that I want to use can be found in the Marshall master volume preamp circuit which can be found at
http://www.firebottle.com/ampage/schems/jcm900.gif

maybe this whole thing just can't be done with 12AU7's
 
The main problem with cathode followers

Their fall time is MUCH slower than their rise time. Negative going signals have a lag time. It really needs an active current source on the cathode but that gets into using large heat sinks with a power BJT or MOSFET. Power mosfets have even high capacitance than BJTs and the thing gets complex. I'd use a White cathode follower maybe a 5687 if I had to use a cathode follower. Ray
 
Southside Ron said:
Thanks for the input!
My understanding that with a direct coupled follower that the cathode resister was used to set the idle current in the tube. The higher the cathode resister the greater the voltage swing avalible with a trade off of reduced cathode current. A good follower should have 10mA current or more.

http://www.glass-ware.com/tubecircuits/Easy_Cathode_Followers_3.html

Although this circuit seems simple enough it is not direct coupled to the plate of the previous stage. A similar circuit that I want to use can be found in the Marshall master volume preamp circuit which can be found at
http://www.firebottle.com/ampage/schems/jcm900.gif

maybe this whole thing just can't be done with 12AU7's

12AU7 Functions just fine in the Marshall circuit... I Own a JCM800 and can just simply stick a 12AU7 in the second tube socket and it works... Just like it should ... Output is reduced a bit and the gain from the 3rd stage is reduced, In the JCM800 Circuit the second stage with the 10k Cathode resistor is when most of the trademark Marshall distortion happens... So the 3rd stage is mainly just amplifying that signal again... and the Cthode follower is just there to reduce the impedance to be able to drive the tonestack with less loading... A 12au7 doesn't buy you much at all over the 12AX7 in this application... Other than a slightly cleaner tone... and Slightly reduced Impedance. Which really isn't necessary as it's already low enough with the 12AX7.

Unless you are going for a very unique topology it's not really worth it...
 
Well I was just thinking that a 12AU7 is a some what of a superior tube to use in an audio application not to mention I have several. It is a tube with higher plate currents, slightly lower interelectrode capacitance, and a suposed more stable operation. It's main use was in television oscillator circuits because of it's stability. A tube with the capability to operate at a higher current running at lower value should last longer. I'm glad to hear it works in your amp as my circuit is pretty much the same as the Marshall preamp. I am using a different tone stack (my design) but the input impedence is close to that of Marshall's according to the tone stack caculator software.

I'll check all my connections again.
 
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
 
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...
 
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
 
Da crunch

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.
 
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!
 
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