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Old 11th October 2006, 10:09 PM   #1
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Default simple 12AU7 cathode follower

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
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Old 11th October 2006, 10:25 PM   #2
DougL is offline DougL  United States
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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
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Old 11th October 2006, 11:28 PM   #3
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Default 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. 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.
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Old 12th October 2006, 03:04 AM   #4
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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/tubecircui...llowers_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
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Old 12th October 2006, 11:38 AM   #5
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Default 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
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Old 12th October 2006, 12:17 PM   #6
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you may want to try to AC couple the circuit first - at least to see if you can get it working.

Take some measurements - what is your cathode resistor voltage? What is the voltage at the grid, plate - etc. Again, a basic schematic would help us troubleshoot for you.

Kevin
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Old 12th October 2006, 12:47 PM   #7
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Quote:
Their fall time is MUCH slower than their rise time.
Just out of curiosity, have you ever actually tried measuring this? You might be surprised.
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Old 12th October 2006, 06:58 PM   #8
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Quote:
Originally posted by Southside Ron
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/tubecircui...llowers_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...
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Old 12th October 2006, 09:10 PM   #9
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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.
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Old 12th October 2006, 09:14 PM   #10
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Actually, it's a lousy tube for hifi purposes (very nonlinear), but that's not at all a concern for musical instrument application where the amp is expected to contribute a characteristic sound.
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