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Why a cathode follower?

8/11/1999 6:15 PM
Steve Jones
Why a cathode follower?
In looking at schems where there are three or more tube stages in the preamp, I am seeing what looks like a cathode follower circuit as the last tube stage. An example is the Fender Tweed Preamp schematic posted on this website. First of all, and I correct in identifying this as a cathode follower, and 2nd, can anyone explain to me why this type of circuit is used for this application?  
The purpose of the question is that I've just acquired an old Kalamazoo Model II amp that has two 12AX7s, however only one side of the 2nd one is used for the tremolo circuit. The preamp uses both sides of the other one. I was considering modding the circuit to use the unused triode as a 3rd tube stage in the preamp.  
Thanks for any help!  
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8/11/1999 9:46 PM
Ken Gilbert
"First of all, and I correct in identifying this as a cathode follower, and 2nd, can anyone explain to me why this type of circuit is used for this application?"
A CF has no voltage gain--in fact, it's gain can only approach 1, so that in practice there is a slight loss of signal voltage.  
The primary application of a cathode follower is as an impedance transformer. The grid has a very VERY high input impedance, especially if it is direct coupled to the previous stage thus eliminating the grid leak resistor.  
The output from the CF is taken from the cathode, which has an impedance of approximately 1/gm, the transconductance of the tube. Thus, tubes with higher gm's will net you a lower output impedance. The output impedance of the cathode itself (1/gm) is also in parallel with the cathode resistor. The cathode resistor primarily sets the idle current through the tube. The cathode's voltage will rise to approximately the grid's DC voltage, plus a few volts. Thus, using V=IR, you can set the idle current through the CF by adjusting R.  
The lower ouput impedance means that the CF is better able to source and sink currents, and will not lose much signal voltage swing when loaded down with shunt impedance. This is in stark contrast to the plate loaded stage, which is very succeptible to loading effects.  
A CF is therefore properly thought of as a kind of impedance transformer. It is often used, historically by Marshalls, as a tone stack driver, since the tone stack bleeds so much signal to ground in order to do it's job.  
Hope this helped,  
8/12/1999 2:40 PM
What would be the purpose of using a CF as the input stage like what Marshall did with their JCM900 SL-X?  
Their schem. shows: Input > CF > 1st gain stage > 2nd gain stage > CF > tone stack > Driver/PI > PA  
I'm wondering why they used a CF right at the beginning of the preamp.
8/12/1999 8:09 PM
Ken Gilbert
To be honest with you, there's NO reason, really. It doesn't serve a useful purpose in my book. If the CF were at your guitar, that would be another story, since it would be better at driving the cable and would present a low impedance drive signal, which is best for noise resistance. But to have one as the input stage is beyond me.  
Are you SURE there's no gain before that CF?  
It could also be just to pump up the tube count.  
8/12/1999 8:26 PM
Ken -  
The schematic from Marshall shows the guitar's input goes right into the first tube which is a CF. If it's just there to pump up tube count, then it's a definite waste. I'm going to trace it on the board and see if it's just like the schem shows.
8/12/1999 8:42 PM

I heard that there was one Marshall that had cathode followers between each gain stage. Sure seems like overkill to me. Maybe the circuit designer has a Conrad Johnson PV-5 preamplifier at home.
8/13/1999 12:22 AM

Or maybe he just likes the gentle rush of the added noise the CF with no gain will give you...

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