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cathode follower, purpose? and effect on tone.

10/21/2000 4:57 PM
Ricky cathode follower, purpose? and effect on tone.
what exactly is the purpose of the cathode folower stage? I know it dosen't have any gain of its own, so is it there to ballance the signal somehow before the tone stack? also, how does it sound when it clips as opposed to a regular gain stage?  
another thing, my amp is set up like this; stage1> volume> stage2> tonestack> long tail PI> cathode-biased PP 6V6s. my amp has too much gain, so if I converted stage 2 to a cathode follower stage, would it bring down the gain to more reasonable levels?  
My goal here is to turn the amps gain down some so I am just getting clean and lightly overdriven sounds from it. I have a soldano preamp I use to get medium and heavy sound with. as it is, I am only using half of the volume knob and the other half is unusable because there is too much gain. this pushes the 6V6s into "flub zone" way too early. I have used lower gain tubes(12ay, 12au7) and they bring down the gain significantly, but I wanted to experiment with other ways to bring down the gain as well(with the cathode follwer) especially if they impart a tone of their own.  
thanks, Ricky
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10/22/2000 2:10 AM
Chris B

A cathode follower is used to lower the impedance of the signal from the previous tube or tubes. I couldn't tell you if a cathode follower will work correctly in your amp. I'll leave that to someone else. Another way to reduce gain is to run the signal through a resistor. I'd start with 100k and go from there. This also cuts treble so you may want to use bypass caps along with the resistors. Look at a Marshall schematic to see what I mean.  
You could also increase the size of the cathode resistor on one of the two gain stages. Start small and work your way up untill you get what you want.  
Hope this helps!:D  
Chris B
10/22/2000 2:30 AM
yes, I have a resistor between those two stages, 500k. the gain thing isn't that much of a problem really, because I can always use a lower gain tube. I'm just really curious as to whether or not I would have enough gain if I ran a cathode follower.  
10/22/2000 2:44 AM
Chris B

I don't know either, for certain. I've run 6BQ5's with a single triode before a long-tailed PI and it had plenty of output, but I don't know if a high impedance is needed for the PI to function correctly. Maybe one of these other guys can help. I'd like to know the answer to that my self!  
Chris B :)
10/23/2000 12:46 AM
I saw in the schematics section on this site, an amp called "the lark" I believe. It was set up close to the amp you're descibing. was your amp mostly clean with a little bit of grind when the volume is close to being maxed? this seems like a way to get almost all power tube distortion, whith hardly any coming from the preamp.  
10/23/2000 2:38 AM
Chris B

The "Lark" is the design that I started with, and then went to the Matchless Spitfire because I wanted more gain, and finaly went to a Fender type circuit with a split load PI because it was easier to wire point to point. There is also an article about the "Lark" in the homebuilt amp gallery here on Ampage and a link to the builders home page, KENZPAGE. He's made several other cool amps as well.  
Chris B
10/22/2000 8:52 AM
A tweed deluxe only has one stage of gain before the driver/PI. The other half of the input tube is used for the other channel. If you were going to only have one input you could use the other stage as a cathode follower.  
I would probably put the CF right after the first stage followed by the tone stack, followed by the volume.  
You're right, the CF won't give you any gain, but it will lower the impedance. Actually, it has a gain of less than 1, so you'll lose a little bit through it, but I believe that with the lower impedance you will lose less signal through the tone stack. I'm not really sure how to explain this, but I think is a pretty good "thumbnail view".  
If I understand correctly, a driver configured as a gain stage and concertina splitter will yield more gain than a long tail pair. (Although there must be something wrong with my understanding here because most amp designs seem to have moved away from the gain stage/split load to the long tail. I don't get it.)  
This setup will pretty much give you a tweed bassman without the very first stage(and a kind of MV), but you will likely be able to generate more signal with the preamp than with a single triode stage.  

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