What is the purpose of a cathode follower?

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dazco6550
Junior tube assistant
Posts: 27
(2/26/04 8:36 pm)
 What is the purpose of a cathode follower?
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 Why do some amps use a cathode follower as tha last stage before a tone stack while others take the signal from the plate? What are the advantages to doing it one way or the other?
 
Casey4s
Senior tube assistant
Posts: 141
(2/26/04 8:53 pm)
 Re: What is the purpose of a cathode follower?
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 The purpose of the cathode follower is to bring the circuit to "Unity" before entering the tone stack.
 
dominotheory
Posts: 8
(2/26/04 9:08 pm)
 Re: What is the purpose of a cathode follower?
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 the cathode follower has a much lower output impedance so it loads tuned circuits less than a higher impedance stage that couples from the plate side. it basically acts as a buffer amp.
 
dazco6550
Junior tube assistant
Posts: 28
(2/26/04 10:00 pm)
 .
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 Ok, but why do some amps use one and some the other. I've seen similar marshall circuits where one uses a CF and the other the plate.
 
dominotheory
Posts: 9
(2/26/04 11:28 pm)
 Re: What is the purpose of a cathode follower?
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the tone stack loading lowers the gain if driven from the plate. if your amp has gain to spare (like most Marshalls) then you might not care. if you drive the tone stack from a cathode follower stage then you don't lose the gain, since the (nearly) unity voltage gain cathode follower buffers the plate stage.

some amps have an extra unused triode (half a 12AX7) so why not put it to good use? :-) you might also get some tone changes by using the cathode follower.
 
dazco6550
Junior tube assistant
Posts: 29
(2/26/04 11:34 pm)
 Re: What is the purpose of a cathode follower?
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 I see now. Thanks...
 
 
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