Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on January 23, 2012, 11:48:27 am
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where is the best position for a cathode follower ? cathode follower( V2a) then tone stack, then V2b OR V2a then cathode follower ( V2b ) then tone stack ?
Colas
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My guess would be anywhere as long as the circuit design is done with that in mind! You have several examples... take the revered tweed bassman 5F6-A or a typicall marshall, where the cathode follow is after the second gain stage, feeding the tonestack. Then, look at the new marshall class 5 with a cathode follower feeding a tonestack imediatly after the first gain stage. Then look at komet concorde where you have a gain stage, a plate fed tonestack, two more gain stages followed by a cathode follower which feeds directly into the phase inverter.
Every single one of these amps sounds nice in it's own way. So it's up to you.
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In fact my question would have been more this one: ''impedance wise, what is the difference'' ? A cathode out to the grid of a 12ax7 then to a power tube vs a cathode out right into a power tube. ( SE in this example). As I can see, after your examples, either ways are good and it is only a question of taste, and that I wasn't quite sure.
Regards
Colas
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Actually, I submit that the question is confusing, by conflating how the tonestack is driven, with its location in the signal path. There can be gain stages before, and then after, the tonestack. This is a concept/design decision: where to put the tonestack? The gain-stage/tonestack topology will have a major effect on the tone and feel of the amp. See: Dave Funk's "Tube Amp Workbook".
How to drive the tonestack -- by plate or by cathode -- is a separate issue.
(@ bruno: I think that Colas is focusing on using the cathode follower specifically to drive a tonestack)
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There can be gain stages before, and then after, the tonestack. This is a concept/design decision: where to put the tonestack? The gain-stage/tonestack topology will have a major effect on the tone and feel of the amp.
Earlier vs later in the circuit yields feelings of bluesier vs crunchier in general terms. Also it can cause the amp to have a sense of greater vs lesser overall impact w/in their settings.
Colas you need to show a basic drawing which designates what V1, V2, and/or v3 are to make any real sense of what you're trying to describe or say as it relates to your designations.
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Perhaps run Duncan's tone stack calculator. Load in the desired tone stack parameters, then play with the input impedance and see what happens. Personally, I'm not using tone stacks very much lately. They're lossy so you have to account for that and make up the gain, + both of my guitars have tone controls which are a lot easier to reach. I'm finding fewer knobs sounds better. Less is more. Bauhaus.
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I have not spoken about using a tone stack. I never use one. Just like you say: less is more. I was wandering about the effect of amplifying or not a cathode follower in a SE amp before the signal reaches the output tubes and if it 'd change the impedance and thus the overall tone of the amp. I have not a particular schems to show, but you know all that lots of amps have their cathode follwer right before the PI ( in case of a pp , or the power tube in a se configuration ) , and lots of amps right b4 a next gain ( second half ).
Cheers !
Colas
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The 5F6A has the dc-coupled pair (with CF stage) driving the tone stack after the channel mixing stages.
Another classic fender bassman, the 6G6B, has a DC coupled pair in its ever-popluar (with surf/rockabilly guitaristists) 'bass channel' as the input stage.
Both amps sound amazing for their own purposes. You have to decide what you want. Maybe an amp with a 5F6A channel and a 6G6B bass channel?, both being mixed into the LTP?
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I have not spoken about using a tone stack. I never use one.
But: "where is the best position for a cathode follower ? cathode follower( V2a) then tone stack"
OK: so it looks like Bruno was on the right track!
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I'm confused by the question. You insert a CF where it is needed or might benefit the circuit. What does a CF do/not do? It doesn't invert and it doesn't have voltage gain. It does lower the output impedance and it does have greater output current available. This makes it conducive to driving longer distances or heavier loads. Some tone stacks get pretty lossy -n- loady. A line out may have a long geetar cord plugged into it. Z can get wonky in summing.
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Ditto