Cathode Follower Feed to Tonestack in Dumble Circuit

[talbany]

Carlton's amp.... Sort of.....

Dave
Iv'e done some experimenting with CF within various gain stages and here is what I concluded..
CF set up like a buffer (Marshall type) lower impedance so in this situation( a low-level signal application), you want maximum voltage transfer, not maximum power transfer (Like output section) in order to achieve this the source impedance must be at least 10 times lower than the load impedance, you're okay, because you'll get 90% of signal transfer so you need to be aware of this when implementing the low impedance source within the gain stages..If not you will end up loosing quite a bit of voltage/gain and the amp will sound choked..Also The lower the source impedance, the less high end you'll lose in the preceding gain stages so the buffer becomes a sort of treble booster (Generally a no no when designing a OD stage)..CF generally work best when using long cables like the front of your pedal board, or effects loops for outboard gear or in the caes of the Bassman driving tone stacks that preceed the OP section and of coarse feeding the OP tubes grids for greater headroom..The only place where a CF might be implemented in an ODS style amp is before the HRM tonestack.. I remember several trying this and commenting that it really didn't buy them much in the way of tone..Everybody is different your ears might be different... Good Luck!!
Layout provided by Normster

Actually pretty "anti-dumble" imho. The whole premise of the D-style OD is a pair of gain stages, each clipping one side of the waveform to produce even order and balanced overdrive. Doing one OD stage and a CF is more Marshall-like, and heaver on the odd-order harmonics, with a harder clip. More aggressive for sure, even when tamed by a post OD stack or trimmer. I haven't much like LC's od tones for the last few years, now I know why...lol.

Andy
It's my understanding CF don't impart a phase shift (hence the word follower) so you should still clip both top and bottom..Not sure what you mean can you elaborate... I know the lower impedance would decrease the load on the preceding stage which would effect harmonic structure no doubt more aggressive top like you say Marshall-ish and a few other things depending on how you set up the follower....

Kind Regards
Tony

[FUCHSAUDIO]

The 'rumor" somewhat substantiated by Scottl hearing what a certain amp builder saw, was that Carlton's favorite Dumble had a single OD stage and CF, not unlike the Bassman and Marshall style circuits. Not sure about post OD stacks or anything else, but Carlton claimed "he never much liked the OD", but his OD was quite UN Dumble like, by a long shot !

In a D-style OD, a pair of similar gain stages in series (compensating for the different signal levels at each circuit), is basically designed to clip the top and bottom of the waveform, producing a somewhat linear distortion, equal in clipping, and more even harmonics as a result. A single stage driven hard (whether followed by a CF or not) would clip in a more harsh fashion, with more third order harmonic, and more clipping on one side than another, due to grid clipping (the AC value starts to affect DC bias).

A properly designed CF basically converts the impedance downward, is unity (actually about a 5% loss) in gain. Although it reduces loading on the previous stage, and makes driving the following device (whether a pot or a tone stack or a power tube grid etc) easier. This changes the circuit's response and touch etc...I don't think it's a bad thing. Many Ampeg preamps did this btw.

http://www.schematicheaven.com/ampegamps/svtpreamp.pdf

It also gives you more latitude as far as tone shaping, as it's tougher to load down than a standard resistance coupled triode might be.

[talbany]

The 'rumor" somewhat substantiated by Scottl hearing what a certain amp builder saw, was that Carlton's favorite Dumble had a single OD stage and CF, not unlike the Bassman and Marshall style circuits. Not sure about post OD stacks or anything else, but Carlton claimed "he never much liked the OD", but his OD was quite UN Dumble like, by a long shot !

In a D-style OD, a pair of similar gain stages in series (compensating for the different signal levels at each circuit), is basically designed to clip the top and bottom of the waveform, producing a somewhat linear distortion, equal in clipping, and more even harmonics as a result. A single stage driven hard (whether followed by a CF or not) would clip in a more harsh fashion, with more third order harmonic, and more clipping on one side than another, due to grid clipping (the AC value starts to affect DC bias).

A properly designed CF basically converts the impedance downward, is unity (actually about a 5% loss) in gain. Although it reduces loading on the previous stage, and makes driving the following device (whether a pot or a tone stack or a power tube grid etc) easier. This changes the circuit's response and touch etc...I don't think it's a bad thing. Many Ampeg preamps did this btw.

http://www.schematicheaven.com/ampegamps/svtpreamp.pdf

It also gives you more latitude as far as tone shaping, as it's tougher to load down than a standard resistance coupled triode might be.

Andy
Thanks for elaborating on the subject and agree..Loading especially in OD circuits produce for me a more desirable tone and feel...
CF shine at
Driving signal through long cable runs
Interfacing with outboard gear
Driving output sections or tonestacks w/master volumes (in some designs)
In that order..My opinion..
All The Best..I love this stuff!!

Tony

[markusw]

In a D-style OD, a pair of similar gain stages in series (compensating for the different signal levels at each circuit), is basically designed to clip the top and bottom of the waveform, producing a somewhat linear distortion, equal in clipping, and more even harmonics as a result.

Sorry if this is a dumb question
I always thought that asymmetric soft clipping would result in more even order harmonics.

[FUCHSAUDIO]

I believe that's what I said...?

[markusw]

I believe that's what I said...?

Isn't it symmetric clipping if the bottom and the top of the waveform are clipped
Sorry, I really don't want to complicate things. Just trying to learn...

Peace,

Markus

[FUCHSAUDIO]

symmetric clipping gives more 2nd order, non-symmetric gives more third order harmonics, especially when driven hard.

[markusw]

symmetric clipping gives more 2nd order, non-symmetric gives more third order harmonics, especially when driven hard.

Thanks a lot for your answer!
But if a D-style OD is designed to clip the top and bottom of the waveform (i.e. symmetric clipping) wouldn't it produce more 3rd order harmonics?
Sorry, I'm really confused now....
Or is my understanding of symmetric clipping simply wrong?

[FUCHSAUDIO]

https://ccrma.stanford.edu/~jos/pasp/No ... rtion.html

[markusw]

https://ccrma.stanford.edu/~jos/pasp/No ... rtion.html

Thanks!
Will take a while to understand....

[talbany]

https://ccrma.stanford.edu/~jos/pasp/No ... rtion.html

Thanks! :D
Will take a while to understand.... :D

Markus
to put it in simpler terms.. Symmetrical clipping occurs when both halves of the waveform clips evenly (although in our application they never really do perfectly)
Asymmetrical refers to half of the waveform clips differently (Harder or softer).. Asymmetrical clipping generates generally 2rd order harmonics (from the fundamental) ... When a cathode follower clips it produces more 2nd order harmonics (which is why you want to push them)... In our application symmetrical clipping produces a thicker tone however not a smooth (especially when playing chords since 3rds play a key role in chord structure)
Much of how the tube clips is determined by the plate and cathode resistor of coarse.. A good example of this is the high plate amps that clip more asymmetrically than the 2 100k low plates

Tony

[markusw]

Thanks a lot for your help, Tony!
Is the more asymmetric clipping of the high plates caused by the difference of the plate resistors (e.g. 220k and 150k) or just by the higher plate resistors. In other words would a 2x220k also produce more asymmetric clipping than a 2x 100k OD?

[talbany]

Thanks a lot for your help, Tony! :D
Is the more asymmetric clipping of the high plates caused by the difference of the plate resistors (e.g. 220k and 150k) or just by the higher plate resistors. In other words would a 2x220k also produce more asymmetric clipping than a 2x 100k OD?

The higher value plate resistors do effect how the tube clips since it puts you in a different part of the curve but primarily speaking it's the difference when your talking asymmetrical..

[erwin_ve]

The higher value plate resistors do effect how the tube clips since it puts you in a different part of the curve but primarily speaking it's the difference..

+1: Looking at only the triode; the higher the plate resistor, more early you get non-linear distortion.

I think it's a balancing act between generating harmonics and clipping.
For me; the 220k/150k plate resistor is more gainy than the 100k/100k plate resistor.

[David Root]

Fascinating stuff and thank you all!

So if I read this correctly, in any two stage triode gain block, whether it is a clean block or an OD block, with or without a tonestack in it, putting a CF in between the two triode stages interferes with the symmetric even harmonic clipping and generates more odd harmonics.

I also think I heard that a single OD stage (presumably either triode or pentode), followed by a CF does not sound too euphonious either, compared with a properly gain equalized triode pair.

Thus if even (mostly second) harmonics generation is what I want I should forget about CFs altogether.

Is that a fair summary?