Cathode Follower Feed to Tonestack in Dumble Circuit

[Max]

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

In regard to "sort of":

As I wrote, I did not meet a "Cathode Follower Feed to Tonestack..." in one of the originals I am familiar with.

But what I did meet in some of the 150W amps have been stages after the tonestack, that did take some special kind of care not to load previous stages to much.

In regard to the "OD stage discussion" here: The ODS 150W uses a 7247 OD-tube.

Cheers,

Max

[llemtt]

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

actually exactly the contrary

even order harmonics requires an asymmetrical waveform

just take a look at some basic DSP textbook...

anyway talking about even vs odd harmonics it's meaningless, we human beings we perceive sound properties (such as timbre...) from the harmonic spectrum as a whole, it's the structure of the harmonic spectrum and it's evolution over time the relevant aspect

cheers
teo

[FUCHSAUDIO]

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

nyway talking about even vs odd harmonics it's meaningless, we human beings we perceive sound properties (such as timbre...) from the harmonic spectrum as a whole, it's the structure of the harmonic spectrum and it's evolution over time the relevant aspect

cheers
teo

Lets say "two stages of distortion sounds more euphonic than one does.." ? One stage being hit hard always sounds harder than two stages being hit not as hard. You have more overall gain, but end up sounding smoother...true though, semantics really. What sounds "better" depeds on what you play, and what YOU think "sounds better"...

af

[talbany]

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

actually exactly the contrary

even order harmonics requires an asymmetrical waveform

just take a look at some basic DSP textbook...

anyway talking about even vs odd harmonics it's meaningless, we human beings we perceive sound properties (such as timbre...) from the harmonic spectrum as a whole, it's the structure of the harmonic spectrum and it's evolution over time the relevant aspect

cheers
teo

#1 Circuit Types: Common Cathode Triode
"preamp distortion", used in the revered Bassman, Marshalls, modern tube preamps; frequency response easily made lowpass (sometimes unintentionally) by grid circuit and stray capacitances. Typically 2% distortion when "clean". Clips asymmetrically, although as typically used, cutoff is mushy and more of progressive compression than clipping, and "saturation" is defined by the grid going more positive than the cathode, with a dramatic drop in grid impedance when this happens, perhaps "saturating" the previous triode stage. Lots of even order distortion products until harder clipping reached, then third harmonic rises Almost no fifth or higher order harmonic production until massively over driven.
#2 Single ended Pentode
"output stage distortion", as in smaller, cheaper amps, like Fender Champ, several Gibsons, others.... Asymmetrical clipping, asymmetric distortion from the single ended transformer..... Distinct second and third harmonics. Very noticeable compression effects from screen grid bias drop, power supply sag.
#3 Double ended Pentode
"output stage distortion" as in large amps, including the Bassman..... Push pull operation cancels even harmonic distortion generated in the output stage, so it can contribute only odd harmonics, principally third. Noticeable bias shift when driven hard can cause compression..... Other compression effects include screen grid bias sag, and power supply sag. This may be the origin of the "tube rectifiers sound better" school of guitar amps, as silicon replacement power supply rectifiers give much less sag on signal peaks and higher power supply voltages, so sound cleaner. Symmetric distortion.
#4 Voltage feedback/biased bipolar - Germanium
"Fuzz Face" distortion. Single high gain stage saturates with guitar signal, is biased in a way that can not hard saturate, but can swing a long way towards cutoff without distortion. Low input impedance loads guitar and pre-filters out highs. Asymmetrical clipping, with prominent second harmonic. Prominent third, and noticeable fourth and fifth harmonics. This is the basic circuit in several vintage fuzzes,
Tony

[ayan]

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

This is entirely true. I submit that one or two stages of distortion doesn't mean symmetric VS asymmetric clipping, furthermore, it's very difficult to do symmetric clipping when there is a gain stage involved. [ You can, however, easily do so with two diodes... like a TS-9, and if you use three diodes, or differnt diodes you can really hear how asymmetric clipping is softer sound, more compressed, etc. ] You definitely cannot undo distortion, so if one stage only were to give you odd order harmonics, you couldn't get rid of them by adding another overdrive stage after it.

A tube can clip in one of two ways: one way is when the input control signal is large enough to clip part of the positive cycle of the signal. The grid voltage overcomes the cathode votage and the tube ceases to conduct, so you clip portion of one half of the waveform only. [ When taken to an extreme, this can cause the grid to begin drawing current and the whole thing sounds really bad, the tube at this point no longer behaves as as small signal amplifier and basically becomes a diode. ] The other way is when there is no voltage headroom left and the output signal gets clipped because it exceeds the plate voltage rails. Having two stages allows you to get more overdrive without making things ugly because each stage can add some distortion without either having the grid conduct (blocking type of distortion) or hitting the voltage rails.

While it is true that two stages allow you to get more distortion in a smoother way than achievable with a single stage, the whole even vs odd order harmonics is pretty much a separate thing and again, once you generate the "ugly" harmonics, there is no getting rid of them by adding more subsequent stages. By way of example, a Mesa Boogie Mark I amp is very smooth ang singing sounding, and it achieves that by adding a stage in front of a Blackface topology, such you basically have three stages before the tone stack. (Basically this is what Dumble did with his FET arrangement) The Mesa Mark IIA and B relocated the extra stage to after the tone stack, but still kept the total number of stagest to contributing to overdrive three and still sounded pretty smooth.

Cheers,

Gil

[thyx]

For me; the 220k/150k plate resistor is more gainy than the 100k/100k plate resistor.

Yes...assuming they're set up properly. And, as a rule, there is less linear distortion with the higher plate values...BUT, what little distortion is there tends to be more third-order. The lower plate values give you more linear distortion, but it is largely composed of second-order...which is at least partly why modern ODS amps sound "cold" to me as compared with the older, low-plate amps. Low plate has less gain overall, but a warmer sound...to my ear, anyway.

[ayan]

For me; the 220k/150k plate resistor is more gainy than the 100k/100k plate resistor.

Yes...assuming they're set up properly. And, as a rule, there is less linear distortion with the higher plate values...BUT, what little distortion is there tends to be more third-order. The lower plate values give you more linear distortion, but it is largely composed of second-order...which is at least partly why modern ODS amps sound "cold" to me as compared with the older, low-plate amps. Low plate has less gain overall, but a warmer sound...to my ear, anyway.

I agree with most of what you write. I don't know if I would say that the 100K sound is warmer per se, as the amps with lower plate values are definitely brighter sounding than the ones with higher plate values. But the sound is definitely more "pleasant" to my ears. The transition from clean to distortion in the 100K amps is different as well, being more gentle and "musical" for lack of a better word. In the higher plate amps, the onset of distortion seems to be more abrupt and with more grind.

Gil

[Structo]

I've been considering changing V1 to 100K/1K5.
Both tubes are high plate and cathode resistors now.

I like it where it's at but it's always fun to try new things.

[markusw]

Gil, thanks a lot!
I believe now there are couple of things clearer

So in theory one could bias the first OD stage to clip the top half of the wave and the second OD to clip the same half it again maybe with some inter stage filtering in between?
Is this done in any amp you are aware of or does it make any sense at all?

[markusw]

#1 Circuit Types: Common Cathode Triode
"preamp distortion", used in the revered Bassman, Marshalls, modern tube preamps; frequency response easily made lowpass (sometimes unintentionally) by grid circuit and stray capacitances. Typically 2% distortion when "clean". Clips asymmetrically, although as typically used, cutoff is mushy and more of progressive compression than clipping, and "saturation" is defined by the grid going more positive than the cathode, with a dramatic drop in grid impedance when this happens, perhaps "saturating" the previous triode stage. Lots of even order distortion products until harder clipping reached, then third harmonic rises Almost no fifth or higher order harmonic production until massively over driven.
#2 Single ended Pentode
"output stage distortion", as in smaller, cheaper amps, like Fender Champ, several Gibsons, others.... Asymmetrical clipping, asymmetric distortion from the single ended transformer..... Distinct second and third harmonics. Very noticeable compression effects from screen grid bias drop, power supply sag.
#3 Double ended Pentode
"output stage distortion" as in large amps, including the Bassman..... Push pull operation cancels even harmonic distortion generated in the output stage, so it can contribute only odd harmonics, principally third. Noticeable bias shift when driven hard can cause compression..... Other compression effects include screen grid bias sag, and power supply sag. This may be the origin of the "tube rectifiers sound better" school of guitar amps, as silicon replacement power supply rectifiers give much less sag on signal peaks and higher power supply voltages, so sound cleaner. Symmetric distortion.
#4 Voltage feedback/biased bipolar - Germanium
"Fuzz Face" distortion. Single high gain stage saturates with guitar signal, is biased in a way that can not hard saturate, but can swing a long way towards cutoff without distortion. Low input impedance loads guitar and pre-filters out highs. Asymmetrical clipping, with prominent second harmonic. Prominent third, and noticeable fourth and fifth harmonics. This is the basic circuit in several vintage fuzzes,
Tony

Copied, pasted, saved
Thanks a lot!

[thyx]

#3 Double ended Pentode
"output stage distortion" as in large amps, including the Bassman..... Push pull operation cancels even harmonic distortion generated in the output stage, so it can contribute only odd harmonics, principally third. Noticeable bias shift when driven hard can cause compression..... Other compression effects include screen grid bias sag, and power supply sag. This may be the origin of the "tube rectifiers sound better" school of guitar amps, as silicon replacement power supply rectifiers give much less sag on signal peaks and higher power supply voltages, so sound cleaner. Symmetric distortion.

This is a good working definition, and is valuable for understanding how the circuit functions...but the reality is that, unless the pair(s) of tubes are exactly matched for performance (in addition to current draw), there WILL be even-order harmonics generated. How much depends on how closely the tubes perform as compared to each other.

[talbany]

#3 Double ended Pentode
"output stage distortion" as in large amps, including the Bassman..... Push pull operation cancels even harmonic distortion generated in the output stage, so it can contribute only odd harmonics, principally third. Noticeable bias shift when driven hard can cause compression..... Other compression effects include screen grid bias sag, and power supply sag. This may be the origin of the "tube rectifiers sound better" school of guitar amps, as silicon replacement power supply rectifiers give much less sag on signal peaks and higher power supply voltages, so sound cleaner. Symmetric distortion.

This is a good working definition, and is valuable for understanding how the circuit functions...but the reality is that, unless the pair(s) of tubes are exactly matched for performance (in addition to current draw), there WILL be even-order harmonics generated. How much depends on how closely the tubes perform as compared to each other.

The output stage will only cancel even order harmonics that are generated _in_ the output stage due to the symmetry and then, only if the AC signal drive, the tubes, and the transformer are balanced like you say (to a point).Just to clarify further even order harmonics generated in the preamp stages will be passed through the output stage to your speakers. If you don’t believe this, crank up a master volume Marshall until the cathode follower clips on the bottom side, indicating a large second harmonic component. Keep the master volume low, and look at the waveform going to the speaker. It will look nearly identical. If the preamp-generated second harmonic was cancelled in the output stage, the output waveform would again be a sinewave at the fundamental frequency, but this doesn’t happen. What actually happens in the output stage is that the ratio of even to odd harmonics shifts; that is, the third becomes larger than the second, in some cases. In others, you end up with a better balance of second and third, as indicated by a symmetrically-clipped top and bottom, but a varying duty-cycle of the waveform. The symmetrical clipping indicates high third, fifth, etc., harmonics, while the asymmetrical duty cycle indicates high second and other even order harmonics. Guitar amps don’t sound good if the distortion is all even or all odd harmonics; rather, you need a balance of both, preferably a dynamically changing balance. This is why diode clipping sounds thin and buzzy. It is predominantly odd (with large amounts of higher order odd harmonics that sound really bad even in small amounts), and the clipping levels are fixed, and don’t vary with pick attack. Single-ended amps are not immune to generating odd order harmonics, either. A single-ended pentode stage biased close to midpoint of the class A range will generate almost all odd order harmonics, while a single-ended triode stage biased at the same point will generate a predominant second order harmonic, followed by a decreasing series of odd and even order harmonics. You can’t always tell what is going on in an amplifier by looking at the oscilloscope; an audio bandwidth spectrum analyzer is the best tool for decoding harmonic content, and, if properly used in conjunction with your ears, can lead to a much better understanding of the tradeoffs between different circuit design techniques and their impact on tone.Bla Bla Bla!! Hope this helps..outta here!!


Tony