My theories on amp tone, why they sound like they sound.

[ELS]

after building many amp clones and experimenting with my own circuits I've started coming up with my own theories on why amps like the bassman, plexi, or blackface fenders sound as they do.
of course there's things like parts values and circuit topography, but some perhaps overlooked things like:
* an AC loaded gain stage will be shifted into a cooler bias, but this makes the sound much more saturated and 'creamy' than just a cold biased gain stages, that's why the bassman when overdriven sounds really warm and creamy (until pushed into output tube distortion) reason being the cathode follower going into grid emission and loading down the stage driving it, and blackface fenders sounding warm all the time reason being the 3 gain stage on the vibrato channel which has an AC load of approximately 50k.
* a sizzly attack that of great sounding bassman amps, and many marshall amps is caused by the output tubes having a slight parasitic oscillation on the attack of a hard note, but putting control grid stopper resistors more often than not reduces this effect or eliminates it entirely in modern amps. that's why I always stray towards a good layout rather than adding grid stoppers to fix oscillations.
* screen current makes the amp sound balsy when going into distortion, while I didn't see much change in overdrive tone in increasing screen resistor current, the way the amp went into overdrive was a night and day difference, large screen resistors made a smooth change from clean to overdriven, but lower ones really gave you the feeling of power, hard to describe the sound but maybe I can compare it to when stevie ray vaughan would have a bold sounding fierce solo, and he would end it with a hard slide down the frets which would sound immensely powerful. but in my experience anything bellow 470ohms for 6L6oid tubes will red plate the screens so I don't recommend going down that low. (although the tubes lasted me a week of gigging with the screens glowing red when I had 330ohm screen stoppers and they were fine, so experiment at your own risk)
* fewer gain stages and higher plate voltage just make a better sounding amp, in my experience adding more preamp stages made the amp sound much worse on clean tones, and it lacked the feeling of power that amps with less stages had. although my reasoning on this is really vague, being that 'it just happens' and I'm not satisfied with that, I still try to get away with a classic, 2 preamp stages and a phase inverter lineup in any amp.

I also have even more vague thoughts about phase inverters:
* the Cathodyne is:
less compressed,
muddy, thick bass, that doesn't seem to break up nicely, it more just 'farts-out' on the attack, when it does break up
slightly brittle on the treble.
* the LTP is:
more compressed,
weaker bass, but distorts better,
more 'bold' sounding.
I love the sound of a cathodyne for some push pull EL84 hifi amps tho, it sounds really 'big', I can't describe why exactly but I haven't recreated that sound with any other PI, for a hifi amp.

I haven't experimented much with differences in tone from part choice, but I am open to discussion about tone capacitors, I think it's reasonable to think that having a different type of capacitor will change the tone of the amp, for example some russian capacitors, in the datasheet there's a chart showing that they are really lossy at higher frequencies while pass more signal on lower ones (all in the audio range).
I use modern film caps and some modern ceramic caps, and metal film mil spec soviet resistors in my amps for testing.

note: these are just my opinions, on things that I've experienced, I'm not saying it is how I say it is, but just spreading my stories.
feel free to add your own opinions on circuit design if you want!

 

[ELS]

I'd also like to add a design flaws I see in many modern amps that annoys me
*when connecting a potentiometer as a variable resistor, connect the unused terminal to the center lug, this ensures that if the wiper fails to make contact, it will just act as if the pot is turned to the maximum resistance, instead of open.
*also try to have the wiper and subsequently the unused terminal connected to the lower impedance side, because although I'm not sure, it would seem obvious that the wiper would attract more noise than the graphite strip.
*always consider wire capacitance, especially for output tube wires, wires running beside each other form a capacitor, you can exploit this too for in place of the plate-plate cap on the phase inverter, but in most cases this fact will just cause problems.
* wire string wrapping is pretty nice, makes for an amp that looks like it was mil spec field equipment.
* don't go all out on filter capacitor values, not only for tube rectifiers there is a max filter capacitance value for the 1st node, but also think of the * sag, there always is gonna be some, which will act as a compressor, the larger the capacitance and impedance that feeds the capacitor, the longer the release time, I recommend PSU Designer 2 by DuncanAmps for seeing how much sag and how long of a release time there is.
and if you're afraid of hum, you don't have to worry about that a lot in push pull audio stages because it cancels out, and it's been found that power supply ripple is a big reason for amps like the tweed bassman sounding so good.
* also ventillation is key, the more ventillation there is the longer the tubes are gonna last, and tubes also emitt a lot of infrared light, which will heat up everything around them, it's best to have some opaque barrier between tubes and heat sensitive parts like capacitors. you also can exploit heat to cool other parts down with a phenomena called convection, which in short means hot air go up, cold air get sucked up bellow.

 

[playloud]

This is really interesting. Thanks for posting.

I wonder if it would be possible to create a "taxonomy of guitar amp circuits", ideally with an accompanying tree diagram? Then you could take the subjective tonal impressions and work backwards to figure out what aspects of the topology account for the differences.

 

[Matthews Guitars]

Ultimately it's all EQ curve and transfer functions. Transfer functions are dynamic and thus variable according to the operating parameters of the tubes and other devices in the amplifier. All the rest are just factors that influence the transfer functions and EQ curves.

What affects tone in a guitar amp?

Everything. Literally everything. Some things have a dramatic effect, some things have no effect that can be detected with your ears. But everything affects tone.

Overdrive character is probably affected more by the class of circuit operation than anything else. Or single-ended vs. push-pull circuits. That's a huge one.

A push-pull stage self-cancels even harmonics. A single ended stage does not. That is really going to make a difference as far as the harmonic character of the overdrive. It's the number 1 reason why low power tweed amps (with single ended output stages) sound so different from low power push-pull amps.

 

[StrummerJoe]

after building many amp clones and experimenting with my own circuits I've started coming up with my own theories on why amps like the bassman, plexi, or blackface fenders sound as they do.
of course there's things like parts values and circuit topography, but some perhaps overlooked things like:
* an AC loaded gain stage will be shifted into a cooler bias, but this makes the sound much more saturated and 'creamy' than just a cold biased gain stages, that's why the bassman when overdriven sounds really warm and creamy (until pushed into output tube distortion) reason being the cathode follower going into grid emission and loading down the stage driving it, and blackface fenders sounding warm all the time reason being the 3 gain stage on the vibrato channel which has an AC load of approximately 50k.
* a sizzly attack that of great sounding bassman amps, and many marshall amps is caused by the output tubes having a slight parasitic oscillation on the attack of a hard note, but putting control grid stopper resistors more often than not reduces this effect or eliminates it entirely in modern amps. that's why I always stray towards a good layout rather than adding grid stoppers to fix oscillations.
* screen current makes the amp sound balsy when going into distortion, while I didn't see much change in overdrive tone in increasing screen resistor current, the way the amp went into overdrive was a night and day difference, large screen resistors made a smooth change from clean to overdriven, but lower ones really gave you the feeling of power, hard to describe the sound but maybe I can compare it to when stevie ray vaughan would have a bold sounding fierce solo, and he would end it with a hard slide down the frets which would sound immensely powerful. but in my experience anything bellow 470ohms for 6L6oid tubes will red plate the screens so I don't recommend going down that low. (although the tubes lasted me a week of gigging with the screens glowing red when I had 330ohm screen stoppers and they were fine, so experiment at your own risk)
* fewer gain stages and higher plate voltage just make a better sounding amp, in my experience adding more preamp stages made the amp sound much worse on clean tones, and it lacked the feeling of power that amps with less stages had. although my reasoning on this is really vague, being that 'it just happens' and I'm not satisfied with that, I still try to get away with a classic, 2 preamp stages and a phase inverter lineup in any amp.

I also have even more vague thoughts about phase inverters:
* the Cathodyne is:
less compressed,
muddy, thick bass, that doesn't seem to break up nicely, it more just 'farts-out' on the attack, when it does break up
slightly brittle on the treble.
* the LTP is:
more compressed,
weaker bass, but distorts better,
more 'bold' sounding.
I love the sound of a cathodyne for some push pull EL84 hifi amps tho, it sounds really 'big', I can't describe why exactly but I haven't recreated that sound with any other PI, for a hifi amp.

I haven't experimented much with differences in tone from part choice, but I am open to discussion about tone capacitors, I think it's reasonable to think that having a different type of capacitor will change the tone of the amp, for example some russian capacitors, in the datasheet there's a chart showing that they are really lossy at higher frequencies while pass more signal on lower ones (all in the audio range).
I use modern film caps and some modern ceramic caps, and metal film mil spec soviet resistors in my amps for testing.

note: these are just my opinions, on things that I've experienced, I'm not saying it is how I say it is, but just spreading my stories.
feel free to add your own opinions on circuit design if you want!

and thank you for a very thoughtful and informative post.

 

[EJstrat&JVM]

You have the knowledge to start a new amp brand.

 

[Eric'45]

I must confess I don't understand much about the internal workings of Guitar Amps, but I have huge respect for those who do.

 

[ELS]

Overdrive character is probably affected more by the class of circuit operation than anything else. Or single-ended vs. push-pull circuits. That's a huge one.

A push-pull stage self-cancels even harmonics. A single ended stage does not. That is really going to make a difference as far as the harmonic character of the overdrive. It's the number 1 reason why low power tweed amps (with single ended output stages) sound so different from low power push-pull amps.

don't overlook the tube type tho, pentodes have a really complex overdrive characteristic. especially in push pull output designs.
and even something as similar as a 6V6 vs an EL84 will have vastly different overdrive, when the tubes are so similar apart for there being beam forming plates instead of a suppressor grid.

 

[Pete Farrington]

an AC loaded gain stage will be shifted into a cooler bias

I don’t see it like that. Rather the bias point remains the same, but the effective loadline becomes steeper, probably reducing the max output voltage swing. And stage gain, as it’s in parallel with the anode resistance and anode load resistor.

the bassman when overdriven sounds really warm and creamy (until pushed into output tube distortion) reason being the cathode follower

Unless a master volume is fitted and turned down, the output valves are the earliest stage to clip. If the amp is to reach its max power output, how could it be otherwise?
By ‘earliest stage to clip’ I mean that given reasonable control settings, as the input signal level rises from 0, it’s the first stage at which clipping occurs.
A typical direct coupled cathode follower (DCCF) stage tends to be fairly linear until very high signal levels, by which point the output valves will already be clipping, probably quite heavily.
For DCCF analysis, see http://www.valvewizard.co.uk/dccf.html

blackface fenders sounding warm all the time reason being the 3 gain stage on the vibrato channel which has an AC load of approximately 50k

The normal channel sounds warm too though. The non reverb 2 channel models have 2 gain stages in the trem channel.
And if the 50k load of the trem intensity control is disconnected, the tone may be perceived as getting warmer, rather than colder, as your thinking might suggest?

bassman … the cathode follower going into grid emission and loading down the stage driving it

You may be thinking of grid conduction/ grid current clipping? That’s a standard clipping mechanism, see section 1.14 of http://www.valvewizard.co.uk/Common_Gain_Stage.pdf
Grid emission is a different thing again.

while I didn't see much change in overdrive tone in increasing screen resistor current, the way the amp went into overdrive was a night and day difference, large screen resistors made a smooth change from clean to overdriven

A screen grid resistor’s primary purpose is to limit screen grid current (and hence screen grid dissipation) at high signal levels, especially overdriven.
It also may act as a grid stopper, as the screen grid is a grid and exhibits transconductance, so a resistor can help with stability.
One side effect of having a non decoupled resistor there is that it will create (local) negative feedback, in a similar manner to an unbypassed cathode resistor. The higher the resistor value, the greater the degree of negative feedback.
Another is that at high signal level, when the screen grid draws more current, the resistor drops the effective voltage between the screen grid and its cathode, thereby reducing anode current. So yes, the lower the resistor value, the higher the peak anode current and hence the higher the amp’s power output.
And conversely, the higher the resistor value, the lower the power output, and a perception of a compression effect.

lower ones really gave you the feeling of power,

Yes, it’s not just a feeling of more power, it really is more power.

ensely powerful. but in my experience anything bellow 470ohms for 6L6oid tubes will red plate the screens

It’s a balance with the anode loadline (and dissipation). The steeper that is, the lower the screen grid current is, hence a lower resistor value may be tolerated.
But as more signal power is squeezed out of a valve, eventually something has to give, either the anode or the screen grid or both will over dissipate.

even something as similar as a 6V6 vs an EL84 will have vastly different overdrive, when the tubes are so similar apart for there being beam forming plates instead of a suppressor grid.

It’s back to the screen grid characteristics for that. Suppressor grid pentode valve types tend to draw more, perhaps about twice, the screen grid current of beam pentodes. So the effects I described above will be that much more prominent.

Your post is regarding ideas, thoughts, rather than theories per se.
Experiments might be used to develop them into a working hypothesis.
Theories are a proven explanation for observed behaviour / phenomena.

 

[ELS]

I don’t see it like that. Rather the bias point remains the same, but the effective loadline becomes steeper, probably reducing the max output voltage swing. And stage gain, as it’s in parallel with the anode resistance and anode load resistor.

reducing the plate load resistor will shift bias more into cutoff. (not related to what I'm saying in the next sentence)
rotating the loadline steeper while having the bias point stay the same will be similar to shifting bias into cutoff, I didn't meant it literally and yes I should've made that clear in the post.

`
Unless a master volume is fitted and turned down, the output valves are the earliest stage to clip. If the amp is to reach its max power output, how could it be otherwise?
By ‘earliest stage to clip’ I mean that given reasonable control settings, as the input signal level rises from 0, it’s the first stage at which clipping occurs.
A typical direct coupled cathode follower (DCCF) stage tends to be fairly linear until very high signal levels, by which point the output valves will already be clipping, probably quite heavily.
For DCCF analysis, see http://www.valvewizard.co.uk/dccf.html
`
not always, and not in the bassman, if you check it yourself, the cathode follower clips right before the output, and that wasn't my point anyway, the point was that the AC Loading of the cathode follower made it sound warmer when that happened.
not every cathode follower circuit. and even in the article you sent it mentions this bassman circuit that does this.
also also, with age carbon comp resistors tend to drift up in value, the more the cathode resistor of the 2nd stage goes up, the sooner the CF clips.

`
The normal channel sounds warm too though. The non reverb 2 channel models have 2 gain stages in the trem channel.
And if the 50k load of the trem intensity control is disconnected, the tone may be perceived as getting warmer, rather than colder, as your thinking might suggest?
`
I wouldn't say so, never liked the normal channel on those amps.
define 'warmer' in that case, because you'd double gain by disconnecting the tremolo pot, which the extra gain you might hear as 'warm'
also that would increase the bass response, also something you could hear as 'warmer'
`

bassman … the cathode follower going into grid emission and loading down the stage driving it

You may be thinking of grid conduction/ grid current clipping? That’s a standard clipping mechanism, see section 1.14 of http://www.valvewizard.co.uk/Common_Gain_Stage.pdf
Grid emission is a different thing again.
`
grid conduction is another name for grid emission, when grid emission occurs the grid conducts. grid current clipping is what happens when you have a high impeadance signal fed into a stage that is drawing grid current.
grid emission sometimes is used to describe the thing that is responsible for the grid to shift to a negative voltage in grid leak bias.


`
A screen grid resistor’s primary purpose is to limit screen grid current (and hence screen grid dissipation) at high signal levels, especially overdriven.
It also may act as a grid stopper, as the screen grid is a grid and exhibits transconductance, so a resistor can help with stability.
One side effect of having a non decoupled resistor there is that it will create (local) negative feedback, in a similar manner to an unbypassed cathode resistor. The higher the resistor value, the greater the degree of negative feedback.
Another is that at high signal level, when the screen grid draws more current, the resistor drops the effective voltage between the screen grid and its cathode, thereby reducing anode current. So yes, the lower the resistor value, the higher the peak anode current and hence the higher the amp’s power output.
And conversely, the higher the resistor value, the lower the power output, and a perception of a compression effect.

Yes, it’s not just a feeling of more power, it really is more power.
It’s a balance with the anode loadline (and dissipation). The steeper that is, the lower the screen grid current is, hence a lower resistor value may be tolerated.
But as more signal power is squeezed out of a valve, eventually something has to give, either the anode or the screen grid or both will over dissipate.

`
yes, but it also shapes the tone of the amp.
by 'sounds powerful' I meant that it sounds powerful, not that it is powerful. I adjusted for volume after the dummy load to have a fair comparison, and it did not sound the same, just with a change from 470 to 2.2k it totally lost the 'powerful' tone

`
It’s back to the screen grid characteristics for that. Suppressor grid pentode valve types tend to draw more, perhaps about twice, the screen grid current of beam pentodes. So the effects I described above will be that much more prominent.

Your post is regarding ideas, thoughts, rather than theories per se.
Experiments might be used to develop them into a working hypothesis.
Theories are a proven explanation for observed behaviour / phenomena.
`
yeah then why with part values accounted for those differences do the tubes sound completely different?
even in comparison videos on youtube you can hear the differences in tone clear as day (albeit they dont change other values in the circuit)
you feel the difference even more in person, even when you can't hear any difference in audio clips, you can feel it in person.

for example:
EVH's brown sound has been said to be the result of his amps running at reduced line voltage.
do you hear the difference in audio clips between full line voltage and reduced? pretty much not.
but the amp responds way slower, it's a totally different beast to play with reduced line voltage, even if to others it doesnt sound different, the fact is that it will sound different when you actually play trough it.

not everything can be explained on a calculator (do not take this 100% literally)
music is a form of art. you don't go to an art museum and try to explain why a canvas splattered with paint doesn't mean crap, it just does, to many people.
call it being brainwashed, or call it what you will. but you will just go crazy trying to explain every aspect of a tube amp in full detail and trying to replicate an amp that it is not, but some 'superstitious' choice in amp parts will have no problem giving you results you want.

 

[_Steve]

after building many amp clones and experimenting with my own circuits I've started coming up with my own theories on why amps like the bassman, plexi, or blackface fenders sound as they do.
of course there's things like parts values and circuit topography, but some perhaps overlooked things like:
* an AC loaded gain stage will be shifted into a cooler bias, but this makes the sound much more saturated and 'creamy' than just a cold biased gain stages, that's why the bassman when overdriven sounds really warm and creamy (until pushed into output tube distortion) reason being the cathode follower going into grid emission and loading down the stage driving it, and blackface fenders sounding warm all the time reason being the 3 gain stage on the vibrato channel which has an AC load of approximately 50k.
* a sizzly attack that of great sounding bassman amps, and many marshall amps is caused by the output tubes having a slight parasitic oscillation on the attack of a hard note, but putting control grid stopper resistors more often than not reduces this effect or eliminates it entirely in modern amps. that's why I always stray towards a good layout rather than adding grid stoppers to fix oscillations.
* screen current makes the amp sound balsy when going into distortion, while I didn't see much change in overdrive tone in increasing screen resistor current, the way the amp went into overdrive was a night and day difference, large screen resistors made a smooth change from clean to overdriven, but lower ones really gave you the feeling of power, hard to describe the sound but maybe I can compare it to when stevie ray vaughan would have a bold sounding fierce solo, and he would end it with a hard slide down the frets which would sound immensely powerful. but in my experience anything bellow 470ohms for 6L6oid tubes will red plate the screens so I don't recommend going down that low. (although the tubes lasted me a week of gigging with the screens glowing red when I had 330ohm screen stoppers and they were fine, so experiment at your own risk)
* fewer gain stages and higher plate voltage just make a better sounding amp, in my experience adding more preamp stages made the amp sound much worse on clean tones, and it lacked the feeling of power that amps with less stages had. although my reasoning on this is really vague, being that 'it just happens' and I'm not satisfied with that, I still try to get away with a classic, 2 preamp stages and a phase inverter lineup in any amp.

I also have even more vague thoughts about phase inverters:
* the Cathodyne is:
less compressed,
muddy, thick bass, that doesn't seem to break up nicely, it more just 'farts-out' on the attack, when it does break up
slightly brittle on the treble.
* the LTP is:
more compressed,
weaker bass, but distorts better,
more 'bold' sounding.
I love the sound of a cathodyne for some push pull EL84 hifi amps tho, it sounds really 'big', I can't describe why exactly but I haven't recreated that sound with any other PI, for a hifi amp.

I haven't experimented much with differences in tone from part choice, but I am open to discussion about tone capacitors, I think it's reasonable to think that having a different type of capacitor will change the tone of the amp, for example some russian capacitors, in the datasheet there's a chart showing that they are really lossy at higher frequencies while pass more signal on lower ones (all in the audio range).
I use modern film caps and some modern ceramic caps, and metal film mil spec soviet resistors in my amps for testing.

note: these are just my opinions, on things that I've experienced, I'm not saying it is how I say it is, but just spreading my stories.
feel free to add your own opinions on circuit design if you want!

Thanks for the thoughtful post. Sorry for my ignorance, but what exactly is an 'AC loaded' gain stage?

 

[ELS]

Thanks for the thoughtful post. Sorry for my ignorance, but what exactly is an 'AC loaded' gain stage?

you know how a gain stage will have a plate load resistor, one going from the plate to the power supply, that resistance would be the DC load resistance, and if you have just the gain stage and don't take signal from it, the AC load resistance will be the same.

but when you have a coupling capacitor (which will pass AC but block DC), any resistance to ground that you have on the other end of that coupling capacitor will decrease the AC Load resistance.

for example if you have a gain stage with a 100k plate resistor, feeding a 100k volume control, the DC load will be 100k, but AC Load will be that 100k in paralel with the volume control's 100k resistance, which would be 50k (you can use 1/resistance + 1/resistance2 = 1/total resistance to calculate the paralel resistance)

in the case of many blackface and silverface fender amps, the vibrato channel's 3rd stage is connected to the vibrato control potentiometer, which is 50k, in paralel with the 100k plate load resistor that would yield and AC load of ~25k, which is heavily loaded.

this low AC load resistance will steepen the load line of the tube, which subsequently will make the bias point be near the cutoff region.
so it acts like a cold clipper, but in a different way. while a JCM800 would have a high cathode resistor to let the bias voltage go more negative, an AC loaded stage would decrease the plate load resistance to force the cathode voltage to rise.