Bruce's next tech article - suggestion

[chickenoolie]

****WARNING SUPER LONG POST****

Hey Bruce (and or Jeff),

It seems like so much would fall in to place for me
in what I've learned about guitar amps if I could
just grasp a few concepts related to frequencies
and tone shaping.

I'm posting this here because you're cool, your
"watts vs. volume" and "sound dispersion" tech
articles were cool, you shoot it straight, and you
have amp seminars where you're willing to indulge
the heathen in your profession. On the flip side, I
know you're also neurotically busy.

Regardless, here goes:

First, a guitar note is obviously not a single
bandwidth tone like a tuning fork or an
electronically generated tone. If it was, playing
guitar would sound like playing that old electronic
simon memory game I used to play when I was 11.
Doot, doot, deet, dote, dote, dote, deet, dote.

So, there's a central frequency but then there's also
harmonic content that makes it sound musical and
cool. In my understanding, the way tubes
handle/produce these harmonic frequencies, even
before clipping, is a central reason they're preferred
over solid state.

That said, I still don't really get how this works.
Let's say I scoop the mids out of my amp. That
should center around certain frequencies right? So,
why is the volume of the notes I play still fairly
consistent up and down the fret board. The tone
changes, but the volume seems relatively unchanged.
Shouldn't the notes that are hanging around that
scooped frequency be much quieter, not just different
tonally?

Is there really that much harmonic content
going on that your ear can still pick out the center
note even when that note's center frequency is
being cut way down?

Perhaps the tone circuit just isn't that powerful by
design. It's cutting it but not too much that it
becomes unmusical?

Second, when you have a passive tone shaping part
of the circuit that is cutting frequencies at a certain
band - is it cutting it proportionally or is it cutting it
down to a certain threshold? I ask this in regards to
using an EQ pedal in front of the amp to boost
frequencies. Obviously, if the tone circuit was cutting
the boosted frequencies down to a static level,
boosting the frequency would have no effect. So, my
guess is that it's cutting it proportionally. Am I on the
right track?

Third question - on master volumes and amp bias.
Again, my understanding - an amps master volume
knob controls the amount of current being fed to the
power amp from the preamp. Turning the volume knob
down does not directly effect the operation of the
power tubes, it's just cutting the signal they're seeing
at their input, correct? Also, the bias is in essence the
multiplication factor for how many X times bigger the
output signal from the power section is than the input
signal? Thus changing the bias does directly effect the
operating level of the power tubes.

Lastly, I think I once found an article or post where you
talked about people's surprise in how subtle the tonal
changes were using the Mix knob on the Rebel. The point
was that the preamp circuit design has much more to do
with the sound of the amp than the power section. Am I
remembering this correctly? Obviously everyone's got there
favorite power tubes. But am I over thinking it if I feel the
need to run my VX module with el84's, my T/D with 6L6's,
my SL with 5881's and my SL2 with EL34's.

I know the common response may be, why don't you try it
and see for yourself. But, there's not really a great way to
try that. Regardless, I feel the opinion of the amp designer
is a thing unto it's own. Which is why I'm asking.

Sorry for the long post. Any wisdom is appreciated.
Also, any wisdom from the board on these topics is also
appreciated.

Nick

 

[jasco]

see below comments for my 2 cents on the first question (though obvioulsy I am not a guru like Bruce! . . . )

****WARNING SUPER LONG POST****

Hey Bruce (and or Jeff),

It seems like so much would fall in to place for me
in what I've learned about guitar amps if I could
just grasp a few concepts related to frequencies
and tone shaping.

I'm posting this here because you're cool, your
"watts vs. volume" and "sound dispersion" tech
articles were cool, you shoot it straight, and you
have amp seminars where you're willing to indulge
the heathen in your profession. On the flip side, I
know you're also neurotically busy.

Regardless, here goes:

First, a guitar note is obviously not a single
bandwidth tone like a tuning fork or an
electronically generated tone [>> only sine tones are in a 'single bandwidth' - i.e., assuming you mean they have energy at only one frequency - i.e., no harmonics (and no energy at inharmonic frequencies). Electronically generated tones like square waves and sawtooth waves - anything other than a sine wave - have energy at other frequencies. But yeah - guitar tones can be very complex in terms of harmonic content, expecially 'distorted' tones]. If it was, playing
guitar would sound like playing that old electronic
simon memory game I used to play when I was 11.
Doot, doot, deet, dote, dote, dote, deet, dote. [>> that would be a sad world indeed ]

So, there's a central frequency but then there's also
harmonic content that makes it sound musical and
cool. In my understanding, the way tubes
handle/produce these harmonic frequencies, even
before clipping, is a central reason they're preferred
over solid state.

That said, I still don't really get how this works.
Let's say I scoop the mids out of my amp. That
should center around certain frequencies right? So,
why is the volume of the notes I play still fairly
consistent up and down the fret board. The tone
changes, but the volume seems relatively unchanged.
Shouldn't the notes that are hanging around that
scooped frequency be much quieter, not just different
tonally? [>> in complex tones, there is energy at all kinds of frequencies. We hear the "fundamental" note not because there is the most energy/amplitude at that frequency, but because our brain implies the fundamental of the harmonic series that is presented to our ear - even if the fundamental isn't there at all! For example, some acoustic instruments (oboe I think is an example) have little or almost no energy at the fundamental - yet we still 'hear' an A440 as A440 even if there is little or no amplitude/energy at 440 hz - the ear hears the harmonic spectrum of A440 and the brain 'knows' where the fundamental is. This is similar to the explanation for the phenomenon known as ghost tones - if you play two sine tones at two different frequencies, you will 'hear' a difference tone at a lower frequency even though it isn't actually there in the vibrating air. I've done the experiments myself on this and it's quite freaky . . . . you know it's not there, but you can clearly hear it! In any event, the net of this re filtering (i.e., 'EQ'ing') is that the 'timbre' of the sound changes but the 'pitch' (i.e., the perceived frequency of the sound) does not. As for the impact of filtering on perceived 'volume', I think the answer is that there is so much energy at so many frequencies in a distorted guitar tone that narrowly cutting one frequency has little impact on the perceived level. That said, take a clean, cranked bass guitar tone and filter out all the low frequencies and you will likely hear a huge drop in perceived volume - i.e., since more of the energy is crammed in the lower frequencies near the fundamental so cutting those will have a more drastic affect on the perceived volume. There are lots of other factors at play too - like that our ears are less sensitive to low and high frequencies at lower volume levels, so the effect that the EQing has on perceived volume can vary with how load the sound is.]

Is there really that much harmonic content
going on that your ear can still pick out the center
note even when that note's center frequency is
being cut way down? [>> - hell yes!! especially in gainy guitar tones!! in a flute, not so much . . . ]

Perhaps the tone circuit just isn't that powerful by
design. It's cutting it but not too much that it
becomes unmusical? [>> per above, you can cut it 100% and, unless it's a sine tone (which has energy at only one frequency), it will still sound like the same 'pitch']

Second, when you have a passive tone shaping part
of the circuit that is cutting frequencies at a certain
band - is it cutting it proportionally or is it cutting it
down to a certain threshold? I ask this in regards to
using an EQ pedal in front of the amp to boost
frequencies. Obviously, if the tone circuit was cutting
the boosted frequencies down to a static level,
boosting the frequency would have no effect. So, my
guess is that it's cutting it proportionally. Am I on the
right track?

Third question - on master volumes and amp bias.
Again, my understanding - an amps master volume
knob controls the amount of current being fed to the
power amp from the preamp. Turning the volume knob
down does not directly effect the operation of the
power tubes, it's just cutting the signal they're seeing
at their input, correct? Also, the bias is in essence the
multiplication factor for how many X times bigger the
output signal from the power section is than the input
signal? Thus changing the bias does directly effect the
operating level of the power tubes.

Lastly, I think I once found an article or post where you
talked about people's surprise in how subtle the tonal
changes were using the Mix knob on the Rebel. The point
was that the preamp circuit design has much more to do
with the sound of the amp than the power section. Am I
remembering this correctly? Obviously everyone's got there
favorite power tubes. But am I over thinking it if I feel the
need to run my VX module with el84's, my T/D with 6L6's,
my SL with 5881's and my SL2 with EL34's.

I know the common response may be, why don't you try it
and see for yourself. But, there's not really a great way to
try that. Regardless, I feel the opinion of the amp designer
is a thing unto it's own. Which is why I'm asking.

Sorry for the long post. Any wisdom is appreciated.
Also, any wisdom from the board on these topics is also
appreciated.

Nick

 

[chickenoolie]

Thanks for the reply jasco!

No wonder there's so much room for voodoo and mojo in guitar playing.
We're all coaxing ghost notes out of glass fire bottles that swirl and sway
their way through the room whispering imagined messages in people's
cerebral ear drums.

Lastly, thanks to anyone whose looked at this post for not flaming me.
I re-read it this morning and it could easily be responded to as one of
those "Why don't you do a little homework before asking questions"
type of posts.

But, the truth is I just got done reading Dave Hunter's Tube Amp Book.
Went cover to cover and there's still some basic stuff I'm not getting.

thanks again jasco.

And in hopes you all have a better Friday I leave you with this:
*make sure you make it to 2:20 for a level of crowd interaction
that would be hard to imagine in today's world.

Rory On!

Nick