tone stack questions

[yunger]

i keep reading that certain tone stacks "load down" the signal and requires additional gain to make up for losses. what exactly causes the losses? is it the signal passing through capacitors that does it....or much of the signal going to ground....or large values of components? i'm sure this is a stupid question that most of you will get a good laugh out of but if someone does decide to dumb it down for me i would be grateful.

on a side note, i'm playing with a single knob tone control and printed a bode plot of the control at halfway turned. it looks quite similar to a fender-marshall tone stack throughout it's rotation but less versitile of course. the mid scoop notch can be reduced or eliminated. i'm hoping the fact that the lower frequencies taper off rather than shelve will allow me to retain some usable low end while not being too muddy.

[cbarrow7625]

It's just a matter of it being a simple voltage divider with the tubes plate output impedance. If you do a Thevenin type equivalent circuit of the tone stack with all of the knobs at mid position you'll find that it comes out as a relatively low equivalent resistance (Just guessing here, but somewhere around 100k or lower for a Fender type stack). The mids & lows are "loaded" more than the HF.

The output impedance from a typical 12AX7 plate stage is somewhere in the neighborhood of 34k (with a fully bypasses cathode). So we have 34k series resistance (plate output) working into a 100k shunt resistance; that equals a pretty good voltage divider (as opposed to 34K plate working into a 1M volume control ala tweed Bassman / Marshall setups where there is very little attenuation).

[Don Moose]

Duncan Munro has a nifty little ToneStackCalculator tool (Windows only) that can show you the attenuation (in dB) and the frequency responses of the common types of tone stacks at various settings.

Playing with this a number of years ago, I got the number 20dB for a Fender stack stuck in my head ... but I am prepared to be misremembering that.

[Groover]

Can anyone help me figure out how to use the Tone Stack Calculator to model the 5F10/6G2/6G3 single control stack. I tried using the E-series style but didn't know how to deal with the bass control and surrounding RC network.

[chipaudette]

If I may, I'm going to reinforce cbarrow's words by by adding some numbers...

The typical Fender blackface/silverface tone stack is indeed very lossy. It is so lossy because, yes, it forms a pretty brutal voltage divider. cbarrow has it totall right there. Totally right. Since many tone stacks are based on the Fender tone stack, many tone stacks are really lossy.

To put some numbers to it, with the tone controls on 7, the Fender tone stack loses ~9 dB at 20 Hz, ~24 dB at 400 Hz, and ~8 dB at 4000 Hz. The 17 dB of extra loss in the mid frequencies is what gives a Fender that classic mid-scoup clean sound. But, note that the lows and highs are also cut down quite a bit. You gotta make up that loss before hitting the power amp.

It just so happens that a fully cathode-bypassed 12AX7 in a typical Fender setup (originating from the RCA handbook and copied by everyone) gives a gain of about 25 dB. That nicely makes up for the losses in the tone stack and gives you a volume control (another voltage divider) that you can set on 3 and still have it sound wicked loud. Fender knew that loud sold amps...so it all worked out well for him. As a result, everyone copied it.

So now we get to the "why". What circuit elements make it so lossy? No it's not the large capacitors. The caps work with the resistors to filter the signal so that the high frequency components of your sound see certain circuit elements, the mids see other elements, and the lows see different elements. As a result, you can use those different circuit elements to change the volume of the highs, mids, and lows independently of each other. What changes their volumes are pots that form the voltage dividers that cbarrow mentioned.

The high frequencies see a voltage divider formed by the 38K output impedance of the tube (plus plate resistor) and passes through the 250p cap, through the treble pot, through the 100n bass cap (around the bass pot) and down to ground through the 10K mid pot. With everything on 7 (assume a pot on 7 equals half the log pot's value), the voltage divider looks like:

Treble: loss = (125K + 7K) / (38K + 250K + 7K) = 0.448 => -7 dB

For the bass, it travels a slightly different path involving the 100 k resistor and the bass pot. With the bass pot on 7 (half its value), there's a pretty good amount of resistance to hold the bass frequencies up off ground and keep it from having TOO much loss:

Bass: loss = (125K + 7K) / (38 K + 100K + 250 K + 7K) = 0.33 => -9.5 dB

The mids, though, have the most brutal path. That bottom cap (47n) ensures that the only thing holding up the mids up off ground is that puny 10K mid pot. So, with it turned to 7 (its a linear pot, so its value is 7K), the loss of that signal path is:

Mid: loss = 7K / (38K + 100K + 7 K) = 0.0483 => -26 dB. Ouch!

So, yeah, it's resistors and pots creating frequency selective voltage dividers. That's what makes the Fender tone stack so lossy. Other tone stacks may have a different configuration that is less lossy, but everyone likes the sound of the Fender one. So, that's what people build.

Hope this helps!

Chip

[Enzo]

But as mentioned above, recovering any of those losses is simple with a triode stage to maintain signal levels. So Loss is not a dirty word. Unless it is maxed, a volume control is lossy.

[yunger]

thanks for all the replies. i downloaded the duncan calculator. very helpful in understanding all this.
seems like the fender/marshall/vox stack would work well between cascading 12ax7s. why the need for cathode followers and paralleled triods?

[tubeswell]

A cathode follower is another way of driving a current-hungry item like a Fender TMB tone stack that results in a very pleasant distortion effect when chucked in a pre-amp circuit (e.g; see the 5F6A, which Jim Marshall liked so much he straight copied it in the JTM45)

The cathode follower has a low-impedance high-current output, so when coupled with a tone stack following it, it forms a voltage divider that isn't as 'lossy' (since this today's new buzzword) as a conventional VA stage

[loudthud]

Any load will reduce the output of a common cathode gain stage. It's usually only one or two dB. Even a 1 meg volume control is about 0.3dB. There is no free lunch. The typical Fender tone stack presents a frequency dependent load and it varies as you tweek the pots. The worst case is at high frequencies where the (depending on the mid control) the slope resistor is in parallel with the treble pot. In the blackface circuit, loss is minimized by the 100K slope resistor. In the tweed Bassman or many Marshall amps, a cathode follower minimizes the loss caused by the 56K to 33K slope resistor. They sound a little dark without it. Often a treble boost is needed somewhere else in the signal path.

Any passive network that affects tone has to do so by attenuating some frequencies more than others. If you want to boost treble by 10dB, you have to do that relative to everything else so the best case is you attenuate the everything except the treble 10dB. The James (sometimes mistakenly called Baxandahl) tone stack has about 20dB of loss when it's set flat. A good example is most Ampeg amps like the B15n. It's that loss that gives it the ability to boost bass or treble by eliminating the loss at treble or bass frequencies. The true Baxandahl tone control stage (where the pots are in a feedback loop) can really have a low input impedance when the pots are rotated towards boost. Best to drive it with a cathode follower.

[cbarrow7625]

Same thing with the paralleled triodes. Anything you can do to reduce the source impedance will result in a "less lossy" tone stack.

This is also why most solid state adaptations of tube tone stacks can use pot values 1/10 the size of the ones we use in tube amps. The source impedance of the preceding op-amp is low enough to prevent these kinds of losses (and/or mimic the losses in tube amps). Lower resistance values also usually = less noise.

[phillhorse]

I've built a SS pre with a Fender Tone-stack, but I am experience treble losses that don't appear on a valve version. any ideas what could be causing noticeable treble loss?

[Suusi M]

i keep reading that certain tone stacks "load down" the signal and requires additional gain to make up for losses. what exactly causes the losses? is it the signal passing through capacitors that does it....or much of the signal going to ground....or large values of components? i'm sure this is a stupid question that most of you will get a good laugh out of but if someone does decide to dumb it down for me i would be grateful.

on a side note, i'm playing with a single knob tone control and printed a bode plot of the control at halfway turned. it looks quite similar to a fender-marshall tone stack throughout it's rotation but less versitile of course. the mid scoop notch can be reduced or eliminated. i'm hoping the fact that the lower frequencies taper off rather than shelve will allow me to retain some usable low end while not being too muddy.

how about putting up the circuit diagram for the tone stack and its driver stage so we can see the problem

[phillhorse]

Currently I cannot post or read attachments for some weird reason but
the tonestack is a Fender copy driven buy a TL072. the output of the tonestack (wiper of treble control) feeds the +ve input of a TL072 with a 1meg resistor to ground configured as a non inverting voltage follower.