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Patrick Turner Patrick Turner is offline
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Default LTSpice, guitar amp tone control

Hi all. At 66yo, I have automatically been granted the license to be a GRUMPY OLD MAN, acronym GOM, aka Experienced Curmudgeon or EC.

Why? because NERDS rule the world, and nerds only know how to "computerize" sensible processes of common sense. But Nerds comprise only 2% of the population.
All Nerds have no idea how common sense works, so that so many supposedly useful programs like LTSpice are never as easy to use as the older versions of say MS paint where it is very easy to draw excellent schematics.

One could hope in vain that a decent simple program like MSpaint, ( Not nerdized ), could be the basis of showing what's connected to what and allow the easy editing and changing of schematic, but no, its far too much to hope for.
So I have great difficulty getting a basic schematic established with LTSpice on the PC screen and with an Input, Output, place for a meter, and for basic aspects of network input and output impedences plus and a few other basic things that have always been wanted by engineers for about 90 years.

You see, if one can draw a schematic effortlessly with MSpaint, then one would think that a simulator program would understand just what is connected to what by reading what you have moused into position.

Look at my schematics at my website at http://www.turneraudio.com.au
Most have been done with MS paint. A large tube amp schematic typically takes 12 hours+ of by-hand work to produce. I should be able to just copy and paste one of my schematics into an LTSpice window, and nominate part values and device type, enter B+ voltages, input, output, and then go CLICK, and get a response, Rout etc, etc, etc. Too easy? NAH, Too hard!

Hmm, I have only got limited time on planet. Not sure I want torture my brain with LTSpice.

Thanks to Stephie for recommending LTSpice to me, but darned if I can proceed.

Anyway, long ago, extremely wise men were able to produce superb amplifiers without the existence of Nerds or any computer aided design, and armed only with slide rules and Book Of Rules written by the GOT ( God Of Triodes ).

I guess we see so little useful information about guitar amp tone controls that it is because so few blokes with IQs above 10 try to analyze it accurately.

At this point of time, aka Now, it seems to me that when all tone controls are set at 5, ie, 50% pot rotation, you get a slightly boosted bass 100Hz at +6dB, then sloping to 0Db at 400Hz, then sloping up to +6dB at 5kHz.
Not much bass or treble cut is ever needed. A very large boost of treble above 1k should be possible, say +15dB at 5kHz, and at least +10dB boost of bass at 100Hz, while 400Hz level remains little changed.
THEN, Mid control means reducing 400Hz by say 9dB max and boosting it by +dB, without affecting 200Hz and 3,200Hz levels very much.

Thus somehow there is way to co-relate the fact that human ears perceive loudness of bass F much less than mid F. And electric guitars don't seem to produce flat responses so you need the amp to boost the declining response
of upper harmonics of all strings. Plus, where you have the string fundamental F at say 0dB, then have 2H and 3H a bit suppressed, but then have 4,5,6,7,8,9H much boosted, then the sound becomes more attractive to many people than that of a well made acoustic guitar with no amp, or amplified with mike held at sound hole with no EQ applied.

So, by having a Mid tone control one is drawn to effectively making a 3 band graphic equalizer. It confuses many musos. Depends a bit on what kind of venue and audience they play to. Perceived loudness of different F varies with general average loudness and there's a famous graph which sums it up in Radiotron Designer's Handbook, 1955, written by wise old men.
The Q of the 3 wanted bands differ; one would seem to want a higher Q for mids. Well, to get that, perhaps the easier way for tone control is to do a simple bass & teble Baxandal, and then in a preceding or following gain tube connect a series resonant LC circuit and pot between cathode and 0V. Thus it is not too hard to get +/- 9dB at 400Hz, Q = 1 or 2, while having it non-interactive with the bass and treble controls. 100uF and 1.6mH become resonant at 400Hz, and
L can be air cored.

Just my 2c,
Patrick Turner.

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Originally Posted by Patrick Turner View Post

Look at my schematics at my website at http://www.turneraudio.com.au
Most have been done with MS paint. A large tube amp schematic typically takes 12 hours+ of by-hand work to produce. I should be able to just copy and paste one of my schematics into an LTSpice window, and nominate part values and device type, enter B+ voltages, input, output, and then go CLICK, and get a response, Rout etc, etc, etc. Too easy? NAH, Too hard!


Just my 2c,
Patrick Turner.
Sorry to send you on a wide goose-chase on the Wolfram app, I forgot to mention that you should just run the applet as it appears on the screen, not actually download and install the code that the author provided, which requires other softwares in order to run on your PC.

Anyway, as popular as LTSpice is, I too hate to use it myself, but if you would like to convert some of the schematics on your site to be used in LTSpice, let me know, I will be happy to draw up some for you (hopefully not all, as there are just too many ;-)

Also a note on the tonestacks, most guitarists are already trained/brainwashed on the Fender-Marshall-Vox type treble-mid-bass tonestacks, it's too late to turn back the clock and design a "proper" tone control for amps - they always revert back to the TMB it seems...

Cheers,
Jaz

Last edited by jazbo8 : February 20th 14 at 03:05 PM
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Stephie Bench Stephie Bench is offline
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Default LTSpice, guitar amp tone control

Hey Patrick,

Was thinking about your musings on a tone stack. On the LC 400Hz mid circuit. A small inductor 1.6mH, only has a reactance of around 5 ohms at 400Hz. The cathode Z is in the hundreds of ohms. Since Q in a series circuit is Q=X/R, you can't develop enough effective resonant Q to be useful. Might save you some disappointing results. I do like the concept of the cathode series tank, but note that will give you a boost only, not boost cut, unless you do a little switching. What might be cool is to implement all the tone stack that way. It might keep the interaction down too.

Also note that significant boost is difficult to achieve with RC only with your proviso that 2 octave away has essentially no change in response. You'll get at least a dB or so change that close. (Thinking of the 100Hz boost vs 400Hz no-boost.

Cheers,
Stephie
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jh[_3_] jh[_3_] is offline
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Default LTSpice, guitar amp tone control

On 20.02.2014 09:33, Patrick Turner wrote:
Hi all. At 66yo, I have automatically been granted the license to be a GRUMPY OLD MAN, acronym GOM, aka Experienced Curmudgeon or EC.




SNIP


So, by having a Mid tone control one is drawn to effectively making a 3 band graphic equalizer. It confuses many musos. Depends a bit on what kind of venue and audience they play to. Perceived loudness of different F varies with general average loudness and there's a famous graph which sums it up in Radiotron Designer's Handbook, 1955, written by wise old men.
The Q of the 3 wanted bands differ; one would seem to want a higher Q for mids. Well, to get that, perhaps the easier way for tone control is to do a simple bass & teble Baxandal, and then in a preceding or following gain tube connect a series resonant LC circuit and pot between cathode and 0V. Thus it is not too hard to get +/- 9dB at 400Hz, Q = 1 or 2, while having it non-interactive with the bass and treble controls. 100uF and 1.6mH become resonant at 400Hz, and
L can be air cored.

Just my 2c,
Patrick Turner.



Hi Patrick,

are you reinventing the SVT/V9 Preamp from Ampeg?


regads

Jochen
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Stephie Bench Stephie Bench is offline
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Default LTSpice, guitar amp tone control

So, I thought as a curiosity, I'd spend a couple hours in Spice land and see if those interesting specs Patrick came up with could be achieved. Lets review them... bass, mid, treble controls. At midpoint on all controls, +6dB at 100Hz, 0 at 400Hz, and +6dB at 5kHz. Max bass, 10dB at 100Hz, min not too important. Mid, + and - 9dB at 400Hz, with little effect at 200Hz znd 3..2kHz. Max treble, +15dB at 5kHz, min not too important. I put a couple added constraints: power=250v, 12AX7s only, overall more or less unity gain, insensitive to input impedance or output loading, supports 1v input level, and no inductors.

What I achieved uses 2 12AX7s and has the following characteristics.
Input Z, 1meg, and insensitive to input Z.
Output: cathode follower and no effect on response by reasonable load.
Bass: mid, +6dB at 100Hz. Max, +10.5dB at 100Hz. Min, -1dB at 100Hz.
Mid: mid, 0dB at 400Hz, max, +10dB at 400Hz, min, -11dB at 400Hz. Effect of the mid pot is less than a dB at 200Hz and 3200Hz.
Treb: mid, +6dB at 5kHz. Max, +15dB at 5kHz. Min, +3dB at 5kHz.

The bass and mid are implemented as tuned circuits, using a section of a 12AX7 each as gyrators. I was actually intrrested in seeing whether that would work reasonably well; and it does.

Incidentally, setting things up for flat rather than boosted response provides symetrical boost/cut for all 3 controls.

The pots for control are all 50k pots, and I alloeed for 1k residual resistance at the pot extremes.

Anyone wanting the Spice file can email me.

-Stephie


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jh[_2_] jh[_2_] is offline
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Default LTSpice, guitar amp tone control

Am 23.02.2014 03:18, schrieb Stephie Bench:
So, I thought as a curiosity, I'd spend a couple hours in Spice land and see if those interesting specs Patrick came up with could be achieved. Lets review them... bass, mid, treble controls. At midpoint on all controls, +6dB at 100Hz, 0 at 400Hz, and +6dB at 5kHz. Max bass, 10dB at 100Hz, min not too important. Mid, + and - 9dB at 400Hz, with little effect at 200Hz znd 3..2kHz. Max treble, +15dB at 5kHz, min not too important. I put a couple added constraints: power=250v, 12AX7s only, overall more or less unity gain, insensitive to input impedance or output loading, supports 1v input level, and no inductors.

What I achieved uses 2 12AX7s and has the following characteristics.
Input Z, 1meg, and insensitive to input Z.
Output: cathode follower and no effect on response by reasonable load.
Bass: mid, +6dB at 100Hz. Max, +10.5dB at 100Hz. Min, -1dB at 100Hz.
Mid: mid, 0dB at 400Hz, max, +10dB at 400Hz, min, -11dB at 400Hz. Effect of the mid pot is less than a dB at 200Hz and 3200Hz.
Treb: mid, +6dB at 5kHz. Max, +15dB at 5kHz. Min, +3dB at 5kHz.

The bass and mid are implemented as tuned circuits, using a section of a 12AX7 each as gyrators. I was actually intrrested in seeing whether that would work reasonably well; and it does.

Incidentally, setting things up for flat rather than boosted response provides symetrical boost/cut for all 3 controls.

The pots for control are all 50k pots, and I alloeed for 1k residual resistance at the pot extremes.

Anyone wanting the Spice file can email me.

-Stephie


Hi Stephie,

....as Patrick is searching for something "guitar related" - is there any
possiblitiy that a real guitar player can taste the reaction and the
sound of an amp incorporationg the gadget?

regards

Jochen
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Stephie Bench Stephie Bench is offline
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Default LTSpice, guitar amp tone control

is there any possiblitiy that a real guitar player can taste the reaction and the sound of an amp incorporationg the gadget?

There is one "non real time" way. If you record a sample of what you'd like to hear into a .WAV file, you can import that into the Spice program. Then, adjusting the controls, you can play that into an output .WAV file and listen to it with different adjustments.

-Stephie
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Patrick Turner Patrick Turner is offline
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Patrick Turner;976269 Wrote:


Look at my schematics at my website at http://www.turneraudio.com.au
Most have been done with MS paint. A large tube amp schematic typically
takes 12 hours+ of by-hand work to produce. I should be able to just
copy and paste one of my schematics into an LTSpice window, and nominate
part values and device type, enter B+ voltages, input, output, and then
go CLICK, and get a response, Rout etc, etc, etc. Too easy? NAH, Too
hard!


Just my 2c,
Patrick Turner.


Sorry to send you on a wide goose-chase on the Wolfram app, I forgot to
mention that you should just run the applet as it appears on the screen,
not actually download and install the code that the author provided,
which requires other softwares in order to run on your PC.

Anyway, as popular as LTSpice is, I too hate to use it myself, but if
you would like to convert some of the schematics on your site to be used
in LTSpice, let me know, I will be happy to draw up some for you
(hopefully not all, as there are just too many ;-)

Also a note on the tonestacks, most guitarists are already
trained/brainwashed on the Fender-Marshall-Vox type treble-mid-bass
tonestacks, it's too late to turn back the clock and design a "proper"
tone control for amps - they always revert back to the TMB it seems...

Cheers,
Jaz

Could you all forgive me for my unnecessary criticisms of the the tone stack program listed at Duncan's pages. It downloads fine, but gave strange results until *I woke up* about the use of the potentiometer slide positions.
When you try to adjust a pot between say 1 and 10, a little window follows the cursor arrow to tell you the % of the resistance between bottom of pot and wiper, which may or may not be connected to the top of the wiper.
So let us say you set a bass control on the amp at No5, ie, center of rotation, then you measure wiper to bottom on a real amp, ie, the re-issue Fender Deluxe now in my shed, you find R pot = 20k, and this is about 7.7% of the total pot resistance, and when you set the slider for 7.7%, then the shown response agrees with the experimental results one will measure.

Duncan or someone@somewhere could have made it more obvious about how to set the controls for the center of rotation, and extremes of rotation.

Usually there is a position for B, M and T pots which give the best looking square wave between say 400Hz and 2kHz and with amp set in for this position the sound is usually Dull&Boring because you'd find the sine wave response is then a fairly flat line. I guess the D&B sound is most likely if the strings on the guitar are old and gummed up with body oils and sweat and other rotting junk from players fingers. I played acoustic guitar a long time ago and noticed how fast a new set of strings became dull sounding. But not all guitarists have damp sweaty hands, I sure don't, but I knew one whose hands would ruin new strings in a couple of long sessions. All the gunk gets into the wound brass around the string. Now just how much this affects electric guitar strings is unknown to me, but maybe it does.

Solid bodied electric guitars are incabable of making their timber structure resonate to any high degree, and must rely on the magnetic pick ups. Its all very different to say an acoustic Maton or Guild or Yamaha folk guitar et all,
and then you have the 1/3 way betweens of jazz guitars.......But I digress.

So, in the sample amp I have in the shed for singing lessons, I'll add a CF buffer between input tube and tone stack which should reduce the IMD ( always sounds bad ) produced by change of gain of HF caused by high levels of LF.
THD is OK, and just adds or subtracts small % of harmonics already in the signal; it can be complex to contemplate because phase of amp H can be opposite of phase of string signal H.

The "proper tone stack" as you mention is in fact the *James* stack which has the exact configuration for a *passive* Baxandal bass and treble network..
It is a little tricky to get R&C values correct so that there is a centre F of say 700Hz and you get a flat response, and good square wave when both B & T are set at No5, ie, 50% rotation, on pure log pots, where the 50% rotation gives R below wiper and 9R above wiper.
Ampeg used the James, ie, Baxandal passive. There was no middle. So if you wanted the 500Hz suck out with an ampeg, you set B up a bit, and T up a bit, and gain down a bit. All a bit guessy, and imprecise, and musos could be confused.

The Baxandal with pure log pot can give lots of bass and treble boost, but there is higher insertion loss than other types of tone stacks which mainly just boost B & T without much cut, and with maybe +2dB and -6dB of middle adjustment centered around 450Hz.

Just a penny's worth,
Patrick Turner.

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Patrick Turner Patrick Turner is offline
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Hey Patrick,

Was thinking about your musings on a tone stack. On the LC 400Hz mid circuit. A small inductor 1.6mH, only has a reactance of around 5 ohms at 400Hz. The cathode Z is in the hundreds of ohms. Since Q in a series circuit is Q=X/R, you can't develop enough effective resonant Q to be useful. Might save you some disappointing results. I do like the concept of the cathode series tank, but note that will give you a boost only, not boost cut, unless you do a little switching. What might be cool is to implement all the tone stack that way. It might keep the interaction down too.

Also note that significant boost is difficult to achieve with RC only with your proviso that 2 octave away has essentially no change in response. You'll get at least a dB or so change that close. (Thinking of the 100Hz boost vs 400Hz no-boost.

Cheers,
Stephie

You are perhaps correct. Maybe make a larger value L of 0.16H, with C = 1uF so Fo = 398Hz. I have not tried the idea.
But a larger L value is a Royal Pain In Arse for most ppl farnarkling around for novel ways of making what is a single band graphic eq. The L is prone to hum pick up, and would use lots of turns of say 0.15dia Cu wire on a bar type of core, so size is a bother.....
Then comes the idea of a gyrator and adherence to normal graphic eq circuit methods, only you use tubes instead of an opamp and a transistor plus R&C for a "fake" L.

Simpler, possibly, is the inclusion of a bridged T between CF buffer after gain control after tone stack and before input to gain tube before power amp.
Only a few R&C and a pot are required, but you only get a notch of lowish Q..
But I cannot see where anyone would want a peaked mid. This would make sound strange where bassy notes go missing, and just a small range of upper notes are way too loud.

Bridged T should be driven with low Rsource, then has R in series with R to high Z grid input of gain tube. R + R is shunted by a C value that is much lower than the pylon C between join of 2 R and 0V. Examples are to be found online if Googled, and see images.
The Fo of the minimum Vo at the dip can be changed with a dual gang pot for each of the R spans of the bridge.
Insertion effect can be governed by a pot somewhere, not sure where/how, but there must be a way for between flat and no dip to full dip.
Patrick Turner.
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Patrick Turner Patrick Turner is offline
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Default LTSpice, guitar amp tone control

So, I thought as a curiosity, I'd spend a couple hours in Spice land and see if those interesting specs Patrick came up with could be achieved. Lets review them... bass, mid, treble controls. At midpoint on all controls, +6dB at 100Hz, 0 at 400Hz, and +6dB at 5kHz. Max bass, 10dB at 100Hz, min not too important. Mid, + and - 9dB at 400Hz, with little effect at 200Hz znd 3..2kHz. Max treble, +15dB at 5kHz, min not too important. I put a couple added constraints: power=250v, 12AX7s only, overall more or less unity gain, insensitive to input impedance or output loading, supports 1v input level, and no inductors.

What I achieved uses 2 12AX7s and has the following characteristics.
Input Z, 1meg, and insensitive to input Z.
Output: cathode follower and no effect on response by reasonable load.
Bass: mid, +6dB at 100Hz. Max, +10.5dB at 100Hz. Min, -1dB at 100Hz.
Mid: mid, 0dB at 400Hz, max, +10dB at 400Hz, min, -11dB at 400Hz. Effect of the mid pot is less than a dB at 200Hz and 3200Hz.
Treb: mid, +6dB at 5kHz. Max, +15dB at 5kHz. Min, +3dB at 5kHz.

The bass and mid are implemented as tuned circuits, using a section of a 12AX7 each as gyrators. I was actually interested in seeing whether that would work reasonably well; and it does.

Incidentally, setting things up for flat rather than boosted response provides symmetrical boost/cut for all 3 controls.

The pots for control are all 50k pots, and I allowed for 1k residual resistance at the pot extremes.

Anyone wanting the Spice file can email me.

-Stephie

Very interesting Stephie.

You can send file you made if it has a schematic and the amplitude responses which are easiest to correlate in a glance. My email address is Just leave out the *nospam* and it'll get here OK..

I should be able to save it in my growing guitar amp files, and open it in LTspiceIV.

Regards, Patrick Turner.


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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default LTSpice, guitar amp tone control


"Patrick Turner"


The program does not allow to change the schematic connections from the
Fender arrangement. All R and C can have values changed, and pots can either
be linear, or log type B, or log type A.


** The pots fitted to 70s Twin Reverbs are variously labelled as being:

A, L, RA & J

What do you take each to mean ?


.... Phil








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Jerry Sze Jerry Sze is offline
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The help file states that "Log type B has 10% of full value at the half way position" vs log type A having 30% at half way. Only on the linear pot, the resistance is at 50% of full value at the "5" position.
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Lord Valve Lord Valve is offline
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Phil Allison wrote:

"Patrick Turner"

The program does not allow to change the schematic connections from the
Fender arrangement. All R and C can have values changed, and pots can either
be linear, or log type B, or log type A.

** The pots fitted to 70s Twin Reverbs are variously labelled as being:

A, L, RA & J

What do you take each to mean ?

... Phil


....

A = audio (10% at mid rotation), L = linear, RA = reverse audio,
and J = audio (30% at mid rotation).

LV


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Stephie Bench Stephie Bench is offline
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Generally, a true log taper is difficult to implement, so the mfgrs implement it by two piecewise linear segments. The 10/90% allows 20 db change from mid to max, a 30/70% allows a 10 db change mid to max. Note that a linear pot only provides a 6db change mid to max.

Patrick, the files I just emailed you are set up for a linear pot. I included response curves for bass max, mid, min, 400Hz max, mid, min, and treb max, mid and min.

I played wiyh the values a little, so, for instance response with all pots set midscale, the response is:
50 Hz +7
100Hz +6
200Hz +2.5
400Hz 0
1k +3.3
2k +5.2
5k +6
10k +6

Bass at 50Hz Varies from +3.5 to +15
Mid at 400Hz varies from -9 to +9
Treb at 5k varies from +3.5 to +15

-Stephie
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Patrick Turner Patrick Turner is offline
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Hi Stephie,
Thanks for sending the schematics and the F response.
There is much food for thought.
The problem with the spice .asc file is that it won't open in anything else so I printed it so I could scan it, but RHS info didn't fit on page and there's no select-image and rotate function to get a better page fit so there's hours of mucking about to make a nice looking schematic.
My LTspiceIV program does not seem to have triodes.

""" triode_nh.txt into the Spice (under LTC/LTspiceIV/lib/sub folder and the triode_dd.asy into the sym directory to have the tube models available""" is a bit beyond my very dumb PC skills. I guess I can work out from what you have which are the a, g, k tube connections and then put in a triode of my own making and generally end up with something that can be put on the bench in front of me so it can be made.

I've never used gyrators. Reverse log pots are mostly unobtanium. They have a purpose in wien bridge oscillators, but need to be quasi log types, ie, between linear and log, to get evenly octave calibrations on a dial.

Today I compared a version of a Baxandal with a mid control I devised with the standard Fender with mid control. I ended up concluding my own mid control with pot and 2 caps attached to work on the R below bass pot to 0V wasn't any better than the Fender stack. The addition of CF buffer to drive the stack gave +3dB gain, but not a huge F response difference.
For the Fender stack, If the bass and treble are set at 10, ie both at max, and mid 10kpot at 1, ie, 10k shunted, then you get -9dB dip at about 320Hz and Q is more than 1 and slopes of dip are greater than -6dB/octave, to the circuit works to exploit phase shift some how, and probably no need for anything else. I'll prepare a set of graphs for my customer to show the B, M &T settings to get various profiles, perhaps that will make him happy and sound will be OK, but this amp has only 2 x 6V6 and is a combo, and maybe he has to change his speaker driver.

Your circuit shows nicely symmetrical peaks and troughs for the mid F. Having thought about it all, when would peaking the mids be ever used? The mid dipping is to make sound less harsh, but boost a sheen on top of everything.. Playing a melody would make upper notes too loud. Chords contain many F from bass to highs, and if the mids are toned down and highs boosted, they are supposed to sound better.

Anyway, I'll be able to ask my customer to bring his axe and we can try a few different settings, and see what he thinks. My concern is to get the best sound, even if it seems a bit irrational and counters the strict conservative approach I have taken for hi-fi amps.

I'll also put in a 2k2 pot in series with 820r GNFB resistor to allow reduction of NFB.

People have been confused with potentiometer properties and their type classification for a long time.

Duncan's tone stack program lists type A log, and B log pots and linear pots.
Afaik, Log A pots have 10% of total R between wiper and bottom at 50% rotation. Such pots suit volume controls, and the Fender tone stack has a "sort of" Log A for bass.

At 50% rotation, the Fender bass pot measured 260k total with 21k between wiper and bottom, so pot goes from 8% of total R to full R between 5 and 10 settings.
This is a type A pot.

The Fender has a treble pot which I measured at also 260k total but at 50%
rotation the wiper to bottom = 60k, so 23% of total. AFIAK, this is a type B log pot.

Linear have 1/2 total R at 50% rotation.

But in the world of pots, there are pots and pots, and type A are usually 10% log pots, and type B is a linear pot, so at most stores where pots are to be purchased you have only A and B type, ie, log and linear.

But then there are "type C audio taper" pots, and then there are all the vast array of pots which differ from what you'd want them to be because the manufacturer is a cheap production site somewhere in Asia and the idea of making a very good pot costing more than $0.10c to produce is quite outlandish, foreign, and way too difficult especially if it is a log pot, where a dual gang type might give +/- 3dB difference between channels at different level settings, and its impossible to compensate for pot errors with a fixed R strapped across one pot or both, somewhere, and hence there's a need for a channel balance control. The pots in the Fender Deluxe re-issue look like $0.08c types and because of vast numbers wanted by Fender, they are so cheap,
and Fender can insist on and enforce some quality control.

But elsewhere in say Jaycar or Dick Smith stores there are log pots which seem to have say 3 sudden kinks in their R increase, thus showing the process of depositing carbon is done in 3 squirts of C at 3 different rates, rather than a smoothly changing variation of the rate of C deposition. Seems like that to me. I ain't no expert on just how log pots are made.

However, many log pots sold in Oz for $5.00 are quite long lasting and usable if you buy 4 and choose the best one, and you seal up the openings with cardboard and tape and you cramp the rivets of lugs to avoid loose lugs.
Long nose pliers helped with a vice does this without over-tightening. Repeated soldering of lugs can soften track material and lead to loose lugs and intermittent operation. In other words, the $5.0 pot needs work to make it passable.

Alps "Black", 27mm square bodied pots seem to me to be very long lasting and quite precise enough without "flat" spots where say 20 degrees of turn doesn't alter the Vo level much.

Best of course are switched attenuators. I've never had troubles with DACT.
But the best of Alps carbon pots and DACT are far too expensive for Fender to include as standard items.

But, I did find that I could fit a standard 24mm dia old style linear 10k pot onto the Fender Deluxe PCB to make a mid control, and placed where I had removed one of the 2 "Normal" channel inputs. It is held to board with turn of Cu wire and then wire links to "Vibrato" channel bass pot some 200mm away. The nut which was on 6mm input jack suited the pot, and hole size was correct. Works fine, and give the owner the mid control he wants like in the old 1965 Twin Reverb amps.

Patrick Turner.

Patrick Turner


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Patrick Turner Patrick Turner is offline
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Default LTSpice, guitar amp tone control

Today I tested the whole Fender Deluxe with its vibrato channel which now has a CF buffer driving standard Fender tone stack but with a linear 10k0 pot for mid adjust. The CF buffer increases the gain by an average of +3dB.

The 1M volume pot following tone stack has 5pF across top of pot and wiper. But standard Deluxe schematic shows a fixed 47pF, so at some volume setting there is a maximum increase of HF because of the combined effect of following 12AX7 Miller input C and the C shunting part of the volume control resistance. Because this particular amp has 5pF, it is perhaps a mod, the HF boost due to 5pF is negligible. I found 47pF at volume pot would boost 5kHz by +3dB,( less at lower F and not much above 7kHz.) But with 47pF, the extra HF varies considerably with volume level because there is an effective CRC filter from tone stack with C drive with a much variable R element of volume pot then Miller C. Some amps have the volume control fitted with a pull switch for some extra HF boost, aka presence. Adding another pot and C to vary presence could be interesting, easier than replacing existing pot. But just remember the speaker may not reproduce much above 3kHz.
There is a danger that with treble turned fully up, and with presence due additional C in path, there can be unwanted HF oscillation via stray C and the high gain of so many 12AX7.Maybe using 47k in series with 47pF across volume pot might avoid such problems

With all controls at 5, mid rotation, the response is not flat, but with
50Hz = 0dB,
100Hz = +1.5dB,
220Hz = 0dB,
500Hz = - 1.5dB,
1kHz = 0dB,
2kHz = +2dB,
3kHz = +5dB,
5kHz = +8dB.
10kHz = +10dB.
This doesn't matter when considering the very variable levels with musical signals from a guitar pick up.

The knob settings for best 1kHz square wave and flattest sine wave response possible with vibrato channel, and **at the amp output** with 8r0 dummy R load is B3, M5, T2, and Volume at 6.

50Hz can be varied +11dB, -8dB.
100Hz can be varied +8dB, - 5dB.
least level adjustment occurs at 320Hz.
2kHz can be varied +14dB, - 5dB
5kHz can be varied +18dB, -2dB.

Maximum mid control is possible with both B and T set at 10.
The mid dip thus created occurs about 320Hz with M at 5.
Dip can be varied +3dB and - 7.5dB at 320Hz, and less than +/- 2dB at 140Hz and 600Hz.
The Q of the maximum dip at M1 = 2.3, higher than 1.0, and slopes of both dip sides is greater than 6dB/octave, and just how well that interacts with perceived sound is anyone's guess. Nobody seems to object to the mid dip.
But to be able to tune the F of the dip and to adjust the Q would at least require an extra triode and 2 pots and some R and C - IMHO.

All F response measurements done without any stage of the amp clipping.

There is no master volume control between preamp 12AX7 just ahead of power amp. Therefore the preamp tubes ahead of the power amp cannot be easily overloaded without power amp clipping first with just 2.5Vrms being made by preceding preamp tubes. To make the preamp tubes clip would need them to produce about 50Vrms.

It would be possible to overload the input tube which drives the tone stack by using an external preamp between guitar and amp input, then set volume at low, so cleanly amplify an already distorted signal, as required by some for venues where low volume is wanted. But other mods are possible to enable devices to work to generate say 7% even numbered H without clipping to thus possibly warm the sound but I won't say just how now because its far too complicated to explain here and now.

Probably, some guitarists would find that a good starting point is setting both B and T to 2.5,
M to 5, and then just boost bass and treble if either of both sound low, and then adjust mid for best sound.

The Deluxe has 2 x 6V6 driven by a 12AT7 in LTP mode. B+ = 400Vdc with fixed grid bias.

OPT has ZR = 6,400r : 8r0.
Maximum Po = 21Watts at clipping onset for 13Vrms to 8r0.
Power amp needs 2.5Vrms input at 12AT7 grid pin 2.

The amount of GNFB = approximately 2dB with 8r0 load.
The NFB network has 820r : 47r so GNFB voltage at 12AT7 grid pin 7 = 0.7Vrms,
and shunting the 47r to remove all GNFB gives Vo increase = +2dB.
I calculated the amp output resistance = 18r, so damping factor with 8r0 0.5 = poor.
But low DF suits such amps where the sound can be allowed to be higher from speaker driver which has peaked Z at bass and around Fs. It also has increasing Z and sensitivity between 300Hz and 2kHz. So the speaker helps to act like a tone control with low bass boost and highish treble boost.

I expect that my customer's complaint about the sound not being as good
as his other old 1965 Fender combo is because the re-issue modern speaker
driver has different ( worse ) sounding characteristics compared to those made 50 years ago.

Having a control to reduce NFB to virtually zero does almost nothing to the
sound or to the measured performance because such a small amount of GNFB
is involved. Much is written about the sound of NFB in guitar amps, but
in the case of Deluxe there is SFA to actually write about.

There is a vast amount typed about guitar amps and by ppl who never ever measure anything or make a graph of responses et all and you may all assume that 90% of what is typed about guitar amps has a very horse **** %.

And nothing the amp can do will make up for the absence of talent in the guitarist.

Patrick Turner.
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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default LTSpice, guitar amp tone control


"Patrick Turner"

(snip tedious stuff)

There is a vast amount typed about guitar amps and by ppl
who never ever measure anything or make a graph of responses
et all and you may all assume that 90% of what is typed about
guitar amps has a very ( high) horse **** %.



** It's much higher than 90%.

Because the folk writing it are all know nothing types full to the eyeballs
with horse****.

Same goes for the whole of consumer hi-fi and most of pro-audio too.


And nothing the amp can do will make up for the absence of talent in the
guitarist.



** Shuuussssshhh - that is one of the BEST kept secrets in the whole
world !!



..... Phil






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Lord Valve Lord Valve is offline
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Default LTSpice, guitar amp tone control

Patrick Turner wrote:

Today I tested the whole Fender Deluxe with its vibrato channel which now has a CF buffer driving standard Fender tone stack but with a linear 10k0 pot for mid adjust. The CF buffer increases the gain by an average of +3dB.

The 1M volume pot following tone stack has 5pF across top of pot and wiper. But standard Deluxe schematic shows a fixed 47pF, so at some volume setting there is a maximum increase of HF because of the combined effect of following 12AX7 Miller input C and the C shunting part of the volume control resistance. Because this particular amp has 5pF, it is perhaps a mod, the HF boost due to 5pF is negligible. I found 47pF at volume pot would boost 5kHz by +3dB,( less at lower F and not much above 7kHz.) But with 47pF, the extra HF varies considerably with volume level because there is an effective CRC filter from tone stack with C drive with a much variable R element of volume pot then Miller C. Some amps have the volume control fitted with a pull switch for some extra HF boost, aka presence. Adding another pot and C to vary presence could be interesting, easier than replacing existing pot. But just remember the speaker may not reproduce much above 3kHz.
There is a danger that with treble turned fully up, and with presence due additional C in path, there can be unwanted HF oscillation via stray C and the high gain of so many 12AX7.Maybe using 47k in series with 47pF across volume pot might avoid such problems

With all controls at 5, mid rotation, the response is not flat, but with
50Hz = 0dB,
100Hz = +1.5dB,
220Hz = 0dB,
500Hz = - 1.5dB,
1kHz = 0dB,
2kHz = +2dB,
3kHz = +5dB,
5kHz = +8dB.
10kHz = +10dB.
This doesn't matter when considering the very variable levels with musical signals from a guitar pick up.

The knob settings for best 1kHz square wave and flattest sine wave response possible with vibrato channel, and **at the amp output** with 8r0 dummy R load is B3, M5, T2, and Volume at 6.

50Hz can be varied +11dB, -8dB.
100Hz can be varied +8dB, - 5dB.
least level adjustment occurs at 320Hz.
2kHz can be varied +14dB, - 5dB
5kHz can be varied +18dB, -2dB.

Maximum mid control is possible with both B and T set at 10.
The mid dip thus created occurs about 320Hz with M at 5.
Dip can be varied +3dB and - 7.5dB at 320Hz, and less than +/- 2dB at 140Hz and 600Hz.
The Q of the maximum dip at M1 = 2.3, higher than 1.0, and slopes of both dip sides is greater than 6dB/octave, and just how well that interacts with perceived sound is anyone's guess. Nobody seems to object to the mid dip.
But to be able to tune the F of the dip and to adjust the Q would at least require an extra triode and 2 pots and some R and C - IMHO.

All F response measurements done without any stage of the amp clipping.

There is no master volume control between preamp 12AX7 just ahead of power amp. Therefore the preamp tubes ahead of the power amp cannot be easily overloaded without power amp clipping first with just 2.5Vrms being made by preceding preamp tubes. To make the preamp tubes clip would need them to produce about 50Vrms.

It would be possible to overload the input tube which drives the tone stack by using an external preamp between guitar and amp input, then set volume at low, so cleanly amplify an already distorted signal, as required by some for venues where low volume is wanted. But other mods are possible to enable devices to work to generate say 7% even numbered H without clipping to thus possibly warm the sound but I won't say just how now because its far too complicated to explain here and now.

Probably, some guitarists would find that a good starting point is setting both B and T to 2.5,
M to 5, and then just boost bass and treble if either of both sound low, and then adjust mid for best sound.

The Deluxe has 2 x 6V6 driven by a 12AT7 in LTP mode. B+ = 400Vdc with fixed grid bias.

OPT has ZR = 6,400r : 8r0.
Maximum Po = 21Watts at clipping onset for 13Vrms to 8r0.
Power amp needs 2.5Vrms input at 12AT7 grid pin 2.

The amount of GNFB = approximately 2dB with 8r0 load.
The NFB network has 820r : 47r so GNFB voltage at 12AT7 grid pin 7 = 0.7Vrms,
and shunting the 47r to remove all GNFB gives Vo increase = +2dB.
I calculated the amp output resistance = 18r, so damping factor with 8r0 0.5 = poor.
But low DF suits such amps where the sound can be allowed to be higher from speaker driver which has peaked Z at bass and around Fs. It also has increasing Z and sensitivity between 300Hz and 2kHz. So the speaker helps to act like a tone control with low bass boost and highish treble boost.

I expect that my customer's complaint about the sound not being as good
as his other old 1965 Fender combo is because the re-issue modern speaker
driver has different ( worse ) sounding characteristics compared to those made 50 years ago.

Having a control to reduce NFB to virtually zero does almost nothing to the
sound or to the measured performance because such a small amount of GNFB
is involved. Much is written about the sound of NFB in guitar amps, but
in the case of Deluxe there is SFA to actually write about.

There is a vast amount typed about guitar amps and by ppl who never ever measure anything or make a graph of responses et all and you may all assume that 90% of what is typed about guitar amps has a very horse **** %.

And nothing the amp can do will make up for the absence of talent in the guitarist.

Patrick Turner.


...

It's a damn good thing that Leo Fender was just a
dumb-ass TV repairman. Otherwise, his amps
would've sounded like ****, having been designed
by some clueless hi-fi/audiophool ****** like you.
It's obvious you have no idea how these amps are
actually used by performers. shrug

You really have no idea why your customer's modern
knock-off sounds like **** compared to his '65, do
ya. And it's obvious that you're gonna do a whole
****load of turd-polishing before you find out it'll
never shine the way he wants it to. So the question
comes down to: WTF did Leo know 50 years ago
that the clueless ****s at FMI have forgotten today?

I shall enjoy watching you hot-shots stepping on your
cranks in your fruitless quest for real tone. TTFN...

Lord Valve
"Muso"


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