Overcoming interaction between control settings in FMV tone stack

[SoulFetish]

My worry was that the active tone control itself has signal gain (at settings where bass or treble is boosted), so its own output will clip, even if the signal coming into it isn't clipped.

To put some numbers to it: the one-knob tone control I'm experimenting with now has up to +12 dB of boost at 80 Hz and 5 kHz (i.e. a gain of about four times). Suppose you're using a 12AX7 with a 300 V B+, and its output can swing, say, 200 Vpp before clipping; the input will only need to swing 50 Vpp at 80 Hz or 5 kHz to cause the output to clip.

So I figure I have to do something to make sure the input to this particular tone control never gets larger than, say, 40 Vpp, to allow some safety margin.

If you drive it from a pentode cathode follower that can cleanly swing 250 Vpp without clipping, that's all well and good for the cathode follower, but that 250 Vpp will clip the tone control terribly.

The only way I can see to keep this from happening is to attenuate the output of the cathode follower down from 250 Vpp to 40 Vpp, i.e by a factor of 6.25 times. Now the cathode follower can be driven all the way to (hopefully good-sounding) clipping, but the active tone control still won't clip.

Does that make sense, or am I misunderstanding what you have in mind?

-Gnobuddy

Okay, I see what you mean. Its a solid point. Which ever way you decide to approach this, overload is going to be a concern.
(I've never designed an active tone circuit, so if I overlook some obvious point or reveal my ignorance, I apologize.)

I suppose this is how I imagine it...
–First, forget the +/- 150V drawn in the schematic above. Since most designs have their own power supply needs, lets assume we'll be designing our own power supply with a separate, stiff, low impedance supply for this section. This gives us some flexibility to set the rail voltages high enough to provide plenty of headroom. I would design the Phase Inverter to provide Plenty of P-P signal swing in order to overdrive the output tubes before the input signal was allowed to clip the PI stage. But we would allow for Phase inverter clipping at some point following the output stage (to taste).
We then set the output of the phase inverter/input of the cathode follower as the open loop gain. Then, we can design a filter using the balanced signal in order set the closed loop gain to -12dB. It almost seems too easy to tap off some of the cathode load resistance and feed the signal right into the cathodes. My hypothesis is this gives us the necessary headroom at the input of the stage and by nature attenuates the signal at the cathode(to the closed loop gain), without risk of overload and defeating the effects of the negative feedback.
So, what do you think? ...So crazy that it just might work?

[SoulFetish]

An active control has gain, as far as I know by definition. As to it clipping, EVERY stage in an amp has potential to clip. But it doesn't clip just because it is a gain stage. It clips because the signal level exceeds the ability of the stage.

I think by definition, an active filter uses and an active device/devices and not just passive components??
But, there's no way to get around your larger point. It wiil definitely have gain, even if it's unity, or negative gain.

[Gnobuddy]

An active control has gain, as far as I know by definition. As to it clipping, EVERY stage in an amp has potential to clip. But it doesn't clip just because it is a gain stage. It clips because the signal level exceeds the ability of the stage.

Agreed on all counts. I guess I just didn't do a good job of making myself clear in my previous posts.

The question then becomes : will you have that level at the stage input?

Exactly. But this is a guitar amp we're talking about - the amp I'm building now will end up in a friend's hands, and I have no guarantee where he will set the knobs, or whether he will try to use a gain pedal ahead of the amp. So it's exceedingly likely that the preceding stage can put out enough signal to overload the active tone control. And it's exceedingly likely that an active tone control will misbehave - badly - if overloaded.

Based on Chuck H's comments earlier in this thread, commercial guitar amps with active tone controls seem to drop the signal level to instrument level (for the FX loop) before sending them through the active tone control. This is an excellent way to keep the tone control out of clipping - if you're building an amp with an FX loop!

Even on a clean amp, we can turn controls up to the point of clipping.

Exactly, to some guitar players, "clean" often means "dirty". I've seen a lot of You Tube videos where some guitarist about to demo an overdrive or distortion pedal says "First let's listen to my clean tone", and then proceeds to play some heavily distorted sounds from an amp that is already well into clipping. (Then he clicks in the distortion pedal as well, but you can't hear what it sounds like because there is already so much distortion.)

This is why I think the fool-proof solution is to turn up the preceding stage till it is in full clip, then attenuate the output by a little more than the maximum gain of the subsequent active tone control. Set up this way, the tone control can never clip. (I'm assuming the tone control uses a similar active device to the preceding stage, i.e. both 12AX7 stages, or both JFETs in my case, and both stages run on the same or similar B+.)

-Gnobuddy

[loudthud]

Presence or Resonance controls actually reduce feedback as they are advanced form their zero position. (Hiwatt is an exception.)

Tubes naturally produce low order distortion that raises with signal level. This sounds great for guitar or bass. (some solid state circuits can also do this.) If you apply feedback around tubes or even local feedback at the cathodes, the distortion spectrum shifts to higher order harmonics, although overall the total distortion level is lower. This doesn't sound as good. Evidence: Compare a Music Man amp to an all tube Fender equivalent. The Music Man just sounds cold and sterile, The Fender is much warmer even if you plug straight in with the Volume set on 2. You can instantly tell the difference in a blind test.

Interaction of controls in the FMV tone stack.

First consider how the FMV stack works. You have a high pass and a low pass filter in parallel. The high pass (series C, R to ground) is the treble cap and the treble control (ignore for a moment that the bottom of the treble control doesn't go to ground. The low pass (series R, C to ground) is the slope resistor and the cap to the Mid pot. The mid pot lets mids sneak through the low pass and changes the low pass into more of a shelving filter. The cap that connects between the slop resistor and the top of the bass control blocks DC and provides some attenuation to very low bass. (Notice it is absent from Music Man stacks.) The bass pot attenuates bass by shunt action. Bass travels up through the bottom of the Treble pot with little attenuation because the treble cap looks like a high impedance at low frequencies. The bottom of the treble pot sees a fairly low impedance at high frequencies, (basically the Mid pot) so it's almost like it is grounded.

The FMV stack tends to show a trench in the mid-range frequencies. The Marshall, not so much, Fender a moderate trench and the Vox a deep trench. One reason for the trench is phase cancellation when the high pass and low pass outputs are combined. The wider you set the break frequencies of the two filters, the deeper the trench. If you change the level of the output of one of the filters, the trench moves. Even if you used separate stages for the high and low pass filters, you would still get a movable trench when they are combined because of the phase shifts.

A twin T or bridged T filter also produces a trench. It's interesting to build a tune-able (got a 4 gang pot?) twin T and play as you tune it. It sounds like FMV interaction of steroids. It might make you swear off the FMV or use it with a Voight, James or Baxandall.

[J M Fahey]

Is there really a community (woo-woo or otherwise) where "FB is Bad"? Am I missing out on something...?

Go no further than thriving (very active, 65000 member) diyAudio

Just as a free sample, I opened a page where they are very excited bout an Audiophile power amplifier, based on a 300B single triode ... direct heated of course and ... you guessed it ... NO feedback.

Of course then you need to use extra super duper beyond linear fairy dust impregnated components.
Such as this transformer, designed specially for that.

Not your average off the shelf generic transformer of course.

This one does not use cheesy silicon steel EI *laminations* , ugh!!!! but a "nickel base Nano amorphous core ".
And no greasy sleazy copper wire !!!!!!!!!! but "6N furukawa wire"

Don´t accuse them of being unrealistic cork sniffers, they are very sad because the *real* wire they wanted to have actually was: "5N mono crystalline silver wire OPT but its too expensive
"

the full thread is: Best SE output transformer for 300b - shootout - Page 2 - diyAudio but as a sample post there:

tube-lover is offline tube-lover Hong Kong
diyAudio Member

Join Date: Jul 2004
Location: Hong Kong

hi,
if U like the amorphous OPT probably I can help U. I was OEM some audio transformers to japan audio shops. this is nickel base Nano amorphous core OPT. with 6N furukawa wire. take a look, I als had 5N mono crystalline silver wire OPT but its too expensive,,. take a look for test result with 5002, 5002 was worst in bass. I use tango X-series winding to nickel core & solve this problem already. & me.

thx
thomas

I bet the transformer cost will match the Fiscal Debt of some small Countries

[SoulFetish]

THREE HUNDRED F*CKING B! The most overrated active device in the history of man. Someday im gonna take some and make a distortion pedal out of them; and only use them as clipping diodes

[J M Fahey]

THREE HUNDRED F*CKING B! The most overrated active device in the history of man. Someday im gonna take some and make a distortion pedal out of them; and only use them as clipping diodes

You are being UNFAIR to the poor tubes , you are an EVIL man.

OF COURSE; you can´t even *mention* them, since you have not complied with the MINIMUM requiirement: listen to them for continuous 100/200/300 hours.
Chained to a column or a dungeon wall of course, can´t imagine any other way aHuman being can dio that:

In Search of the Perfect 300B Tube Page 3

If one is to give a reliable evaluation of a 300B, it is essential that the tube be burned in for an appreciable amount of time (footnote 4). One hundred hours are not enough; 200 hours seems to be a workable minimum (footnote 5). In doubtful cases, we added an extra 100 hours

Read more at https://www.stereophile.com/content/...F7Vfw8bVo0P.99

I bet after 300 hours you will sign ANYTHING, even your own Death warrant, screaming "just turn that goddamn amp OFF".

Unless you are a masochistic weirdo and actually enjoy the ordeal, that is

Of course, you can surrender in the first 5 or 10 hours or whenever you start feeling sleep deprived and signe the "I Believe" statement.
No guilt involved, after all the ¨"should" sound good, otherwise they wouldn't:

Prices for new 300B tubes ranged from US$175 to $2,000 per matched pair.

[SoulFetish]

Can you imagine paying $2000 just to listen to triodes sound like shit as output tubes??

[Leo_Gnardo]

Chained to a column or a dungeon wall of course, can´t imagine any other way aHuman being can dio that:

I bet after 300 hours you will sign ANYTHING, even your own Death warrant, screaming "just turn that goddamn amp OFF".

Unless you are a masochistic weirdo and actually enjoy the ordeal, that is

Oh no, leave those 300B amps ON, they're the only heat in my dungeon.

Remember in the late 1990's no less a company than Lucent (formerly Bell Laboratories) contacted ex-employees who had worked at the old Western Electric tube plant in Kansas, offered them new jobs in sunny Georgia building brand new 300B's. I think the "genuine item" was offered as matched pairs, in velvet lined wood presentation boxes like bottles of rare Cognac. IIRC they started out at $850 a pair. Should'a picked up a truck full... Guess I'll have to limp along with the batch of funky ol' 211's I have.

Can you imagine paying $2000 just to listen to triodes sound like shit as output tubes??

Have you listened to music played through a good 300B amp? Mmmmmm, yeh, I didn't think so. Me neither. But I wouldn't knock 'em 'til I tried 'em. The hi fi snobs have been nutty about them for decades. I figure where there's smoke, maybe there is fire. Then again it could be some audio geek tokin' on his hookah, then writing equipment reviews once he's gotten gassed on his hubble-bubble. Personally I reckon it's kinda like some expensive esoteric food, say truffles. Maybe they're terrific but I just don't know.

Also if you think you're an expert on triodes and how they suck because you're not happy with what you hear in guitar amps that have a switch that can select between normal and triode mode, I can definitely tell you I have LOTS of customers who DO like that subdued tone. Maybe you want to slag them off as cloth-eared idiots too. But I can't complain when I hear them play. If they make a decision to avoid razor sharp squeaky high frequency tone via triode mode, that's a perfectly valid artistic choice. I'm listening to what they play, the notes and the tones, not watching what position their switch is in. And the ones that can really play well sound good to me regardless of triode, tetrode, pentode or whatever.

[Gnobuddy]

Tubes naturally produce low order distortion that raises with signal level. This sounds great for guitar or bass. (some solid state circuits can also do this.) If you apply feedback around tubes or even local feedback at the cathodes, the distortion spectrum shifts to higher order harmonics, although overall the total distortion level is lower. This doesn't sound as good.

I agree, in fact, there is usually enough feedback in an active tone control to push all distortion below the audible threshold. So what you get is a sterile-clean tone control: it adds no warmth or musical distortion or "valveyness / tubeyness" to the sound at all. But what it *does* do is a very good job of manipulating EQ curves, in much more subtle and sophisticated ways than an FMV passive tone control can.

To put some rough numbers to it: a typical 12AX7 common-cathode stage might have a voltage gain of 50 times (34 dB) and add maybe 3% - 5% of (mostly 2nd and 3rd harmonic) distortion. In my experience, this is actually a very small amount of distortion, and most people, most of the time, with a guitar as input and the same EQ curve in both, won't hear any difference between 5% and 0.01% distortion. But if you have several gain stages each contributing 5% distortion, then people with good ears will start to hear "Fender clean" or "tubey clean", which sounds sweeter than solid-state "sterile clean".

So now let's take that +34 dB, 5% distortion, triode gain stage, and wrap an (active) Baxandall tone control network around it. With the Baxandall network in place, the closed-loop gain is reduced to unity (0 dB) with tone controls flat. This means you now have 34 dB of negative feedback. Simple feedback theory predicts that the distortion from the tube will be reduced by 34 dB, or fifty-fold. That means the distortion is now down to 0.1%!

Even for Hi-Fi, with excellent full-bandwidth Hi-Fi speakers and a sine-wave test tone, 0.1% THD is undetectable to most people. Now replace the sine-wave with an electric guitar, replace the full-bandwidth Hi-Fi speaker with a guitar speaker, and I guarantee you that nobody on earth can detect 0.1% THD!

So yeah, an active tone control is almost certainly going to be "sterile clean" by itself.

But this doesn't mean the entire guitar amp has to be sterile-sounding. It just means you have to make sure the gain stages before and after the active tone controls provide the "valveyness" you want the amp to have. The tone control itself will be sterile-clean. But the amp won't.

This is exactly like using a graphic EQ pedal along with a good valve guitar amp, something many, many pro musicians do. The graphic EQ pedal is sterile-clean. But the combination of amp + EQ is not!

Compare a Music Man amp to an all tube Fender equivalent. The Music Man just sounds cold and sterile, The Fender is much warmer

I believe you (I have never heard any Music Man amp). But it just means the Music Man engineers didn't get the details right: they should have added one or more triode gain stages to get "tube warmth" into their amp.

It's interesting to build a tune-able (got a 4 gang pot?) twin T and play as you tune it.

My version of that is to put a Danelectro Fish-n-Chips 7-band graphic EQ pedal in between guitar and amp, and fiddle with the EQ while listening to the effect on guitar tone. You can move any slider down to get a notch at that frequency. Or you can move several to create a more broad-band change in "voicing".

It's absolutely amazing how much you can improve the sound of a guitar with some careful EQ tweaks using a $30 guitar pedal!

-Gnobuddy

[Gnobuddy]

...the ones that can really play well sound good to me regardless of triode, tetrode, pentode or whatever.

This is the reason why I don't believe in magic devices (300B, guitar pedal of the day, amp of the moment, 50 year old vintage Les Paul guitar, whatever.)

In the end, the magic is in the musician, not the lifeless dead bits and pieces he/she is playing through.

The trouble is that magic is in short supply, you can't buy it at the store, and for some of us, thirty years of playing is only long enough to realize that you are never going to be great. So we start looking for magic devices that might make us sound great even though we actually aren't great.

At least I can find comfort in the thought that I can always get better. Great may be out of reach, but better isn't.

Then again: a musician friend recently had to quit playing her violin because of increasing arthritis and neurological pain in her arms, hands, and shoulders. She is 87 years old.

So I guess it's not true that I can always get better, either. There will come a time when the limitations of old age will outweigh whatever increased experience and expertise I have. From that point on, I will get worse, rather than better, no matter what I do.

That's sobering. I guess it's a reminder to enjoy making music while I still can!

-Gnobuddy

[Gnobuddy]

There are several LTSpice models for a logarithmic pot and one of them is directly in LTSpice (but it's difficult to find it).

So sorry, I completely forgot about this post - my apologies!

I never found that log pot in LTSpice! There is still a lot I don't know about LTSpice's capabilities. I just followed a few of Simon Bramble's online tutorials, which was enough for me to do the simulations I've wanted to do so far.

Please run it and tell me whether it runs OK without the symbol delivered additionally. I think it will run correctly.

You're right, it runs just fine. I just tried it.

I understand the equations but I was worried about the symbol "1 m" (with a blank in between). I think that if you write it with a blank, the "m" symbol will be ignored and you get 1 ohm instead of 1 milli-ohm as you wanted.

Hmm, interesting! Here's a weird thing: I went back and looked, and found no blank between "1" and "m" in the .asc file I had loaded up in LTSpice.

But I'm running LTSpice in Wine, on a PC running Linux. Run this way, the fonts may not be the same ones you see when run natively on Windows. It's possible that the font LTSpice used to render "1m" simply suffers from bad kerning. This was a fairly common problem on Linux a few years ago, because the free fonts available were not up to the same standards as the good professional ones. However, those good professional fonts are very expensive, and cannot legally be used without paying for them, so couldn't be used in Linux.

I think I *have* left gaps between value and SI prefix in other schematics I've simulated (i.e written "10 uF" rather than "10uF"), which should have produced a factor of a million error (!) if LTSpice truly wasn't reading the SI prefix. So I think LTSpice does read the prefix in either case. But I should check, just to be sure.

By the way, I forgot to mention, the 40k resistor in my schematic of the Voight tone control actually represents the output resistance of the 12AX7 stage driving it (assuming the usual 100k anode load and internal anode resistance in the 60k - 70k region). The point of inserting that in the schematic was to see if the Voight circuit would behave well without an input buffer. And it does!

Maybe you could verify it? The equation that I use is directly in the subcircuit definition.

It seems to produce the identical EQ curves to my DIY version. I think the log pot equations are actually mathematically identical to the pair of equations I came up with, just written slightly differently, with the nonlinear factor "x" put into a separate equation by itself.

i.e., I put the full nonlinear mathematical definition into the value of each of the two resistors comprising the pot, while the LTSpice equation has a linear equation in "x" for those, but then you notice that "x" itself is nonlinear...

You said also that you don't know how it sounds. But I hope that you know that in LTSpice you can specify a *.wav file as a source signal and save the output signal. In this way you can "hear" the circuit without building it.

Nifty! No, I didn't know that. Thanks for telling me!

-Gnobuddy

[SoulFetish]

Have you listened to music played through a good 300B amp? Mmmmmm, yeh, I didn't think so.

Um... Of course I've never listened to one. NOBODY HAS ACTUALLY EVER LISTENED TO THEM! Yet, they'll line up one after the other to tell you how great they sound. But really, would you expect anything else? I mean the whole experience is so linear when it comes to 300Bs. I guess that's why I call bullshit on the whole thing. I'm starting to doubt they actually exist. I think the government fabricated the whole thing as part of the MK Ultra mind control experiments. I hope they do exist though. I'm counting on them to sound great inside my Chandler Tube Drive.

Also if you think you're an expert on triodes and how they suck because you're not happy with what you hear in guitar amps that have a switch that can select between normal and triode mode

Ha! What can I say, Leo. You're so dead ass right on this one, it's like you were there.
Fair enough, this application is probably an incomplete sampling to draw sharp distinctions on Triodes in output stages. I'll keep an open mind.

[Chuck H]

AFAIK the application of "triode mode" to guitar amps is never done for tone, it's done to reduce power. Whereas in hi fi triode operation is typically hard wired in and done because designers wanted more triode like performance out of the lesser performing (WRT linearity) pentode tubes. Forcing a pentode into triode operation may be less than ideal in either case, but still adequate for getting the job done most of the time. But I think my point is that a true triode for the goals in hi fi may actually be better. Just as operating pentodes as pentodes is better for guitar amps.?. Just an outside observation. I've never heard a 300B amp either. But I don't doubt there may be an advantage to a PROPERLY designed triode amplifier for linearity and low distortion. I wouldn't use a tractor as a commuter vehicle any more than I would try to plow a field with a Celica. Both have four tires, a steering wheel and a motor, but they are intended for different jobs.

When I was starting in this genre of tubes the 300B was made of unobtanium. No one was making new ones and old ones were selling for a thousand bucks each. It was intriguing, but never a game I wanted to play.

[Mike Sulzer]

I first heard of the 300B maybe fifty years ago. It had a fabulous reputation as a tube for high fidelity applications, but even then it was kind of out of my price range. I did make several stereo amps using other triodes, and as far as I was concerned triode amps were the standard to beat until SS did a couple decades later. It is the same sort of thing: if you are going to have distortion, it should be low order. But for HiFi, it is without doubt better to have none. Except for those folks who cannot wean themselves from that warm, hazy sound in order to learn what good source material reveals with a really good amp.