So, non-expert here with what is probably a dumb question. Related to analyzing a guitar amp, where it uses a 12AX7 for first an input amplifier, and then a tone section, it doesn't make sense that I can first amplify a guitar signal of let's say +/- 2 V, into a much larger voltage, and then couple that voltage into the grid of the tone stage. If the load line of the 12AX7 expects grid voltage swings of a couple of volts, which OK for the guitar output, how isn't the amplified version then way too much for the input of the tone section? There's probably an obvious explanation so I appreciate any insight!
You sort of answered your own question ie your right it can get overloaded...... Depending on the pickup, figure 200mV to 500mV is normal range of light strumming.... hard banging can produce peak transients between 1 and 2 volts... The example you mention sounds like classic BF /SF input stage... The first stage is working into a very lossy tone stack ...The signal attenuation is significant ....Since this is a guitar amp..the front end can easily be over-loaded to make for some nice harmonics ..if you turn the BASS up just a tad too much you can get overload very easily..
Thanks for the response - but I think you may have missed my point. Using easy numbers, and assume a well behaved guitar signal (within limits), if the guitar signal is +/-1 Vpk, and the amplifier gain set by the plate resistor is 20, then the output of the input amplifier would be +/-20V? If so, then that is sent into the grid of the tone stack? I know the tone circuit will attenuate the signal, but it's before that happens I'm curious about. Hopefully that's better and thanks again...
It all depends on the circuit that you have in mind. Schematic would help.
Seems to me, the situation you listed, is designed for a passive gain controls (high and low) before the tone stack stage.
Seems to me, the situation you listed, is designed for a passive gain controls (high and low) before the tone stack stage.
OK, so I think I was confusing in my wording. I was calling the 2nd state the tone. In this circuit, if i understand correctly, if the Volume pot is all the way up (arrow at the top of the wiper), isn't the amplified signal coupled directly to the grid of the 2nd stage? If so the would seems to violate the load/grid voltage swing the should be allowable?
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I have found my guitar is way less than +/- 2 volts.
Its just a few 10's of millivolts.
I can only guess you have a different type of pick up.
The 12ax7 is used in hundreds of guitar valve amps so should work ok.
If your guitar really is putting out +/-2 v then it might need a volume control on the input to tame it.
So gain is roughly 59 in the mid band....neglecting the tone pot..
59 x 200mV light to medium strumming = 12V across Volume pot pot..
LOG taper pot varies depending on maker and many other factors...
Let say you set the pot to half-way ... figure .1 of 1MEG AUDIO taper pot by PEC (measured)
.1 x 12V = 1.2V
2nd stage biasing roughly at 1.5V for 5F2 circuit..but can vary..
Yes. The 2nd Stage will begin to saturate (aka: Overdrive) before it reach the top. Isn't that what that circuit is designed for?
> how isn't the amplified version then way too much for the input of the tone section?
I think the key point is: *It has a Volume knob!* If it is too loud and distrorted, and you don't want that, TURN IT DOWN!
What is "too much" for a network of resistors pots and caps? They are likely 400V caps.
1V peak is possible from e-guitar but would be REALLY STRONG. 200mV peak is more likely.
However, guitarists like to be able to overdrive the output with 20mVrms (30mV peak) at the input, everything on 10. Any lower sensitivity (higher voltage needed) means the guitarist has to work too hard. Why have an ox and pull the plow yourself?
The gain of that stage IS 50 for practical purpose.
So for this very strong signal we have 10V coming out of the first stage.
Work from the speaker toward the front. The 6V6 needs about 20V peak on grid. The AX7 before it, in the original unbypassed form, has gain of 25. So *at the Volume pot wiper* the overload level is 0.8V.
If you are slamming 10V out of the preamp it is going to be 12X overloaded. This is gross distortion but not unusual *for e-guitar work*.
Or you can turn the Volume knob down to 1/12X (maybe "4" on a log pot) and have just-clipping.
Or you can leave it full-up, do NOT strum like John Henry hammering spikes, an easy 10mV-20mV out of the axe brings the poor 6V6 just to the edge of clipping, so small adjustments of finger technique bring-in overload as an accent on desired notes.
I think the key point is: *It has a Volume knob!* If it is too loud and distrorted, and you don't want that, TURN IT DOWN!
What is "too much" for a network of resistors pots and caps? They are likely 400V caps.
1V peak is possible from e-guitar but would be REALLY STRONG. 200mV peak is more likely.
However, guitarists like to be able to overdrive the output with 20mVrms (30mV peak) at the input, everything on 10. Any lower sensitivity (higher voltage needed) means the guitarist has to work too hard. Why have an ox and pull the plow yourself?
The gain of that stage IS 50 for practical purpose.
So for this very strong signal we have 10V coming out of the first stage.
Work from the speaker toward the front. The 6V6 needs about 20V peak on grid. The AX7 before it, in the original unbypassed form, has gain of 25. So *at the Volume pot wiper* the overload level is 0.8V.
If you are slamming 10V out of the preamp it is going to be 12X overloaded. This is gross distortion but not unusual *for e-guitar work*.
Or you can turn the Volume knob down to 1/12X (maybe "4" on a log pot) and have just-clipping.
Or you can leave it full-up, do NOT strum like John Henry hammering spikes, an easy 10mV-20mV out of the axe brings the poor 6V6 just to the edge of clipping, so small adjustments of finger technique bring-in overload as an accent on desired notes.
OK, thanks everyone so far, we're almost there. I should just mention that my questions are circuit related, as in, I'm not actually saying I have a problem, so of course, turn down the volume if it distorts too much. I'm just curious from an electrical standpoint. So, with that said, hear me out one more time. IF, using cerrem's numbers, we have a 12V swing from the input stage, then according to my eye, virtually all of that amplification cannot be used if I apply it to a 12AX7 chart. This chart, with an added arbitrary load line, would seem to indicate the ABSOLUTE limit of allowable grid voltage swing is +/- 2V (can't go above 0 right?)? If so, two things - what then is the point of amplifying to +/- 12V? Also, with this being a 5F2A Princeton circuit, which I've built, how is it I can turn it up all the way, albeit with distortion, but it's still musical - as in it doesn't cutoff, etc.? Thanks! Am I making sense?
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So what actually happens if you feed it a sine wave, from a high impedance source (previous triode), with an unloaded peak to peak 12 volts? What happens when that absolute limit is exceeded?
The next triode has Vgk maybe (-1.5 V). As you said, the grid can't go above 0 V (more or less). So the triode responds more-or-less linearly to the signal swinging positive from -1.5V to 0 V, then clips off the rest of the positive half cycle. At the anode, you get a negative-going half-cycle of signal, heavily flat-topped (or should that be flat-bottomed?)
And for negative input half-cycles, once the signal swings from -1.5V to roughly -4V, the triode cuts pretty much completely off. It's anode voltage has now risen to nearly the full B+ voltage, and there it will stay, until the input signal once more rises above -4 V on its journey back to zero volts.
So the output is heavily flat-topped on both positive and negative peaks. But it's not as if anything explodes, or the universe ends, or the world stops turning. You just get a distorted output. Which we may like.
(As PRR said, if you don't want that, just turn down the volume.)
By all accounts, Leonidas Fender didn't want to create a distorting amp. He just didn't know any better, being an accountant and not an electronics engineer. This amp was one big mistake from his point of view.
But, surprise, musical tastes changed, guitarists and audiences both began to like the sound of distorted e-guitars, and Ye Olde 5E3 became wildly popular for some genres of music. Fender Corp. still makes reissue versions, because guitarists still want them.
Originally, an accident due to Leo's ignorance. Today, a desired and valued trait. Newer Fender products like the Blues Junior use a similar topology to make sure you can overdrive and distort that second triode if you want to. Most buyers want to.
(Other guitar amp brands intended for more distorted genres of music take this much further, distorting and clipping the heck out of the guitar signal in multiple successive gain stages.)
It probably does cut off, on every half-cyle. And saturate, on every other half-cycle. Unless you have a very low-output guitar pickup, play with a very light touch, or have the guitar's own volume control turned well down. It's precisely because it's saturating and cutting off that it sounds distorted.
Not yet, but we're all trying to find out exactly what's bothering you. 😀
-Gnobuddy
"...up all the way, albeit with distortion, but it's still musical - as in it doesn't cutoff, etc.? "
It is only the top and bottom of each cycle that gets "cut off". Not the whole sound/signal through the stage.
It is only the top and bottom of each cycle that gets "cut off". Not the whole sound/signal through the stage.
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See Hamm, R.O 1973 21(4) JAES "Tubes Versus Transistors-is there an Audible Difference"
and
Rutt, T.E.: Vacuum Tube Triode Nonlinearity as Part of the Electric Guitar Sound, AES 76th Convention, New York, USA, 1984 October 8-11. (Particularly figure 7 & 8 )
R.O. Hamm said:
R.O. Hamm said:
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#1 The first stage pre-amp is capable of driving to +-12V only if the loading is a high impedance passive load. The 2nd stage loading in the circuit posted will prevent it from reaching much pass zero voltage. As the output voltage of the 1st stage go near 0V, the 12AX7 of 2nd stage grid will begin to draw current. This will begin push the 1st stage output voltage down (clipping). The 100K anode resistor is doing that. The 2nd stage input will never see 12V.
#2 An easy way is put a say 10K or other resistor in series of the 2nd stage grid (aka: Grid stopper) . This will drop the input voltage in a more linear fashion (a different sound).
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All above are excellent explanations.
Let me add just one detail: yes, strumming hard and with volume pot on 10, next grid WILL clip.
To be more precise, it will chop any positive swing above +2V because you have a "hidden in plain sight" Grid to Cathode diode, the Grid being its anode; you can swing down as much as you like in the negative side because it will be reverse biased.
Yet "it doesn´t matter"
Why?
Because way before that, the power stage will be clipping , so any added earlier stage clipping will be masked by it.
Besides the fact that for better or for worse, "tube distortion sounds nice" 🙂 so we forgive a lot of it.
Let me add just one detail: yes, strumming hard and with volume pot on 10, next grid WILL clip.
To be more precise, it will chop any positive swing above +2V because you have a "hidden in plain sight" Grid to Cathode diode, the Grid being its anode; you can swing down as much as you like in the negative side because it will be reverse biased.
Yet "it doesn´t matter"
Why?
Because way before that, the power stage will be clipping , so any added earlier stage clipping will be masked by it.
Besides the fact that for better or for worse, "tube distortion sounds nice" 🙂 so we forgive a lot of it.
I think that, for almost the entire history of Western music, people have gradually been trained to accept sounds that were more and more harsh, compared to what came before.
In the early days of Western christianity, somewhere nearly two thousand years ago, it was only acceptable to hear one single sung note at a time - plainsong was monophonic. Two notes at once was considered too harsh for singing God's praises.
A century or two later, the octave was considered acceptable. Two notes at a time, but one octave apart. All other musical intervals were too harsh to be tolerated in church.
Another century or two later, the perfect fifth was now acceptable, along with the octave.
Another century or two, and major and minor thirds were finally included, along with fifths and octaves. So there is the beginning of what we now call triads - the simplest chords, the "cowboy chords" every beginning guitarist learns. But it took centuries of "that is too harsh!" before people were able to accept three notes at a time. And there were still unacceptable intervals triads - the "devils interval", the diminished fifth, and chords built on it, was still banned - literally - by the Church.
A few more centuries, and four-note chords became acceptable. Major seventh and dominant seventh chords were now found in classical and worship music.
The golden era of jazz quickly created even more complex chords, like the flat 9ths and flat 13ths routinely used in jazz now. But no distortion from the instrument - all the harshness came from dissonant musical intervals, not from distorting electronics.
It seems like it was only in the 1950s and 1960s that outright guitar distortion began to be acceptable - only to the young and outrageous. Their parents could not stand it at all.
Fifty years after that, even the formerly "smooth" genres - country, pop - include harsh distorted guitar.
We are now trained from birth (maybe before that, listening while we are still inside our mother's body) to like the sounds of distorted guitars. If mom listened to Cream while pregnant, the baby is born already conditioned to like distorted guitar sounds.
In the 1980s we arrived at the point where some music genres had so much distortion that nothing was left of the sound of the guitar itself. People who grew up with metal hear that as the normal sound of a guitar.
I don't know what happens to babies whose mom and pop listened to Alice In Chains. Maybe they listen to music made with power tools:
(1) YouTube
2) YouTube
But it took about 2000 years to get here from there.
-Gnobuddy
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Leo first made the Champ with a 6SJ7, changed it to a 12AX7. Why? Not enough gain for the pickups of the day. You also have two holes (mostly) to plug into on his amps, a higher gain and a lower gain. The reason being to limit drive to the input if you have distortion or use the gain available if not. Also two inputs were there to use two instruments at a time, heck even a microphone. And when doing that you can load each other's output somewhat. And then there is the Princeton tone control. There is no treble boost unless you have the volume knob turned down. So if you have an instrument which is dull sounding you use some gain to get the treble up.
Now is the amount of gain of a 12AX7 needed in a Champ circuit? Maybe not but Leo, being an accountant, probably did not want to keep a 6SJ7 in stock when the rest of his line changed over to those new darn tiny small signal tubes.
I bet two triodes in one 12AX7 worked out a lot cheaper per gain stage.
Buy one 12AX7 and one tube socket, get two gain stages. Buy one 6SJ7 and one tube socket, get only one gain stage.
As long as the 12AX7 plus its socket costs less than twice as much as a 6SJ7, you've spent less money per gain stage. An accountant would definitely notice that!
-Gnobuddy
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