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Old 4th October 2011, 01:30 AM   #11
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by Alexrmc92 View Post
So lets use gain instead of amplification factor. In the champ amp, a calculated gain is about 60. So theoretically lets take 100mv X 60 = 6v, which is still higher than the grid bias voltage of the champ. As far as i know, this still causes the second stage to clip. Why does it not clip? Unless typical guitar voltage is more aroun 15 - 25 mv.


As I said above, there is NFB. but I was not so clear abut how it works. The second triode's cathode is connected to the speaker terminal. So let's assume you are correct and the signal is at 6V but if so, then the cathode voltage is also high and what matters is the difference. The second triode is actually a difference amplifier.

Also you might wonder why your tube with mu of 100 only has a gain of 60. Again it has to do with feedback. The cathode voltage (and hence the grid bias) is not fixed but depends on the input signal because the voltage drop across the cathode resister depends on the current. So the more the tube conducts the less it is biased.
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Old 4th October 2011, 02:07 AM   #12
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Which Champ circuit? The 5E1, 5F1 has two 12AX7 stages with 100k plate resistor and 1.5k on the cathodes with no bypass capacitors. Just a 1M pot between them. Also the second stage is within the negative feedback loop reducing gain.

The Champ before these models use a pentode rather than 12AX7 so it can sit this one out. The Champ after the above modeled have Fender's gain sucking tone stack (which give about a drop of 6dB when bass and treble are at their max and midrange dropped 20dB) in between the two triode stages.


I just bread boarded a 12AX7 with both stages at 220V, 100k plate and 1.5k cathode resistors, no capacitor bypass. I put 1.5V p-p into the input stage (close enough to 1V rms for me) and I got out a 30V p-p signal. So gain is 20 (remember no cathode bypass as the original 12AX7 Champ).

Then I attached the second stage, the signal at the 1M (I did not use a pot but a 1M resistor as I wanted the full signal to the second stage but wanted the 1M loading on the first stage) input to the second stage then appeared clipped in one half of the waveform. I then reduced the signal to the first stage till there was no visible clipping at the input of the second stage. I had to reduce the input to 100mV before the sine wave had no sign of clipping, and it just so happened the signal level at the 2nd stage input was about 2V p-p.

Both stages were biased at about 0.85V on the cathodes. So conceivably you should be able to put a 1.5V signal in before any input clipping, seems we can put a bit more than that.

Now just for kicks I checked how much signal was needed at the input of the first stage before the signal at the output of the second stage starts to distort. Well the two stages can put out 30V p-p (with a 220V supply) with a 60mV p-p sine wave at the input. Since this is more than enough to drive a 6V6 to full output any guitar should have enough signal. The extra gain, well you can use it as you will. Add bypass caps and you have more gain.

Last edited by Printer2; 4th October 2011 at 02:09 AM.
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Old 4th October 2011, 02:10 AM   #13
twystd is offline twystd  United States
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To put this in real simplistic terms, as others have pointed out, guitar pickups rarely put out 1V, the attenuation of tone and volume controls, the 12ax7 real gain (in an actual circuit) is typically much less than the theoretical gain, voltage dividers, and negative feedback, all these factors keeps things more in control.

If you've ever heard a guitar amp that doesn't clip, like with an audio amp, you'd be surprised how it sounds, very campfire like (for lack of a better term). Guitar amps should clip, while audio amps never should, the clipping (overdrive) is an essential character for guitar amps.

I have a friend that sometimes designs guitar amps, he says his sounded like crap until he made preamp stages that had at least 6 to 12dB of overdrive capability into the driver stage, and the driver stage should be able to overdrive the power stage as well.

Typically, audio amp preamp and driver stages never go into clipping, only the power stages, and then only when the driver stage is completely maxed out (swinging maximum voltage).

When an audio amps starts to clip, we turn it down, when a guitar amp starts to clip, well that's a good thing. It's important to realize the difference in design criteria for the two different amp genres.

twystd
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Old 4th October 2011, 02:31 AM   #14
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If you scope a guitar pickup you will see an initial attack that can be a volt maybe two with humbuckers. But the body of the signal is not all that high. So when the attack is shaved off by the clipping it is not really noticed all that much since it is full of high harmonics anyway. And what does clipping add, harmonics. It is mainly when we boost the signal when the rest of the signal is clipped that we think it as being a distorted guitar.
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Old 4th October 2011, 02:50 AM   #15
twystd is offline twystd  United States
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Quote:
Originally Posted by Printer2 View Post
If you scope a guitar pickup you will see an initial attack that can be a volt maybe two with humbuckers. But the body of the signal is not all that high. So when the attack is shaved off by the clipping it is not really noticed all that much since it is full of high harmonics anyway. And what does clipping add, harmonics. It is mainly when we boost the signal when the rest of the signal is clipped that we think it as being a distorted guitar.
That's interesting, I didn't know that, but certainly makes sense.

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Old 4th October 2011, 05:19 AM   #16
ChrisA is offline ChrisA  United States
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Originally Posted by Printer2 View Post
Now just for kicks I checked how much signal was needed at the input of the first stage before the signal at the output of the second stage starts to distort. Well the two stages can put out 30V p-p (with a 220V supply) with a 60mV p-p sine wave at the input. Since this is more than enough to drive a 6V6 to full output any guitar should have enough signal. The extra gain, well you can use it as you will. Add bypass caps and you have more gain.
The results of your bread board experiment are valid but the breadboard does not represent a Fender Champ. NFB changes "everything" and you left it out of the breadboard.

I just happen to have a couple Champs. One is a 5E1 clone I built and the other a Fender Champ 600. ( I built the 5E1 because I was slightly disappointed with the 600) Just this last weekend I took the feedback off the 5E1 in order to trouble shoot a problem. While I had it disconnected I thought I'd try plugging in a guitar. It dramatically changes the amp.

Last edited by ChrisA; 4th October 2011 at 05:37 AM.
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Old 4th October 2011, 05:49 AM   #17
ChrisA is offline ChrisA  United States
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Originally Posted by Printer2 View Post
If you scope a guitar pickup you will see an initial attack that can be a volt maybe two with hum buckers...
It also makes a huge difference it you are playing chords or a single string lead. It should be clear that six strings are louder than one string.

My theory is that the champ does not distort so easily because there is less gain than you'd think because only the first triode is outside of the NFB loop.
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Old 4th October 2011, 09:06 AM   #18
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Originally Posted by Alexrmc92 View Post
So why does the second stage not clip like crazy? is there some huge grid impedance or something else im missing?
Do you have any evidence that it doesn't clip?


IMHO, if you have a Champ that only plays clean, it needs new tubes!
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Old 4th October 2011, 04:03 PM   #19
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Quote:
Originally Posted by Keriwena View Post
Do you have any evidence that it doesn't clip?


IMHO, if you have a Champ that only plays clean, it needs new tubes!
If the volume pot is turned down then there isn't clipping. I was just using this as an example if the input to the tube was 60v then there would be constant clipping at almost any volume level.

Thanks for all the help guys, i have a much better understanding of this now.

My new question is if i were to make a tube distortion pedal (essentially an overdriven preamp in a pedal) what way is best to reduce the output of the second stage back to guitar level, lets say 100mv?

I was thinking of using a voltage divider but when you put the input of the next amp in parallel with the voltage divider it will change the amount of voltage dropped by the divider.

i guess It really depends on the impedance of the input section of a guitar amp, but they could all be different...
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Old 4th October 2011, 04:06 PM   #20
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If you'd use 2 volume controls, 1 after the first stage and the 2nd after the 2nd stage, the first one will determine the amount of distortion and the 2nd one the overall volume.
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