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Alexrmc92 3rd October 2011 10:26 PM

I need an explanation.
 
In the fender champ amp schematic, the 12AX7 pre amp has 2 class A stages. Both stages are biased the same.

A 1v signal comes in to the first stage and is amplified to 100v (the 12ax7's gain according to the datasheet). 100 volts hits the grid of the second stage but the second stage is biased for 1.5v just like the first stage.

So why does the second stage not clip like crazy? is there some huge grid impedance or something else im missing?

sreten 3rd October 2011 11:11 PM

Hi, around 100mV in is a lot more reasonable than 1V, rgds, sreten.

HollowState 3rd October 2011 11:32 PM

Quote:

Originally Posted by Alexrmc92 (Post 2733607)
A 1v signal comes in to the first stage and is amplified to 100v

Incorrect. Your misunderstanding of "amplification factor", or mu () is what's throwing you off. The amplification factor is the ratio between a small change in plate voltage and a small change in grid voltage which results in the same change in plate current. It is an indication of the effectiveness of the control grid voltage relative to the plate voltage in controlling the plate current.

If a tube is said to have a of 100, it means that the grid voltage change required to produce a certain change in plate current is 100 times less than the plate voltage change required to bring about the same change in plate current. In other words, the grid voltage is 100 times more effective than the plate voltage in it's influence upon the plate current.

An example would be that if a plate current change of 1ma is produced by a plate voltage change of 10V, and a grid voltage change of .1V produces the same 1ma change in plate current, then the amplifiaction factor is 100. ( - 10 .1 = 100) Emphasis should be placed on the fact that it is the change in plate and grid voltages that are important and not the individual values.

Joshua_G 3rd October 2011 11:49 PM

Most electric guitars pickup voltage are rated 100mV. High outputs ones are rated 1V.

On the Fender Champ Amp schematic I have, there are two inputs, High and Low. The High input goes through a voltage divider, which attenuates the voltage coming to the grid if the first tube (or the first half of the dual triode).

After the first tube there is a tone control which has some attenuation. After the tone control there is a volume control, which may attenuate on normal use.

Also, 12AX7 has an amplification factor of 100, but the actual amplification is lower than that. The actual amplification depends on the circuit topology and on the value of the anode resistor.

Alexrmc92 3rd October 2011 11:49 PM

Quote:

Originally Posted by sreten (Post 2733638)
Hi, around 100mV in is a lot more reasonable than 1V, rgds, sreten.

How come its acceptable for amps to start clipping around volt? not that i am saying your wrong, because the output voltage of a guitar pickup seems to anywhere between 10mv and 5v. But just wondering.

Quote:

Originally Posted by HollowState (Post 2733656)
Incorrect. Your misunderstanding of "amplification factor", or mu () is what's throwing you off. The amplification factor is the ratio between a small change in plate voltage and a small change in grid voltage which results in the same change in plate current. It is an indication of the effectiveness of the control grid voltage relative to the plate voltage in controlling the plate current.

If a tube is said to have a of 100, it means that the grid voltage change required to produce a certain change in plate current is 100 times less than the plate voltage change required to bring about the same change in plate current. In other words, the grid voltage is 100 times more effective than the plate voltage in it's influence upon the plate current.

An example would be that if a plate current change of 1ma is produced by a plate voltage change of 10V, and a grid voltage change of .1V produces the same 1ma change in plate current, then the amplifiaction factor is 100. ( - 10 .1 = 100) Emphasis should be placed on the fact that it is the change in plate and grid voltages that are important and not the individual values.

Isn't that what the grid is supposed to do in this case? I have always viewed the grid as a current valve, hence why i thought they were called valves. Also doesn't the output voltage all depend on the plate resistor? The larger the input signal you have, the more variance you have in your output current which drops more / less voltage at the plate resistor. Unless im wrong of course.

Another question i have is if the cathode resistor adds in series to the plate resistor. Because current flows from negative to positive, wouldn't Rk, Rp and the tubes internal resistance all be in series with each other?

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.

Joshua_G 3rd October 2011 11:54 PM

Quote:

Originally Posted by Alexrmc92 (Post 2733674)
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?

1. There is the unavoidable attenuation of the tone control and the probable attenuation of the volume control.
2. At high volumes and full volume, the amp probably clips, which some guitarists want.

Alexrmc92 3rd October 2011 11:57 PM

Quote:

Originally Posted by Joshua_G (Post 2733678)
1. There is the unavoidable attenuation of the tone control and the probable attenuation of the volume control.
2. At high volumes and full volume, the amp probably clips, which some guitarists want.

I understand that some attenuation is unavoidable. But after 0v on the grid no more clipping can occur right? so lets say attenuation brought it to 3v, then there is still about 1.2v over 0v. (if were talking 100mvrms and 3vrms) if it smore like 100mvpp and 3vpp then the problem is solved.

3vpp would result in max clipping because it goes into grid current and to 0 grid volts, right?

Joshua_G 4th October 2011 12:04 AM

Quote:

Originally Posted by Alexrmc92 (Post 2733684)
I understand that some attenuation is unavoidable. But after 0v on the grid no more clipping can occur right? so lets say attenuation brought it to 3v, then there is still about 1.2v over 0v. (if were talking 100mvrms and 3vrms) if it smore like 100mvpp and 3vpp then the problem is solved.

3vpp would result in max clipping because it goes into grid current and to 0 grid volts, right?

It was answered above.

ChrisA 4th October 2011 01:03 AM

Quote:

Originally Posted by Alexrmc92 (Post 2733607)
In the fender champ amp schematic, the 12AX7 pre amp has 2 class A stages. Both stages are biased the same.

Not quite true. The second triode has strong negative feedback applied to the cathode resistor.

Some versions of the Champ have a tone stack or a tone stack made up of fixed resistors. The early tweed Champs do not have the tone stack but there is a 1M pot between the two triodes and YES it does clip if you crank the pot to "12" and have a hot guitar signal going it.

But after that volume control pot the rest of the amp is inside a NFB loop. To talk in greater detail we have to agree on a scematic. I'd say the 5F1 is "the" classic Champ.

http://myfenderchamp.com/wp-content/...-schematic.png

Alexrmc92 4th October 2011 01:18 AM

Quote:

Originally Posted by ChrisA (Post 2733740)
Not quite true. The second triode has strong negative feedback applied to the cathode resistor.

Some versions of the Champ have a tone stack or a tone stack made up of fixed resistors. The early tweed Champs do not have the tone stack but there is a 1M pot between the two triodes and YES it does clip if you crank the pot to "12" and have a hot guitar signal going it.

But after that volume control pot the rest of the amp is inside a NFB loop. To talk in greater detail we have to agree on a scematic. I'd say the 5F1 is "the" classic Champ.

http://myfenderchamp.com/wp-content/...-schematic.png

i agree as well, i never considered the negative feedback. That changes things a lot. Honestly i feel much better understanding that guitars aren't really outputting 1v constantly. I using this knowledge to design my own preamp and i just wanted to understand why the champ doesn't clip at all volumes with 100v into stage 2. I knew it wasn't 100v but i wanted to know why.

I guess my only question is how far positive can you safely go on the grid, or should you never go past 0v?


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