What are 12AT7/ECC81 power drivers ? - diyAudio
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Old 18th September 2011, 01:51 PM   #1
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Default What are 12AT7/ECC81 power drivers ?

Hi,

Excuse my newb questions but I am looking to replace my tubes on this amp:
.:JoLida: manufacturers of quality audio equipment

and found that there are 12AX7A pre-amplifier tubes and 12AT7 power drivers.

From what I understand the pre-amplifier are the most important for the sound quality. So I guess I should invest more in these tubes.

What are the role of 12AT7 power drivers?
Also this amp is it a class AB push-pull?

Here is the schematic:
http://www.drtube.com/schematics/jolida/sj-502a.gif

Thanks in advance!
Simon
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Old 18th September 2011, 03:05 PM   #2
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IMO don't buy any tubes from anywhere other than the amp supplier. They should know which tubes are best. Nice looking amp, it would be a shame to spoil it with dodgy tubes.

The 12AX7 in layman terms is a voltage amplifier, typically 100 times. (This will make a 500mV signal into 50volts.
The 12AT7 is a current amplifier (very little voltage gain) and phase inverter to give enough signal to drive the output tubes to full power.
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Old 18th September 2011, 03:06 PM   #3
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Quote:
I am looking to replace my tubes on this amp
Why ? Are those already exhausted ?

Quote:
What are the role of 12AT7 power drivers?
It is a phase inverter. It generates opposite phase ( 180 deg) signals to the output tubes EL34.
It's gain is some 25 dB.

Quote:
Also this amp is it a class AB push-pull?
Yes

Quote:
From what I understand the pre-amplifier are the most important for the sound quality.
I think that the output stage determines the final performance of the amplifier.
Especially when the amount of negative feedback is low, like in this case (5 dB as per specs.)

Last edited by artosalo; 18th September 2011 at 03:11 PM.
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Old 18th September 2011, 03:30 PM   #4
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According to the manufacturer's specification the negative feedback (global?) is less than 5 dB, but according to schematic it is 10 dB.
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Old 18th September 2011, 03:33 PM   #5
DF96 is offline DF96  England
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Many things contribute to sound quality, so many things can detract from it too. To a first approximation, preamp valves determine the noise and interference. Later valves, including driver and phase splitters, have more effect on distortion because the signal is larger here.

12AT7 is not a current amplifier. It is a voltage amplifier (mu=60) but can be run at higher currents than a 12AX7. It has a lower output impedance so in some circuits may be better as a driver.

You don't need to buy valves from the amp supplier, but this may be a good thing to do if you don't know anything about valves and you have deep pockets.
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Old 18th September 2011, 04:31 PM   #6
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Most of the distortion is generated at the output stage.
The phase splitter can be constructed to give the full signal required by output tubes with less than 0,3 % of THD.
The voltage amplifying stage can be done with less than 0,1 % at the same situation.

When all these three stages are in the global feedback loop, the stage that requires correction by the negative feedback is the output stage.

The noise levels at typical integrated amplifiers, where input level is some 0,5 V, are negligible. Signal to noise ratio of 80 dB is easy to achieve without any special endeavour.

But still the soud quality can be spoiled in many places of the amplifier.
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Old 18th September 2011, 09:51 PM   #7
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Quote:
Originally Posted by DF96 View Post
12AT7 is not a current amplifier. It is a voltage amplifier (mu=60) but can be run at higher currents than a 12AX7. It has a lower output impedance so in some circuits may be better as a driver.
I think I answered the question correctly. "What are the role of 12AT7 power drivers?"

The anode and cathode resistors are similar is value in his circuit so the Voltage gain is low. The output impedances of this stage are lower than the previous stage so there must be some current gain.

Last edited by payitforwardeddie; 18th September 2011 at 09:55 PM.
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Old 18th September 2011, 09:57 PM   #8
DF96 is offline DF96  England
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OK, when you said "12AT7" you meant the function of the 12AT7 in that particular circuit. I think what sent me off on the wrong path is that you said that the 12AX7 has a typical gain of 100 - this clearly refers to the valve itself rather than the circuit as typical gains for 12AX7 in a circuit are more like 60-70.
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Old 18th September 2011, 10:39 PM   #9
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That's the problem with writing stuff and trying to keep it brief. We know what we mean, but does a noob like REALFLEO or have we confused him even more?
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Old 18th September 2011, 10:43 PM   #10
DF96 is offline DF96  England
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Always a risk. The difficulty is that newbie can mean anything between not knowing how to wire a battery to light a bulb, and an experienced microwave chip designer who is getting started with valves at the weekend.
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