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
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
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.
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.
Why ? Are those already exhausted ?
It is a phase inverter. It generates opposite phase ( 180 deg) signals to the output tubes EL34.
It's gain is some 25 dB.
Yes
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.)
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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.
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.
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.
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.
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.
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.
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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.
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?😱
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.
Thanks all for your anwsers and NO I am not that confused ;-) I am learning that is why I came here.
The tubes are not dead but I wanted to improved the sound as discussed on this site:
Jolida SJ-502A Integrated Amplifiers Reviews
Plus, it is not that expensive so why not try to improve it.
I am hesitating about these tubes:
Genalex - Gold Lion ECC83 / B759
or
Mullard 12AX7 / ECC83
What do you think about this Prefered Series...
Preferred Series 7025
The 12AT7 are they considered part of the output stage? Because some people say it has huge influence on the performance of the amp.
Thanks
Simon
The tubes are not dead but I wanted to improved the sound as discussed on this site:
Jolida SJ-502A Integrated Amplifiers Reviews
Plus, it is not that expensive so why not try to improve it.
I am hesitating about these tubes:
Genalex - Gold Lion ECC83 / B759
or
Mullard 12AX7 / ECC83
What do you think about this Prefered Series...
Preferred Series 7025
The 12AT7 are they considered part of the output stage? Because some people say it has huge influence on the performance of the amp.
Thanks
Simon
I think that was the first tube I rolled in my 302b. I first tried EH12AT7's, which weren't much better than the original Shuguangs. Next came some very nice NOS Mullard CV4024's. Then some equally as nice NOS GE 6201 "5-star" tubes, which are still in the amp. I could live "long term" with either the GE's or the Mullard's.
I played around with a few different 12AX7's for the front end, but eventually settled on some JAN GE 5751's, not "5-star" tubes but millitary spec JG's (I think).
Hope that helps.
jeff
That design is using the 12AT7 as an LTP voltage amp/phase splitter. The 12AT7 is a high gain dual small signal triode that was designed for operation at highish plate currents. ( Pd= 2.5W(max) ) unlike 12AX7s that were designed for high voltage gains and much lower Q-Point currents.
This type appeared often as oscillator/buffers, frequency doublers/triplers, and Class C, PP intermediate drivers. Being able to pull larger Q-Point currents improves the current sourcing capability to charge the Cgk + Cmiller + Cstray of the power finals to reduce the problem of slew limiting at the higher audio frequencies.
(He shoulda used active tail loading instead of passive loading and forcing balance with unequal plate loads.)
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