newbie question : driver/output relationship, input sensitivity - diyAudio
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Old 17th August 2008, 06:50 PM   #1
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Join Date: Nov 2007
Default newbie question : driver/output relationship, input sensitivity

Hello there,
I am planning to build my first diy tube amp, after making several fullrange loudspeakers.
I intend to build the 6SL7/2A3 JE Labs Simple amplifier. Parts should be here in a few days.

I have read a few articles on the web and I hope to now quite understand how a triode stage works. (thanks especially to the valve wizard and boozhound labs who helps make things very understandable for non-tech persons)
I did find very few things about the two next things :

> How a driver and an output stage works together? (SE/class A only concerning basic RC 2 stages with one valve cathode bias triode each)

I now guess the AC output voltage swing of the driver comes as input at the grid of the output stage.
But what about current and impedance, miller effect etc...determining if the two valve stages work together...simple to understand

> How to calculate input sensitivity of the global amplifier ?

For example in the JELABS for theorical 2v CD input, from the 6SL7 i get a 92 v output at 1,5 mA (-48,+44V output from bias point) and on the 2a3 (grid bias 0 to -120v), this 92v is well inside the graphs and never reach clipping even at full power. I calculate here 2.35w at full power, as the bias is under-optimised for 2a3 to allow a 45 with the same schematics.
does this means i am above 2v input sensitivity, then my potentiometer never reaches clipping point ? and i then miss a lot of the desirable tube apparent power (because it clips gently, and you barely hear it) ?

My problems went when I then tried to understand the decware zen amplifier (i was hesitating between these two) and i theorically got 50v swing out of 6922 driver plate for a 2v source and grid of EL84 is from 0 to -20v then I imagined about less than 1v input sensitivity maximum, but decware states 2v for full output... meaning i understand nothing !?

As a driver never seems overdriven, i guess the sensitivity is determined at output stage ?

Thanks in advance for your help (please remember i am not a specialist)
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Old 17th August 2008, 08:10 PM   #2
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Join Date: Apr 2007
Welcome to highly addictive world of tubes.

"> How a driver and an output stage works together? (SE/class A only concerning basic RC 2 stages with one valve cathode bias triode each)"

Buy Morgan Jones book "Valve Amplifiers", it is THE book for a beginner trying to understand tubes.
But in short, the driver stage needs to supply the needed voltage swing to drive the output tube to clipping (full power). It also need to have sufficiently low output impedance to put the HF roll off caused by miller capacitance significantly above the highest fequency of interest (20kHz).

"> How to calculate input sensitivity of the global amplifier ?"

Just calculate backwards. If the output tube is biased with a grid-kathode voltage of say -45V you know that you will need approx 45Vpk or 32Vrms to drive it into clipping. Then you just divide this voltage with the total voltage gain of the previous stage/stages. If, for simplicity, we assume your driver stage gain is 32 your input sensitivity is 32/32=1Vrms for full power.

/Olof
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