06-11-2016, 01:54 PM
Mark, apologies for derailed the thread somewhat with semantics and saturation!
DA, good you're quite chuffed with it, give yourself a pat on the back!
Ref queries... First, don't put the low-pass filter between the ECC83 and the EL84. Why? Because this is INSIDE the NFB loop. So, the NFB will fight to straighten out any top-cut. And secondly, top cut or bass cut adds phase shift, which can play havoc with stability. It's best to put OUTSIDE any NFB path. You could have a simple RC filter preceding the volume control. But excess HF going in tends to be less of a problem than excess LF going in, so it may be academic anyway...
Capacitor for g2 of the EL84? Just calculate it so that the voltage stays steady at the lowest frequency of interest, with a safety margin. The slope resistance at g2 is somewhat analogous to the ra of a triode, but that is unlikely to be specified. So, as a rough approximation, take it as Vg2/Ig2. And then make C equal to 1/10 of this at the lowest frequency of interest. Often, g2 is taken to HT+ so there isn't a problem; and sometimes there is a small value resistor in series which is low enough not to give significant AF voltage swing. It's there to prevent parasitic RF oscillations.
Bypass capacitor for the ECC83? You can leave it off, but if you do there are two significant feedback paths, one from the loudspeaker via the feedback resistor, and the other, more local current feedback within the ECC83 stage itself. Analysis would be horrendous, so I'd suggest put it in so you've got just the one feedback path.
Calculate the feedback resistor and capacitor? One way is as you've done it. Set the resistor to give you the reduction of gain (which is also the reduction in percentage distortion) that you can accept. And then play with the capacitor to give the best square-wave performance, with regard to overshoot, droop, and ringing, possible. It's not very mathematical, but it can give you a big insight into the behaviour and interaction of circuit elements.
DA, good you're quite chuffed with it, give yourself a pat on the back!
Ref queries... First, don't put the low-pass filter between the ECC83 and the EL84. Why? Because this is INSIDE the NFB loop. So, the NFB will fight to straighten out any top-cut. And secondly, top cut or bass cut adds phase shift, which can play havoc with stability. It's best to put OUTSIDE any NFB path. You could have a simple RC filter preceding the volume control. But excess HF going in tends to be less of a problem than excess LF going in, so it may be academic anyway...
Capacitor for g2 of the EL84? Just calculate it so that the voltage stays steady at the lowest frequency of interest, with a safety margin. The slope resistance at g2 is somewhat analogous to the ra of a triode, but that is unlikely to be specified. So, as a rough approximation, take it as Vg2/Ig2. And then make C equal to 1/10 of this at the lowest frequency of interest. Often, g2 is taken to HT+ so there isn't a problem; and sometimes there is a small value resistor in series which is low enough not to give significant AF voltage swing. It's there to prevent parasitic RF oscillations.
Bypass capacitor for the ECC83? You can leave it off, but if you do there are two significant feedback paths, one from the loudspeaker via the feedback resistor, and the other, more local current feedback within the ECC83 stage itself. Analysis would be horrendous, so I'd suggest put it in so you've got just the one feedback path.
Calculate the feedback resistor and capacitor? One way is as you've done it. Set the resistor to give you the reduction of gain (which is also the reduction in percentage distortion) that you can accept. And then play with the capacitor to give the best square-wave performance, with regard to overshoot, droop, and ringing, possible. It's not very mathematical, but it can give you a big insight into the behaviour and interaction of circuit elements.
