Arkless Electronics
Trade: Amp design and repairs.
I completely agree with both of your posts toprepairman, basic theory. What I don't agree with is that it is particularly relevant to the final result.
A typical valve amp draws its main HT through the centre tap of the output transformer. HT to earlier stages is independently RC filtered and decoupled and so irrelevant. The dynamic impedance seen by the amp at this point is almost entirely defined by the value and ESR of the final smoothing capacitor which feeds the centre tap (ESR less important than in a SS amp.. low current, high voltage here) and that is the important point.
In the bad old days when 8uF smoothing caps were common then a choke was helpful in getting ripple down to sensible proportions. Today (and for well over 30 years in fact) we have much bigger caps available and don't need chokes at all.
A typical 1953 or so supply with a pi section, a pair of 8uF caps and a 10H choke has several times the ripple voltage and dynamic output impedance of a more modern set up with a pair of 220uF caps and 100R resistor in the same pi configuration. So what if we use the 10H choke with the modern 220uF caps? The large caps so dominate everything that it makes bugger all difference. In fact the choke version would have slightly higher dynamic output impedance as frequency goes up as the inductance of the choke will isolate the capacitance of the first smoothing cap more than the resistor will... by a pedantically tiny amount...
A choke input filter would make very little difference for the same reasons as above when modern large caps are used. It can of course draw power much more smoothly and so generate less EMI but this is of highly tenuous advantage IMHO.
A typical valve amp draws its main HT through the centre tap of the output transformer. HT to earlier stages is independently RC filtered and decoupled and so irrelevant. The dynamic impedance seen by the amp at this point is almost entirely defined by the value and ESR of the final smoothing capacitor which feeds the centre tap (ESR less important than in a SS amp.. low current, high voltage here) and that is the important point.
In the bad old days when 8uF smoothing caps were common then a choke was helpful in getting ripple down to sensible proportions. Today (and for well over 30 years in fact) we have much bigger caps available and don't need chokes at all.
A typical 1953 or so supply with a pi section, a pair of 8uF caps and a 10H choke has several times the ripple voltage and dynamic output impedance of a more modern set up with a pair of 220uF caps and 100R resistor in the same pi configuration. So what if we use the 10H choke with the modern 220uF caps? The large caps so dominate everything that it makes bugger all difference. In fact the choke version would have slightly higher dynamic output impedance as frequency goes up as the inductance of the choke will isolate the capacitance of the first smoothing cap more than the resistor will... by a pedantically tiny amount...
A choke input filter would make very little difference for the same reasons as above when modern large caps are used. It can of course draw power much more smoothly and so generate less EMI but this is of highly tenuous advantage IMHO.
