SE Power Amp, is it really this simple?

[jmaf]

steve, i've always heard this called the operating resistance, and is usually much lower than the "proper" primary impedance.

have you found good success in using primary Zs in the same range as operating resistance?

i don't pay attention much to primary impedance in MI amps anyway... i consider it another tone switch.

I've seen this time and again and I'm confused. At DC the inductive load is just a DC wire resistance, right? How come idle current is used to calculate the primary impedance, if there is no reactance at dc? Might be a stupid question, I've seen this on several books and perhaps I'm missing something basic.

[kg]

there's no direct calculation.

however, some generalizations or rules of thumb can be based off that "operating resistance" (Vp/Ip). usually they are focused around triodes.

i have a page on my site with an email from paul joppa, and links to some articles he wrote.

Triode Operating Points (Paul Joppa) | ken-gilbert.com

keep in mind paul is approaching the subject from the perspective of minimizing distortion first and maximizing power output second, which is probably not what we're after here... but there is still much to be learned from his explanation.

[jmaf]

Thanks for the pointer, kg. I've read your blog entry but not the pdf's yet.

I bought a book recently and it simply ignores reactance and calculates the impedance for the primary on the OT without mentioning frequency, like a 4k primary would offer 4k resistance to DC quiescent current, which I assume it does not(a hammond with a 4k primary would have around 85 ohms DCR IME).

I asked the author a question on this forum and he didn't reply.

[kg]

Thanks for the pointer, kg. I've read your blog entry but not the pdf's yet.

I bought a book recently and it simply ignores reactance and calculates the impedance for the primary on the OT without mentioning frequency, like a 4k primary would offer 4k resistance to DC quiescent current, which I assume it does not(a hammond with a 4k primary would have around 85 ohms DCR IME).

I asked the author a question on this forum and he didn't reply.

with transformer/inductive loads, generally impedance calculations are done in the midrange, where imperfections in the coil characteristics can be hand-waved off as irrelevant.

obviously these characteristics get messy at the frequency extremes.

one thing to consider is that the classical load-line analysis of transformer/inductive loads uses TWO loadlines: one for AC, one for DC. the DC line is much steeper, nearly vertical. also keep in mind that paul joppa's comments are focused around triodes, and he mentions that at least certain parts of the analysis are based on resistive loads (not inductive) which simplifies things to some extent. also remember the DC part of the plate curve loadline is basically useless in a multigrid tube, since vg2 has so much more influence than vp.

regardless, the optimal resistive load can be taken as the optimal reflected primary impedance in the midband range of frequencies. the only difference is that the B+ rail voltage need not be ~2x vp at the quiescent point as it would need to be with a pure R load.