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Exactly!!!

Posted by Paul Joppa (P) on February 28, 2005 at 21:00:37

In Reply to: Your other choice posted by VoltSecond on February 28, 2005 at 19:09:36:

I've been saying this for many years - you cannot speak meaningfully of the operating point without including voltage AND current AND load impedance.

"Deep class A" simply means the load line does not come close to zero current. Starting from a design optimized for power but still Class A, you can achieve deeper class A by reducing the plate voltage, increasing the plate current, or increasing the plate load impedance.

The standard RCA operating point calls for 250v (plate to cathode), 60mA, and 2500 ohms load. The bias is said to be -45 volts, but their plate curves show operation with -43.5v bias. At zero volts bias (maximum current before grid current starts) it draws 120mA, and at -87v bias (twice the quiecent) it draws about 12mA. Clearly Class A, with a margin of 20% of the quiescent current. The Paramour operates at 300v, 50mA and a 4000 ohm load. Bias is said to be -60v, but is actually (on the curves) -57v. At twice the bias, -114v, the current is 10mA or - you might have guessed - 20% of the quiescent. Their "depth of Class A" is identical.

Now if you replaced the power supply so you could operate at the RCA point (250v, 60mA) but kept the 4000 ohm load, you would find a minimum current of about 30mA at -87v bias, giving a margin of 50% of quiescent. I'd call that "deep Class A". You can get exactly the same margin, and therefore I think the same depth of Class A operation, by keeping the Paramour voltage and current, but changing the output transformer impedance to about 8k ohms.

Son of a gun, and how about that - there's already an 8k ohm tap on the Paramour output transformer! Give it a try, it won't hurt anything.

But you might not like it. All of the above has completely skipped over the issue of properly designed iron. Higher impedances call for higher plate choke and transformer inductances. Unless you change the plate choke and output transformer, it's not really a fair comparison.

This effect is probably quite significant. When you run a tube at lower voltage and higher current with the same output transformer, the problems caused by inadequate inductance are much reduced. I am convinced that's one of the reasons people think "deep Class A" sounds better - it's not the tube operation that sounds better, it's the improved transformer margin. You can improve the transformer margin just as well by increasing the choke and/or output transformer inductance. That's what happens with the Paramour or SEX kit upgrade transformers.

Another way to say this is that deep Class A lets you get away with a lower inductance transformer.

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