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Load lines and manufacturers figures (newbie)
Load lines and manufacturers figures (newbie)
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Old 12th August 2017, 01:05 PM   #21
20to20 is offline 20to20  United States
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What it really means is that if you set your quiescent current at 100% of plate dissipation in a Class A amp then apply maximum signal for full volume, you will drive the plates over their power dissipation rating and they may red plate.
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Old 12th August 2017, 09:37 PM   #22
PRR is offline PRR  United States
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> if you set your quiescent current at 100% of plate dissipation in a Class A amp then apply maximum signal for full volume, you will drive the plates over their power dissipation rating and they may red plate.

No. A class A amp runs *cooler* when you work it HARD.

This was only theoretical until I ran an older tube into dummy load. It creaked when it got hotter or cooler. Going zero to max, I could hear the cooling creak. Simple metering confirmed the DC power hardly changed while the dummy resistor was smoking its paint. IR probe confirm the plate ran cooler at full sine, even cooler pushed to hard clipping.

> couldn't work out how zero-signal plate current being 72mA and maximum-signal plate current at 79mA

That sounds like a push-pull condition.

72mA per _pair_ is 36mA each. At idle, a steady 36mA. Under large signal, one side swings down to zero and up to 72mA, the other side swings up to 72mA and down to zero.

It is a see-saw on a stump. Idle, both sides level above the ground. Under children, one side goes to zero (hits the dirt) and the other side goes twice as high.

> 72mA ... 79mA

Perfection would be 72mA. Tubes are curved. The "79mA" is a DC reading and not the peak. Tubes turn-on better than they turn-off, so under heavy signal they tend to conduct a bit more.
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Old 13th August 2017, 08:45 AM   #23
jazbo8 is offline jazbo8
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Load lines and manufacturers figures (newbie)
Quote:
Originally Posted by PRR View Post
> couldn't work out how zero-signal plate current being 72mA and maximum-signal plate current at 79mA

That sounds like a push-pull condition.
It's actually the spec's for SE operation. See Load lines and manufacturers figures (newbie)
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Old 13th August 2017, 10:39 AM   #24
JoeAlders is offline JoeAlders  Netherlands
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PRR wrote:

"IR probe confirm the plate ran cooler at full sine"

But that is quite understandable because out of the total plate dissipation fed to the plate, a maximum of AC power
is delivered to that resistor leaving the plate less to dissipate.
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