Hi Rich,
I wrote:
> >Cathode saturation current of a 3-500Z begins at approximately six
> >amperes, and at 500 mA of meter indicated anode current, PEAK cathode
> >current (which includes grid current)is only about 2 amperes. There is a
> >3 to one safety margin.
Rich Measures wrote:
> To see if Mr. Rauch's statement washes, refer to Eimac Curve #4364 for
> the 3-500Z. If you do not have a copy, see Figure 17 at the
> aforementioned Web site.
> The items of principal interest are the
> plate(anode) current=1.4a and 1.2a peak current curves. For linear
> amplifier service, to convert peak anode/plate amperes to average
> amperes,, divide by 3.
Peak anode current is not "three times the indicated current" in a class
AB PA.
A Chaffe analysis must be done, since the current waveshape is made
complex by the less than 360 degree conduction angle. Even forgoing the
Chaffe analysis, and assuming a sine wave, we certainly should agree on
the following.....
With a perfect 180 degree sine shaped waveform, peak current is 2.828
times average current. With 360 degree conduction angle and a sine wave,
peak current is 1.414 times average current. The conduction angle is
about 215 degrees (class AB) at 500 mA effective current, so peak
current is
(.6/1 * 1.414) less than 2.36 times indicated anode current. 500mA *
2.36 is 1.180
amperes.
Actual peak current is even less than that 1.18 amperes, since the
transition into cut-off isn't perfectly sharp. A Fourier analysis ( or
the more simplified Chaffe analysis ) would be needed to calculate the
exact peak current. It is actually in the range of 1.15 amperes with 500
mA of anode current.
The "times three" rule you used sounds like something from a class C
amplifier. Where did you get it from?
> Thus the 1.4 a curve is equivalent to about
> 466mA, and the 1.2a curve equivalent to about 400mA.
That also is incorrect, since the multiplier "three" is wrong.
> The 1.4a curve
> stops abruptly at 500v while the 1.2a curve continues down to 250v. Why?
> If the 1.4 a curve were to continue below 500v, the curve would hook and
> shoot off the graph into Saturationville .
That is also incorrect. The performance curves stop there because the
grid dissipation limits are reached when anode voltage is held below 500
volts
during the curve plotting process. Saturated plate current is far off
the chart.
The tube is actually operated at about 1.2 amperes peak anode current,
when a full blown Chaffe analysis is done, with 535 mA indicated plate
current. After all, the conduction angle is not less than 180 degrees.
73, Tom
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