[AMPS] Re: 3-500Z - ZG
Tom Rauch (W8JI)
Sat, 29 Mar 1997 08:53:47 -0500
> I have to disagree a bit with the above. The standard Eimac or clone
> tantalum plate tube has a published amplification factor of 130. The
> Amperex graphite version has an amplification factor of 200 according to
> their 1969 spec sheet.
Carl, does the Amprex 3-500Z require a reduction of bias when replacing
a 3-500Z? When you plug in am Amprex tube does the gain increase?
Neither of these things occur in any PA's I've seen.
The spec sheet I have indicates the Amprex tube has the same
amplification factor as a 3-500Z.
Amplification factor is often "compared" to gain in a grounded grid PA.
The main determinate of gain is the ratio of driving to operating
impedance, not amplification factor. Tubes with low cathode impedances
(8877, 3CX800) compared to output impedances (plate voltage over
operating current) have more power gain. Tubes with high cathode driving
impedances (3CX1200, 3-1000, 4-1000) have less gain in grounded grid.
The gain of a grounded grid stage is directly (almost perfectly) related
to the ratio of cathode signal voltage to anode signalk voltage, both
are tied to input and output operating impedance.
That's why adding screen voltage and grid bias to a 4-1000A in grounded
grid barely changes circuit gain when compared to a 4-1000A in straight
grounded grid service. The amplification factor changes greatly, but the
gain does not.
In a grounded grid PA, the output and input are in series. This produces
negative feedback, and the feedback is controlled by the ratio of
cathode driving to anode load impedance, mu is a secondary effect.
I've never seen any PA get "unstable" from Amprex tubes, but I have seen
plenty of graphite tubes arc from gas. The graphite anode traps gas and
slowly releases it on the shelf. When the tube is plugged in, pow....it
arcs. An arc in the tube has nothing to do with RF instability. That's
Labeling a Chinese tube Made in Russia is fraud, as we all agree. And it
is being done.
One fellow did weekend jail time for re-marking used 8877's (and other
tubes)as new. All the new Eimac tubes on the market are not new, some of
the Penta Labs Chinese 8877's were tubes from a batch of rejected Eimac
8877's with cathode and grid defects.
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