> If your theory is correct concerning tube gain (Mu) how do
> you explain this:
> A gs-35b requires around 40 volts bias for zsac of 150ma
> with 4kv Ep...A
> 8877 at the same Ep requires much less for similar zsac.
> The gs-35b is a
> lower gain tube than the 8877...Did you mean: It takes
> more negative bias to
> reduce anode current as the mu is DE-creased? The
> inquiring minds want to
> know..73, Jim..
Hi Jim,
Yes. For a clearer example, look at tubes with very similar
internal geometry and ratings but different mu. Try the
3CX3000A1 and 3CX3000A7
You will see the higher the mu, the less grid bias it takes
to maintain a certain Ip. When the geometry and overall
design is the same or very similar, the lower mu tube has
higher quiescent current at a given anode voltage and bias
voltage. What people usually say is a higher mu tube of the
same type has more current at fixed bias when mu is higher.
That is backwards thinking.
Gain in grounded grid is more related to the ratio of load
impedance to driving impedance than anything else. This is
because the output is in series with the input, time-varying
anode current flows through both load and source resistances
in series, and so the negative feedback is proportional to
the ratio of those resistances. This is why gain is reduced
when the PA is loaded heavier and why the PA becomes cleaner
(oh oh, there comes that overloading of the PA during tuning
and bad engineering thread again!).
An increase in driving impedance does the same thing as a
reduction in anode load impedance. This is why a 4-1000A
with 100 ohms of cathode driving impedance has poor gain
whether you apply screen voltage or not in grounded grid,
and why a 50-60 ohm 3-1000Z at the same anode voltage has
more gain in GG.
If you look at really high gain GG tubes they have very low
input impedances. Case in point, the 3CPX5000A7.
While I'm not a tube design engineer and haven't had a
course on vacuum tube design since the days of Janis Joplin
and Jimi Hendrix, my recollection is the only way to have
more current at a given anode and bias voltage and more gain
in a good tube is to make the emission area and grid longer
or otherwise change geometry in the tube. So I'm wondering
what the deal is with the Chinese tubes.
Ameritron had to actually derate the power gain of the AL82
when Eimac glass bottles went away.
Those Swetlana 572B that oscillated in FL2100's did so at
the fundamental frequency not because gain was higher, but
because the tube remained connected to the input and tank
circuits and had substantial idle current from less mu, but
load Z went though the ceiling when the antenna was
disconnected and the tubes gain in GG went through the roof
with very high anode Z and the same cathode drive Z.
73 Tom
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