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New 811A tubes have inconsistent
cut-off characteristics. This is similar to the Svetlana 572B behavior
differences in Yaesu amplifiers, where tubes oscillate on standby. People blame
this on higher tube mu or transconductance, but the problem really centers
around cutoff bias characteristics. Some off-shore tubes behave differently than
older USA transmitting tubes behaved. Making things worse, some off-shore
transmitting tubes behave differently from batch-to-batch. These tubes act as if they are lower mu for
cutoff, very much like a semi-remote control grid used in tubes designed for AGC
controlled stages.
Amplification Factor
Amplification factor is represented by the Greek
letter µ (also called “mu”). Grid-to-cathode voltage controls
cathode-to-anode current. For example, a 10-volt change in grid voltage might
affect anode current as much as a 100 volt change in anode voltage.
Amplification factor, or mu or µ, tells us how much
more a grid voltage change affects anode current than an anode voltage change
affects anode current. For example, if a 1-volt grid change, from -20 to -21
volts, affects anode current the same as a 20-volt anode voltage change, say
from 800 to 820 volts, µ (mu) is 20/1 = 20. We do this over a small range of
anode voltages.
We can also say mu is ∆Ep / ∆Eg for
∆Ip = 0.
We would determine µ by setting anode and grid
voltages and looking at current, adjusting anode-to-cathode voltage a certain small amount
while carefully watching the change in anode current, and adjusting grid voltage
to maintain the original anode current. This would tell us delta Eg for a certain
delta Ep.
This is not exactly the same as stage
amplification or gain, it is amplification factor.
Amplification factor (or µ) can, and usually does, vary through different
operating voltage ranges. Mu or µ is most consistent over wide operating ranges
in tubes like beam power tubes, or pentodes, and least consistent in triodes.
This is not the same as gain. Gain will be some amount less than µ.
Semi-remote and Remote Cutoff
Semi-remote grid behavior is also
like having a significantly variable amplification
factor, or variable mu, as we change grid voltage. This is accomplished by
winding the grid with a variable wire pitch, or by allowing electrons to “leak
around” areas of high electrostatic charge in the grid. Sometimes this happens
in unintentional cases out through the grid’s open ends.
In a variable-mu tube, the cathode has a path to anode through areas where the
grid does not have the same control.
As
bias moves toward cutoff, denser electric field areas of the grid cut completely
off. Only the more open areas with less control determine current. The influence
of the grid becomes less as bias increases.
Remote cut-off or variable mu is intentionally used in AGC controlled
(automatic gain control) stages.
Variable mu increases the useful gain range of the stage, allowing smooth AGC control
without serious distortion.
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