Technically, the statement below is only partly true, and not enough to
worry about in any case.
Case 1: The extra signal added to the grid is something like hum or or
voltage variation that's not related to the input signal. This will not
cause distortion unless it has so much amplitude that it causes the
amplifier to enter the non linear region (overdriven). Let's assume a
60/120 Hz hum, the hum will simply linearly add to the driving signal and
produce a hum on the output, nasty but not non-linear or causing IMD's.
Case 2: the extra signal added to the grid is dependant on the input signal
such as a bias that fluctuates with drive. This create a non-linear net
grid voltage and definitely creates intermodulation distortion. But even
then one has to look at the effective frequency response of the bias
fluctation. If the PA causes the line voltage to sag and the bias supply to
have some variation at the voice syllabic rate, then the intermods created
will only extend on each side of the voice spectrum by the frequency of the
syllables, about 10-30 Hz.
So, unregulated grid bias is not likely to create IMD on the amplifier
output that anyone would notice.
On the subject of the test question, response to impulse or steps....
Think about an amplifier idling with unregulated plate voltage, regulated
screen voltage, and unregulated grid bias. Hit the amp with an abrupt key
down (step response test). The amp with begin to transmitt with higher than
normal plate voltage and then over the next 50 msec or so the Ep will sag
down to normal operating condition. The PA output will run a bit higher for
those msec and will operate slightly mismatched due to having been tuned for
peak power output at the sagged plate voltage.
The grid bias supply also operates unregulated off the AC line and so it too
sags as the load causes the AC line to drop a few volts, but the time
constant is much longer, probably around 1 second or more because the
designer used a large capacitor to minimize hum and the grid load is so
So, the operating condition definitely change during the step response test,
but what's the effect on intermods? Not much. Intermod performance doesn't
depend on plate voltage very much at all (for the tetrode case we thinking
about here). And the grid bias changes a little bit around the AB1
operating point, but that's well known to be a non critical setting.
----- Original Message -----
From: "Dr. William J. Schmidt, II" <email@example.com>
Look. It does not matter if the tube draws grid current or not. If there
any signal impressed on the grid voltage via any means, it can lead to
distortion. It's a simple concept. Regulation of the bias voltage is one
way, then, to ensure that it does not happen. The better regulated the bias
supply is (e.g. the lower the Z of the supply) the less likely this is to
happen. Here is a test: will the amplifier's unit impulse or step response
will be different with poorly regulated supplies?
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