>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.
** Which why it's better to tune up a SSB amplifier with a string of
pulses instead of AØ / NØN
>
>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
>light.
>
>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.
** It seems to me that a sag in bias potential would simply increase
ZSAC slightly -- which would make little difference in IMD if the ZSAC
was OK to start with.
>
cheers, Jeff
>
>
>----- Original Message -----
>From: "Dr. William J. Schmidt, II" <bill@wjschmidt.com>
>
>Look. It does not matter if the tube draws grid current or not. If there is
>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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