k7fm wrote:
>"When the amplifier was first constructed, the distortion was found to be
>quite bad (about -30 db). This was caused largely by the fact that the
>current meters and the circuit breaker coils in the negative leads of the
>power supplies were not bypassed for audio frequencies, causing an audio
>voltage to be superimposed on the power suply voltages. By bypassing the
>meters and circuit breakers coils with electrolyticcapacitors, the
>distortion was decreased considerably.
>
>
Right. All the voltage need to have low impedance across the audio
spectrum.
The classic test for uses uses the two tone intermodulation test method and
sweeps the spacing of the two tones from about 300 Hz to 3 KHz.
If the level of IMD varies with tone spacing, then one or more of the
voltages needs better filtering.
In solid state amplifiers, thermal effects can through off the tuning
because the
surface of the die can have a very fast thermal time constant. The
transistors
actually change temperature at the beat frequency between the tones. In the
literature, this is call "memory effect".
>The distortion was decreased still further by leaving the unused turns on
>the plate tank coil unshorted when switching from band to band, instead of
>shorting them as was done when the amplifier was first constructed.
>
>
Interesting... It's not discussed very much, but IMD products come from
multiple sources. People describe 3rd order IMD usually as 2*F1-F2. But
the amp will generate the same spur frequency from 3*F1-2*F2, 4*F1-3*F2,
etc.
Each of these components have their own phase and can either add or cancel
with each other. It's possible for a two tone test to show very
different IMD
results from slight variations (0.1 dB) in the power to one tone. The
FCC has
changed the test method for PA's to require modulated signals
specifically to
prevent manufacturers from hunting around for a sweet spot in IMD.
It's possible that the change in results from a short vs open turns
comes from
the different impedance the Pi-Net presents to the tube has the higher
harmonics.
>The 8179 data sheet states that the tube must be operated in a vertical
>position only. During testing, the amplifier was operated on its side and
>it was noted that the distortion figures became slightly degraded, probably
>caused by misalignment of the control and screen grids due to sagging of the
>elements when the tube is operated on its side."
>
>Later in the article, they summarized design considerations for a low
>distortion amplifier. In summary, they are:
>1. Any impedance in series with the power supplies must be bypassed for
>audio as well as rf.
>
>
yup
>2. In grounded grid operation, second harmonics of the driver must be
>suppressed, normally with a tuned input.
>
>
yup
>3. The screen and control grid power supplies should be well regulated.
>
>
yup
>4. The excited distortion should be at least 20 db better than the
>amplifier in order not to increase the amplifier distortion by more than 1
>db.
>
I'd say at least 10 dB better for less than 1 dB effect
>If extra drive power is available reisistive swamping is advised for
>grounded grid operation.
>5. The plate tank coil should be progressive opened instead of shorted when
>changing bands.
>
>
Probably depends on the details of the frequency response of the network at
harmonics...and probably not a general rule to follow.
>I brought up these points a number of years ago and someone told me I was
>nuts for saying that shorting out the turns would affect distortion. Well,
>I am not saying that - two employees who worked for Amperex said it.
>
>
Possible...but not easily controlled at design time.
>The inference is that they had a spectrum analyzer and hard data to support
>their statements - but I do not know. Perhaps they were janitors and just
>made it up.
>
>I mentioned it, because if Tom is designing the perfect amplifier he at
>least ought to have the input, and it can be tested easily enough.
>
>73, Colin K7FM
>
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>
>
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