Someone asked whether components such as capacitors can cause distortion in
linear amplifiers. Somehow the e-mail got side-tracked, but I have some
comments in response. The response is not quite on point, but raises some
interesting points.
The comments are not mine, but those of W2LPC and WA2PDI, both who worked
for Amperex at the time they wrote the article. The article was published
in 73 Magazine in May of 1963. The article is entitled "Low Distortion All
Band Linear Kilowatt" and described about the design of an amplifier using a
Amperex 8179, which may be similar to the 4-400. They mentioned all of the
normal things, such as regulated screen and bias - but let them finish the
story:
"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.
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.
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.
2. In grounded grid operation, second harmonics of the driver must be
suppressed, normally with a tuned input.
3. The screen and control grid power supplies should be well regulated.
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. 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.
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.
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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