Tom's question was:
> May, 1963 issue of 73 magazine by Bert Green, W2LPC. Bert wrote the
> article while working for Amperex. He made distortion measurements of the
> amplifier after construction and stated that "the distortion was decreased
> even 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".
Losses and circulating currents don't cause non-linearity or
distortion. More than likely he was seeing normal measurement
scatter.
How much change did he measure, and what was the test setup?
I have the article, but do not have a scanner available. The article did
not report the change nor the test setup. Since the amplifier was a design
using the then new Amperex 8179 tetrode, my assumption was that that the
design and testing was done at Amperex. It is interesting to note a number
of statements in the same article - such as:
"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 supply voltages. By bypassing the
meters and circuit breaker coils with electrolytic capacitors, the
distortion was decreased considerably."
Also, another comment was:
"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. This is
normal and is probably caused by misalignment of the control and screen
grids due to sagging of the elements when the tube is operated on its side."
The article even has information on the famed parasitic suppressor debate:
"When first placed into operation, the amplifier was found to parasite at
about 120 mc. This could have been cured by placing a parasitic suppressor
in the plate lead of the tube, but this would have inserted considerable
resistance in series with this lead at 30 mc. Due to the high output
capacitance of the tube, a good portion of the plate tank current flows
through the tube and any resistance in the plate lead would cause high
losses resulting in low tank circuit efficiency at 30 mc. For this reason,
parasitic suppressors were placed in the control grid and screen grid leads
instead of the plate."
The concluding paragraph of this article was:
"Parasitic suppressors should, if possible be kept out of the plate tank
circuit in order to keep the unloaded Q and, therefore, the plate tank
efficiency as high as possible. At high frequencies, where the tube
capacity becomes a good portion of the total tank capacity, this precludes
the use of the normal parasitic suppressor in the plate lead. As much
suppression should be done in the control grid circuit as possible with only
as much suppression as is absolutely necessary in the plate and screen
circuits. Suppressors should be made with the lowest value of resistance
and inductance that will suppress the parasite to avoid excessive loss on
the operating frequency."
In this case, I am just reporting an article that appeared almost 30 years
ago. I have neither confirmed nor disproved statements made - just wanted
to stir up the pot a bit.
Colin K7FM
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