"Amperex 8179, which may be similar to the 4-400"
The 8179 or QB5/2000 is actually more of an European 4-1000A.
It has comparable plate voltage and plate dissipation ratings, and is specified
for
linear amplifier service.
73/
Karl-Arne
SM0AOM
----- Original Message -----
From: "k7fm" <k7fm@teleport.com>
To: <amps@contesting.com>
Sent: Tuesday, January 10, 2006 5:24 AM
Subject: [Amps] Components Causing Distortion in Amplifiers
> 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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