Hi Colin,
That is some interesting info!
I was not aware of the need for audio bypassing in the areas you mentioned.
Also, I have always wondered about shorting the final's tank coil for band
changes. I figured it lowered Q and I usually just tapped it as suggested.
But on the higher bands with tapping, there is a Tesla coil effect and the
voltages can arc bandswitches, etc. So I've usually made the amp for 160-40M
and got away with tapping, not shorting.
In addition, I've found that when using a "shorted turns" jumper, it
sometimes got very warm, if not hot on the higher bands. Maybe I needed a
larger conductor. There appears to be large circulating currents in the
shorted turns none the less.
The driver needs to be clean for sure, though I think someone here said that
the driver must be at least 6db cleaner than the final for a 1db final stage
degradation. They suggested 20db better, which is still easily obtainable in
class A drivers.
Swamping a GG amplifier is interesting too. I once measured the swing on my
GG amplifier input impedance and found it varying about 25% or so.
Anyway, I'll be watching to see comments on your points and maybe someone
will add more to it.
I'm in the metal and mounting construction stage on the new amplifier right
now and can make major changes if need be.
BTW, I was told today that in order to successfully apply 17db of RF
negative feedback around two stages, the phase has to be within ONE degree
of 180 degrees for reasonable stability. Pretty difficult! I'd like to do
20db NFB. I'd imagine a lot depends upon layout, etc, but it appears that
it's a good thing I am building a 75M monobander that will stay around the
same freq in the DX window rather than QSYing from band to band.... :-)
Thanks for the post, Colin!
73,
Tom, K1JJ
--------------
> 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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