>Subject: [AMPS] Re: Alpha models: 91B vs. 89: TUNED INPUT?
>Sent: 5/8/97 11:17 PM
>Received: 5/9/97 7:09 AM
>From: morel grunberg, morel@shani.net
>To: gdaught6@leland.Stanford.EDU
>CC: amps@contesting.com
>
>Hi,
>
>I don't know if the sole demand is only a 50 Ohms load
>for exciter.
The grid terminating R can also serve to improve VHF stability. To
explain: Every tetrode or pentode has a finite amount of feedback C
between the anode and the grid. (1pF is a typical amount. At 110MHz,
1pF has 1440 ohms of X.)
If a low VHF-R path exists between the grid a ground, VHF feedback
between the anode and the grid is decreased. However, I have seen Class
AB1 amplifier designs that place an inductor in series with the 50 ohm
grid termination R. The intent is do semi-cancel the reactance of the
grid on the higher freq. bands. However, adding an inductor in series
with the grid R obviates the VHF attenuation ability of the grid R. The
fix:
1. Put the grid termination as close to the grid as is practical and use
copper ribbon leads to reduce L. (I like to feed the grid bias voltage
through the RF-bypassed end of the grid termination R.)
2. Neutralize the amplifier on the higher bands to make the SWR
essentially unity wherever the amplifier is used.
note---Tetrode-neutralization is much easier that most grounded-gridders
seem to realize. Neutralization is explained on my Web site.
>Please check with a spectrum analyzer
>the spectral purity of a non-tuned input amplifier vs.
>a tuned one with same tubes. I did it and I'm convinced
>that the few more bucks invested and some little losses
>in a tuned input network will worth the improvement in
>output IMD products! In my views every 1-2 dB is very
>important.
Eimac says that for lowest IMD, Class AB1 amplifiers should be
neutralized and operated from a regulated screen supply
>Maybe I'm too conservative or old fashioned to beleive
>some of the myths and unbeleiveble claims of those who
>want to justify cheap solutions or poor engineering
Good engineering is not about about cheap. It is about addressing all
issues and possible situations.
>No, I do not promote the over-designing, all I ask is
>proper design.
In the minds of some manufacturers, getting though the warranty period is
all they seem to worry about. This is pretty much the way American
companies have done business. However, our own Dr. Demming believed that
quality would eventually pay off in increased sales, customer loyality,
and decreased return cost. Demming was widely ridiculed and dismissed by
corporate America. However, the Japanese listened to Dr. Demming. So
instead of building products that needed service, the Japanese tried to
build products that didn't break. In Japan, Dr. Demming's birthday is
widely celebrated.
>I learned that quality costs money and cheaper prices
>are usually mean cheaper results. Also, bad engineering
>costs you a fortune in field returns and your reputation
>as manufacturer. Am I wrong ?
No.
Rich, ag6k
Sue Measures
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