[160m] 160M Amplifier

[Tom Rauch]

> Thanks for your prompt reply to my query.  Well, your
> comments lead me to have 2nd thoughts about the ACOM
> 1000 that has rave reviews on eHam.net.  It's lure is
> that it's a 6M-160M 1000W amp.  It has a single
> 4CX800A(GU74B)Svetlana high perfomance ceramic-metal
> tetrode w/ plate dissipation of 800W(forced air
> cooling,grid driven). It is a bit pricey.

It's very easy to for me build anything I want. Despite the
fact I can have any amplifier I want, the last grid driven
tetrode amp I built for my own use was a 4-1000A amp.


Over the years Svetlana gave me tetrodes for amplifiers, and
I never went beyond prototypes. The reason was availability
of tubes, and cost of the amp. The money saved by buying a
cheap old stock tetrode tube was almost entirely offset by
increased power supply and protection circuit costs, and
that was when the tetrode would cost ~$100 compared to a
$300 triode of similar power.

Tetrodes require screen and bias supplies, and to keep
intermod products low both screen and control grid voltages
should be very stable both long term and dynamically with
power level variations from speech or CW keying.

Tetrodes also lack the very heavy negative feedback inherent
in cathode-driven designs, so the same tube has
significantly higher IM products when grid driven compared
to cathode drive. That's why, if you look at ARRL test
reviews for IM, you'll find tubes like 3CX800's are 10-20dB
cleaner than 4CX800's.

Another thing I consider is replacement. If I use a triode
and availability goes away, I can generally cobble in some
other type of tube with only a mechanical change for
socketing and cooling and a filament supply change. For
example with an AL811 I can use 572B's. I could use other
surplus triodes or even some pentodes (connected as hi-mu
triodes in grounded grid) without much work.

If the amp used screen and grid supplies, I'd probably have
to significantly change the circuitry. Only the HV supply
and tank circuit would likely be usable. In most cases none
of the input circuitry would work, the bias wouldn't, the
screen would be useless, the socket, and so on.

I just never saw cost advantage, reliable long term tube
availability, and didn't want the risks if a tube wasn't
available.

In 100 watt drive 10 kW output commercial amp it's
economical to use a 4CX5000. In a 100w drive 1 kW amateur
amp, a triode makes way more sense.

73 Tom