The National NCL2000 with 8122 tetrodes ran a swamped grid input that could
be configured for either 10-20 or 50-100W drive. IMD was nothing fancy,
~ -28 or so, but this was a 1963 design. The FCC CB rule change killed that
amp. The retrofit using 4CX400A's that I designed for Svetlana improved that
to -32 dB.
The Tokyo Hy Power HL-1K/A ran a pair of 4CX250B's in a cathode driven setup
with the control grid at RF ground. Typically 9-10dB stage gain and up to
600W out. A review gave -34 dB IMD. Battleship construction and a relatively
high landed price limited US sales but they sold good in Asia/Oceania.
Something to consider for a home brew circuit since those tubes are
everywhere and the special Eimac/Johnson socket is not needed.
Both the above amps could be driven with a SS xcvr without resorting to an
internal/external tuner.
Carl
KM1H
National Radio 1963-69 Member of NCL2000 Design Team
----- Original Message -----
From: <TexasRF@aol.com>
To: <kdutson@sbcglobal.net>; <amps@contesting.com>
Sent: Thursday, April 26, 2007 12:52 PM
Subject: Re: [Amps] Triode vs Tetrode Amp
>
> Hi Keith, many, if not most, engineering decisions are $$ based. A
> source
> of cheap 4CX800 tubes from Russia could cause a company to use them
> instead of
> expensive 3CX800 tubes in an amplifier design for example.
>
> The tetrode tube has to be cheap enough that the cost of the screen
> supply
> and tube socket has to be more than covered by the cost savings.
>
> Performance wise, the cathode driven triode will typically have lower imd
> than the grid driven tetrode (due to the heavy negative feedback).
>
> Where the tetrode has an advantage is in the rare case that one wants to
> use
> very low drive power, such as a few watts from a QRP transmitter. The
> input
> grid circuit can be designed to allow full output power with very low
> drive
> power levels. The FCC will not bless a commercial amplifier like this as
> the
> rules don't allow such a design.
>
> 73,
> Gerald K5GW
>
>
>
>
> In a message dated 4/26/2007 11:16:22 A.M. Central Daylight Time,
> kdutson@sbcglobal.net writes:
>
> Something that has been bugging me...
>
> Why do some designers use a triode versus a tetrode for linear amplifier
> finals? A perfect example is the use of a pair of 3CX800's compared to a
> pair of 4CX800's.
>
> Thanks and 73,
> Keith NM5G
>
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