[AMPS] Question : 4CX800 Amplifier 6-10-12-15-17-20 meters

[W9GT]

Thanks, Colin.  Not the literature or the context at fault, just my memory.
Yes, I knew tuned input was an important consideration for a GG amplifier.
I have been sufficiently dressed down for my incorrect reference.

73,  Jack, W9GT
----- Original Message -----
From: "Colin Lamb" <k7fm@teleport.com>
To: "Amps reflector" <amps@contesting.com>; "W9GT" <w9gt@home.com>
Sent: Tuesday, December 04, 2001 11:34 AM
Subject: Re: [AMPS] Question : 4CX800 Amplifier 6-10-12-15-17-20 meters


> Jack said:
>
> "Much of the literature contends that a tuned input is better for IMD,
> however, that leads to possible requirements for neutralization and other
> complicating factors."
>
> Either you are reading the literature wrong or it is wrong.
>
> There is nothing to be gained by using a tuned input when the amplifier is
> in a grounded cathode configuration.  It is only when it is cathode
driven.
> With a grounded cathode and a resistor load on the grid, the only concern
is
> adequate grid voltage.
>
> Sometimes hams take facts out of context.  When driving a cathode, usually
> the full 100 watts (more or less) is required to drive the amplifier.
> Without a tuned input, the amplifier does not present a proper load over
1/2
> of the cycle - which reflects back into the driver and increases driver
> distortion - leading to increased distortion out of the amplifier.  The
> pi-network or variation acts as a flywheel to reduce distortion by 3 db
more
> or less.  But, the same thing could be done a number of other ways.  A
diode
> and load resistors on the alternate half cycle would do the same thing, or
a
> specific coax length (which makes it a single band amplifier).
>
> Using a grid driven amplifier with a passive input presents a proper load
to
> the driver at all times.  Changing from a swamping resistor to a tuned
input
> in that case will not improve distortion at all.  The passive grid is
> perfect because of the excess of drive when driving the grid.  You could
> actually use a passive cathode input to eliminate any tuned circuit in the
> cathode - but the drive requirements would at least double.
>
> The question is one of choosing between grounded grid (with a tuned input)
> or a passive grid.   The grounded grid circuit has one advantage in that
it
> has inherent feedback - which may reduce distortion products by up to 10
db.
> Knowing that, you would then look at the tube characteristics.  The
4CX-250
> series would be a poor choice for grounded cathode operation because it is
> rated for approximately -22 db distortion products.  Assuming -30 db is a
> minimum desired goal, you would have to use the 4CX-250 in grounded grid
or
> provide negative feedback or degeneration of some sort if used in a
grounded
> cathode configuration.
>
> The 4CX-800 tube is cleaner than the 250 series, so it is a good candidate
> for grounded cathode with a resistive grid input.  George Daughters has
> designed a few amps using the Svetlana tubes and inserted degeneration in
> the cathode by using an unbypassed resistor.  They perform nicely.  The
> degeneration consumes some of the excess drive and decreases distortion
> products a bit.  The articles are available on the ARRL website.
>
> One of the advantages of passive input is that the amplifier can be used
on
> a different frequency simply by tuning the output.  When we consider there
> are 8 hf ham bands where you would use the amp, the passive grid circuit
has
> a lot to recommend it.
>
> 73,  Colin  K7FM
>
>


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