May I add my 2cents worth.
Some causes of imd are not in fact due to the amplifier! In the G2DAF original
circuit
the impedance "seen" by the driver is approx. 300 ohms in order to develop
sufficeint
screen-grid voltage rectification. In those days - 1960's - the transistor was
a far
distant device in amateur rigs and practically all transmitters used a Pi tank
and a pair
of 6146's or similar which could happily load into the high Z of the typical DAF
amplifier.
Today things have changed considerably. Most transceivers have solid-state
finals. When
a solid-state final is run into a mis-matched load, such as a DAF input
circuit, the imd
generation is severe. So switching out the DAF amplifier often causes the
signal to
clean up as the rig is now feeding an antenna which looks sort-of 50 ohms and
it is
happy. Putting the DAF back in circuit causes splatter - so You immediately
pin the
blame on the amp - when in most cases it is the transceiver generating the
splatter and
the amplifier faithfully amplifies it!
For what it's worth.
Have a peaceful Xmas and a prosperous New Year.
John ZS5JF
----------
> From: Lamb <k7fm@teleport.com>
> To: measures <measures@vcnet.com>; AMPS <amps@contesting.com>
> Subject: Re: [AMPS] More and More and More and More - drive derived screen
> potential
To: <amps@contesting.com>
> Date: Monday, December 20, 1999 7:28 AM
>
>
> Rich:
>
> You keeping adding new twists to assure failure.
>
> Your statement . "The DAF runs heavy grid current" is no more true than the
> statement that any grounded amplifier draws heavy current.
>
> At maximum drive, the plate, screen, control grid, and filament of both the
> grounded cathode and the G2DAF circuit will have the same voltages (when
> adjusted correctly). So, the G2DAF circuit will draw no more grid current
> than the standard grounded cathode configuration. By defining the G2DAF as
> drawing grid current at maximum signal, you assure failure.
>
> According to Bill Orr, in his article "Intermodulation Distortion in Vacuum
> Tubes", "as far as IM distortion goes, (the perfect tube) is a poor device
> to use in equipment designed for linear amplification". However, he
> explains that the practical tube in fact will provide less distortion.
> Speaking about the curve of the practical tube, he states "the central
> portion is fairly linear and the lower portion resembles a parabola". The
> G2DAF circuit simply extends the parabola. Compression at the top end of
> the curve is the biggest cause of distortion, and the G2DAF circuit may
> actually reduce this by increasing the screen voltage at the peak.
>
> The Orr article assumes a constant screen voltage, but does state that (at
> least for a portion of the bottom end of the curve) the parabola is not only
> not harmful but exists in practical tubes which actually have less
> distortion than the "perfect" tube.
>
> Orr states that "the magnitudes of the intermodulation distortion products
> are significantly affected by almost everything: changing heater or filament
> voltage by only a few per cent; slight shifts in bias voltae, idling
> current, screen voltage, plate or grid tuning, neutralization, loading - all
> of these factors and others even more obscure enter into the determination
> of intermodulation distortion". Varying the screen voltage is simply one of
> a number of factors to be considered.
>
> The fact that the two amplifiers you have listened to had splatter does not
> prove that the circuit will not work. To the contrary, your reports of the
> 813 amplifier in the G2DAF circuitry were consistent with what one would
> expect using the same tube in stabilized screen grounded cathode circuitry.
>
> Colin K7FM
>
>
>
>
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