[AMPS] More and More and More and More - drive derived screen potential

[Lamb]

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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