"clean sound." Each group obviously desires sound which
simulates the original, but each rejects the elusive and
unmeasureable distortions which characterize the tube type preferred
by the opposition camp. A new type of tube, none of which has been
put on the market for many years, might be the thing which could
reconcile these diverse views of listeners who all look for the same
thing but seek it in different ways.
The requisites for such a
new tube can be listed readily:

1.
Low internal impedance, such as is offered
by the
triode.
2. High power sensitivity of the tetrode so as to

minimize drive problems.
3. Lower harmonic and
intermodulation
distortion than either
triode or tetrode at both
high and low
levels of operation.
4. Sufficiently high efficiency to permit

adequate output without undue hulk or
cost.

Since no such tube is available, the only recourse is
to seek a mode of operation of existing type tubes to approximate
the desired qualities and then to see whether the theory is
justified by listening tests.

Linearizing the Output
Stage
The
physical difference between the triode and tetrode is, of course.
The screen grid. This gives the tetrode its efficiency on the one
hand, but also increases the plate resistance and contributes toward
the "tetrode sound" which is so violently disliked by triode
favorers. Therefore, the screen grid seems to be the element which
gives the tetrode its advantages and its disadvantages compared to
the triode. In fact, when the screen is connected to the plate, the
resultant tube is a triode which is excellent in many respects
though handicapped by limited power output and low permissible
dissipation. Control of the screen is a logical step toward
extracting the favorable attributes of the tube and discarding the
unfavorable.

Experimentally it was found that the goal of improved
operation could be achieved through energizing the screen with DC
through a special winding on the output transformer and combining
the effects of both plate and screen current in the output
transformer. This is illustrated at (A) in Fig. 1 with an
alternative and simpler method shown at (B). It has been found that
the screens must be fed from a low-impedance source or the benefits
of this arrangement cannot be realized. This eliminates the
possibility of doing the same job with resistive bridge networks or
voltage dividers.

Fig. 1.
Arrangements for energizing the screen grid to improve tube
linearity.

The screen load impedance is somewhat critical if
optimum results are desired. As the ratio of screen load impedance
varies from zero (tetrode operation) to unity (triode
operation),
important effects show up:

1. The internal
impedance takes a sharp drop
and then levels
off.
2. Maximum undistorted output drops slightly

at first, then decreases rapidly.
3. Intermodulation
distortion at high level
operation drops to a
minimum and then soars
upward.
4. Low
level IM decreases somewhat and
then holds almost
level.

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