"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 
4. Low level IM decreases somewhat and
    then holds almost level.


pg. 2


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