. Electronic apparatus for biological research. Electronic apparatus and appliances; Biology -- Research. LC OSCILLATORS difficult to vary the frequency over a wide range without severely influencing the output. When these simple circuits are followed by a buffer valve, the frequency is substantially independent of the load. The output voltage is generally smaller if the buffer valve load is a resistance but not if it is a transformer or another tuned circuit. The output power depends on the buffer valve. There is the cost of the extra valve. The purity of the output waveform is good. HT+. -Pr
. Electronic apparatus for biological research. Electronic apparatus and appliances; Biology -- Research. LC OSCILLATORS difficult to vary the frequency over a wide range without severely influencing the output. When these simple circuits are followed by a buffer valve, the frequency is substantially independent of the load. The output voltage is generally smaller if the buffer valve load is a resistance but not if it is a transformer or another tuned circuit. The output power depends on the buffer valve. There is the cost of the extra valve. The purity of the output waveform is good. HT+. -Preferably a tuned circuit for l^^^X, good waveform Figure With the electron coupled oscillator the frequency is independent of the load and may be made independent of the HT voltage, over a range. The output voltage may be large, and also the output power if the valve is a beam tetrode or power pentode. The purity of the output waveform is poor, and requires resuscitating with a further timed circuit. Danger—red hot screens In designing Hartley or Colpitts oscillators to deliver appreciable powers— of the order of watts and above—there are good reasons for employing beam tetrodes or power pentodes because of their greater efficiency compared with triodes. If this is done it must be borne in mind that when the anode swings negative, the anode potential will be extremely low—possibly even lower than the cathode—and that this state of affairs coincides with maximum positivity at the grid and hence maximum cathode current. Under these circumstances nearly all the cathode current will flow to the screen, an electrode normally required to pass only a fraction of the cathode current. It is necessary to pay special attention to the maximum aflowable screen dissipation for the valve, and to check the product of average screen current—as measured by a moving coil instrument—and screen potential employed. Failure to remember this, and the use of a screen potential which
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