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ELECTRON TUBES 265
quired, either the directly interchangeable Western Electric 350B, the commercial 1622, or the mechanically different 807 are definitely preferred. Fortunately there are few if any points in a sound-film recording system where the maximum power delivered by a pair of such tubes in push-pull is ever required.
In the case of electron tubes that operate in high impedance circuits, glass tubes are usually more satisfactory than their metal counterparts despite higher interelectrode capacitances. This seems especially true in circuits in which the impedance exceeds about 1/2 megohm. Although metal tubes may provide equal performance with regard to tube noise when new, they usually become more noisy more rapidly and show appreciably higher noise levels at the end of as little as 200 hours of operation. A 6H6G, for example, would be a better choice than a 6H6 as a diode rectifier in a compressor, noiseless recording, or other signalcontrolled circuits.
Electron tubes of high mutual conductance designed for television purposes such as the 6AC7 are not ordinarily applicable to sound-film recording equipment. Despite the advantage of high mutual conductance, their microphonic sensitivity is relatively so high that they represent a poor design choice under usual circumstances of application.
In preamplifiers, where signal level is low and leakage and noise must be kept to a minimum, electron tubes with the grid connection at a grid cap are preferable to single-ended tubes where the grid connection appears at the base. If a glass tube is used, not only is the physical separation of the grid from other tube elements great, but also the path external to the tube is of high resistance. Thus, a 6J7 would be a better selection than a 6SJ7 ; a 6J7G would be still better.
Special tubes are available in both metal and glass that are designed for low sensitivity to microphonics (noise resulting from the mechanical movement of the tube elements caused by vibration) and for low internal noise such as hiss, hum, and random "pops." The most common are the 1603 (glass) and the 1620 (metal) ; these are electrically equivalent to the 6C6 (glass) and the 6J7 (metal), respectively. The 6J7, 6J7G, and the 1620 are octal-base tubes; the 1603, like the 6C6, has a six-pin base. All five have similar electrical characteristics except for noise and for differences in interelectrode capacitances. In designing a preamplifier to use one of these five tubes in the first stage, the choice would be the 1603. Oftentimes, two-stage preamplifiers make use of a 1603 in the first stage and a 1620 in the second stage; because the 1603 type tube