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ELECTRON TUBES 263
facturers often stock replacement tubes for their apparatus. The performance claimed for the apparatus can be obtained not only with the tubes supplied as original equipment, but also with those supplied as manufacturer-furnished replacements. Selected tubes carry a higher price than unselected tubes ; the price difference customarily reflects the additional cost of selection.
In some instances equipment manufacturers may offer special electron tubes for sale that provide appreciably better performance compared with the run-of-the-mill commercial product. Typical examples are the Western Electric 348A and 350B ; these are quite superior to the commercial 6J7 and the commercial 6L6 — their respective equivalents. Oftentimes as in the case of Western Electric equipment such special tubes are subject to sale restrictions and are not available for 16-mm sound-on-film recording purposes except as part of a piece of equipment leased by Western Electric or RCA on a royalty basis.
Where long life is important as in industrial tubes, RCA markets special tubes known as the "red-top" types. The 5691 (equivalent to the 6SL7GT) and the 5692 (equivalent to the 6SN7GT) are rugged and can withstand shocks of 500 g for short periods, and 2.5 g for hundreds of hours continuously; these tubes require 0.6 ampere for the heater. The 5693 (equivalent to the 6SJ7) is a long-life rugged tube using the same heater current as its commercial equivalent ; apparently its outstanding characteristic is low gas current, since under prescribed conditions it is capable of operating with as much as 40 megohms in the circuit of grid #1.
Should a recording equipment manufacturer supply selected tubes for his equipment, the selection criteria are not at all mysterious. Selection in usually made for :
1. Tube noises — such as hiss, hum, and random "pop" noises.
2. Microphonic sensitivity — the ' ' bong ' ' or howl sound heard when a susceptible tube is mechanically tapped.
3. Special electrical tests — these are dictated by the specific need — they may be such tests as uniformity of \i (amplification factor), Gm (mutual conductance), Zp (plate impedance), "drift," grid circuit gas current, etc.
The Tube Department, RCA Victor Division, Harrison, N. J. advises as follows: "Types such as the 1603 and 1620 which are sold as non-microphonic types are specially tested for such items as hum, noise, and microphonics. Other items such as gm, rp, mu, and heater-cathode voltage rating are held to the same limits as the prototypes.
' ' The special tests include hum measured with a low-frequency amplifier, and hiss, noise, microphonics, and pops all measured with a wide-band amplifier. Limits on each of these items have been established to correlate as well as practical with the results obtained in commercial high-gain, high-quality audio amplifiers. In general, heater-cathode insulation tends to improve with age.