Main article: Vacuum tubes fall into three mainly non-interchangeable categories regarding their heater or filament voltage/current, though some tubes' heaters run at both a standardized voltage and current, e.g., 6.3 V at 300 mA or 13 V at 200 mA, making them suitable for either operation. • Battery types, with a low-power filament operated usually from 1 to 2 V; all filaments in a design are rated at the same voltage and are connected in parallel.

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They are usually directly heated to save heating power (therefore requiring a current), except if the (due to the voltage along the filament) poorly defined cathode potential impedes the function of the device, as is the case e.g. Keil c 4 keygen torrent. With AM diodes with filament voltages of 1.9 V or more; in such cases, indirect heaters were employed and marketed as or unipotential cathode at the time. • 1.25 V for one () • 1.4 V for one, later for one () • 1.7 to 1.9 V for one with a variable filament resistor (: – ) • 2 V for one lead-acid cell: 12ff.: 8: 5 () • 4 V for two lead-acid cells, a 1910s UK standard which e.g. The, the ran on. Major manufacturers kept a directly heated 4VDC product line throughout the 1930s: 14: 9: 8f.

• 6.3 V, indirectly heated, for four dry cells or for three lead-acid cells for use in 6-Volts () • 12.6 V, indirectly heated, for six lead-acid cells for 12-Volts car radios () • Types for -only equipment with a mains; all tube heaters are rated at the same voltage and fed in parallel from a transformer secondary winding; In the US, a 2.5 V standard was conceived in 1929; elsewhere, nominal heater voltages from 1920's lead-acid/' era (multiples of 2 or 2.1V) were kept, 4V RMS in the UK, but eventually the 6.3V and 12.6V car radio standards prevailed everywhere. Most are indirectly heated. • However, most are directly heated, so for the abovementioned reason, the heater voltage must be much smaller than the signal voltage on the grid and is therefore in the 1.1 to 25 V range, drawing up to hundreds of Amperes from a suitable heater transformer. • Types for equipment designed to run on either AC or DC mains power () with no mains transformer; they are always indirectly heated and all heaters are connected in series across the mains power supply, possibly with an additional () or power resistor chosen so that the sum of the heater and ballast voltages equals the mains voltage.

All tubes must be rated at the same heater current such as,,,,, or mA, depending on mains voltage and device tube count, and have a comparable warm-up time. • A notable precursor was 1920's European 180mADC standard used in municipalities with a then-still commonplace DC mains system, which ruled out not only mains transformers, but also polarized, as the sets had to tolerate accidental reverse-polarity feeding, making it notoriously difficult to filter out or howl. Compatible product lines include the,: 19 the, the,: 7 the Telefunken REN(S)18xx series, some, the,: 10 and the. Most of these tubes had 20- or 21-Volts heaters; suitable barretters were available. The currents drawn by parallel-connected types, and the voltages across series-connected types, vary widely according to the tube's heating power requirements. Otherwise identical tubes were manufactured in several variants with different heater characteristics (but usually the same power, e.g.