Care of Series-Parallel Filament Circuits: Part 1
This note applies to indirectly heated tubes and their circuits only.
Consumer radios and TVs use either parallel (mostly 6.3V, EU:E) or series (mostly US:150/450mA, EU U:100/P:300mA) connected filaments. In many complex radios (such as the Collins 51S-1, S-Line, 618S-1, R-390/URR, Heathkit SB/HW-Series and many others) mainly standard 6.3V tubes are used, but the filaments connected in a series-parallel circuit configuration providing the correct voltage needed to each tube used. This is done to reduce filament current (wire size) and/or adapt to mobile/avionic DC supply voltages (e.g. 12.6V, 26.5V).
In order for this concept to work, the filament circuit branches need to be balanced so that the current and voltage for each tube (and dial-lamps!) is correct for their rating. Sometimes additional parallel/series resistors are used to balance out differences in filament currents. Often this filament circuit is shown separately on the circuit diagram, also indicating current regulators, balancing resistors, RF bypass capacitors or RF chokes. A nice example of such an arrangement is shown below. These applications deserve to be studied.
Example design: Collins 51S-1 heater circuit.
Replacement, Substitution and "Improvements"
To maintain the designed balance, all tubes and dial-lamps need to be replaced by exact or truly equivalent types. Unfortunately this aspect appears often overlooked, even in literature, quoting "substitutes", e.g. 12AU7@150/300mA vs. 5814@175/350mA. While both will work in a simple parallel filament circuit as all other key data are indeed identical, it is of significance in this case.
When implementing mods this parameter must be considered just as all other criteria. E.g. replacing a 6HS6@450mA with a 6AU6@300mA in a SB/HW-101 upsets the filament circuit balance as does the replacement of the 6BF5@1200mA with a 6AQ5@450mA in the 51S-1/75S-3 or changing dial-lamps to other types or LEDs. If such mods are implemented, the filament circuit needs to be rebalanced and correct heater voltages on all tubes checked!
Some tube suppliers in the past have misbranded TV-tubes (in this case designed for series heating with 450mA) as 6.3V tubes. Infamously EL-MENCO supplied in the 70ies "6AU6" to Heathkit which were really rebranded 4AU6@4.2V/450mA which found their way into new SB/HW-kits. International "6EA8" and "6KE8" in my current stock bought in the 90ies are really 5EA8@4.7V/600mA and 5KE8@5.6V/450mA. Needless to say these tubes upset the balance (in my case these "6EA8"s did in a HW-101), overheat other tubes and don't last long. This just being samples of what you might get, it is best to check the tube filaments before use with a DC power supply, raising the voltage and measuring the current (e.g. with the 5EA8 the listed 450mA for the 6EA8 will be reached with less than 6.3V applied).