From FB group Bell Telephone…
While I see a discussion about connecting phones in a party line, I’d like some input on this. It’s easy to connect two phones because they can be connected in series, and both phones have to be off hook. But more than two phones, they can’t be connected in series because if only two of the three phones are used, the third would block the current if it was on hook. I’ve seen more than one solution. Obviously one is to connect them in parallel with a separate load resistor for each, and coupling capacitors between them.
Another solution is to connect them in series with a zener diode in parallel with each phone. If one phone is on hook, the current can pass through the Zener when the voltage across it rises above its breakdown voltage. The current will then go through the other phone when it is taken off hook, and all three phones will work. The one disadvantage I see is the changes in voltage could cause a loud noise in the earpieces.
Any ideas how this is usually done?
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Yes, I can see tying all phones together. But the power supply has to be a current source. If one or more phones are connected to a voltage source such as a normal power supply, any audio will be filtered out. So there has to be a load resistor to make the supply look like a current source.
But differences between phones, or between the lengths of the loop, can cause one phone to get most of the current, or very little current. This isn’t a major problem with Bell 500 or other sets, but for other phones, the sound levels may be too loud or soft. So I would give each phone its own load resistor and put coupling capacitors between the phones.
Ed Vaughn said “The most perfect source for common talk battery is a 401A card in a 1A2 system. It provides proper filtering and coupling for multiple stations off-hook.”
Glenn Peters said, “In real world that is not how it was done. Sometimes you would have up to 8 parties on a single pair back to the CO in rural areas. Other times you can have up to 4 different pairs tied to a single OE for 4 partys or more. Most in use today are mainly 2 party, which can be either 2 separate cable facilities tied at the OE or a single feeder pair bridged at a cross box or a distribution terminal. H88 cable loading maintained a maximum of -8.5 DBM loss at 1000 Hz to the station. This provided proper signal levels across the National DDD Network. No provision was made for more than one station to be off hook at a time per tariff.
Selective ringing was handled by a couple different ways depending on the telco.
More selective ringing methods were introduced using various technologies.
In the system of divided ringing, the ringing circuit was separated from the talking circuit by adding a ground connection between the central office and the subscriber stations for ringing. On the same subscriber line, one party used the tip side of the line and ground for ringing, whilst the other party on the same line used the ring wire and ground for ringing, to achieve full selectivity for two-party lines, in which only the selected station would ring. These names for the wires are derived from the paired cord plugs—used on a manual switchboard—composed of three parts: the tip and the sleeve separated by a narrow metal band called the ring, each of these three components being insulated from one another.[39] In the Bell System, the two stations were thus called the tip party and the ring party, In combination with code ringing, this method could be extended to four and eight subscribers to reduce the number of disturbances. In several variations of divided ringing, also called grounded ringing, the bells were activated with polarized current, so that full selectivity was achieved for up to four parties.
Another selective ringing system was based on using different ringing frequencies for each station on the party line. In North America, this was used mostly by independent telephone systems, while the Bell System abandoned frequency selective ringing in the early 1900s. Initially four frequencies were used, which were based on a system of harmonic multiples of a frequency of 16 2/3 Hz. Combined with divided ringing, this provided fully selective service for up to eight stations.
All fully selective ringing systems on party lines still brought the inconvenience of finding the line in use occasionally, by hearing talking, when one picked up the phone to make a call. All party lines also required special equipment to complete calls to another party on the same party line.”
