ka5piu
02-29-2008, 10:02 AM
Hello.
On some of the really old telephone runs, people would have subscriber amplifiers.
One old gent who lives in West Texas is such a case.
The twist is that there are 2 telephones.
One, a western electric 500 set, has a transistor amplifier.
But, the other, a 300 set, the first one installed, had no amplifier, or so I thought.
But, hidden under a cabinet, there it was.
It was installed around 1942, and all but forgotten.
Inside are 5 western electric tubes.
Service notes say that one tube and a cap were replaced in 1953, and that was it, or so it would seem.
Now, name any other electronic device that can work, day in and day out, for over 50 years, without any service.
True, the tubes are not doing anything most of the time, and a circuit puts everything in idle state between calls, but still.
One trick is there are 2 relays.
One is in the DC loop and does more than provide increased talk battery.
Going off hook causes the relay to remove a resistor that is in line with the filaments.
All of the tubes are rated at 2 volts for the filaments and 45 volts for the plate.
Plate voltage comes from the central office.
But, how can one get 48 volts DC while off hook?
Simple, B+ is ground in telephony, but this is normally a balanced circuit.
Just like older payphones, a ground is employed to operate things.
But, in 1953, the phone company changed things, so a local supply of B+ was provided.
The tube change was to provide a dual-diode tube in place of the original single unit.
Turns out, western electric has provided light bulbs for switchboards that have worked from the time the things were installed to the time they were replaced by something newer, sometimes 80 years.
But, that is not the really odd part.
In order to provide talk voltage in a power failure, a dry battery is provided.
The burgess battery, made for western electric, still provides voltage, and has a 1952 date code.
On some of the really old telephone runs, people would have subscriber amplifiers.
One old gent who lives in West Texas is such a case.
The twist is that there are 2 telephones.
One, a western electric 500 set, has a transistor amplifier.
But, the other, a 300 set, the first one installed, had no amplifier, or so I thought.
But, hidden under a cabinet, there it was.
It was installed around 1942, and all but forgotten.
Inside are 5 western electric tubes.
Service notes say that one tube and a cap were replaced in 1953, and that was it, or so it would seem.
Now, name any other electronic device that can work, day in and day out, for over 50 years, without any service.
True, the tubes are not doing anything most of the time, and a circuit puts everything in idle state between calls, but still.
One trick is there are 2 relays.
One is in the DC loop and does more than provide increased talk battery.
Going off hook causes the relay to remove a resistor that is in line with the filaments.
All of the tubes are rated at 2 volts for the filaments and 45 volts for the plate.
Plate voltage comes from the central office.
But, how can one get 48 volts DC while off hook?
Simple, B+ is ground in telephony, but this is normally a balanced circuit.
Just like older payphones, a ground is employed to operate things.
But, in 1953, the phone company changed things, so a local supply of B+ was provided.
The tube change was to provide a dual-diode tube in place of the original single unit.
Turns out, western electric has provided light bulbs for switchboards that have worked from the time the things were installed to the time they were replaced by something newer, sometimes 80 years.
But, that is not the really odd part.
In order to provide talk voltage in a power failure, a dry battery is provided.
The burgess battery, made for western electric, still provides voltage, and has a 1952 date code.