634A or BA or 684A or BA subsets

[Fennec]

Hello:

I was always (probably erroneously) under impression that in WE 634A or BA or 684A or BA designation, "A" stands for "sidetone" and "BA" for "anti-sidetone"...

However, this reputable resource, http://www.telephonecollectors.org/resources/abc.htm , claims that all of the above are anti-sidetone subsets.

Is this indeed the case? And if so - what exactly is the difference between "A" and "BA" ?

[unbeldi]

Western Electric usually used the A suffix for the most common or basic variety of an apparatus or part.  Modifications of that for special services, with special parts, or simply later versions, that were equivalent but used newer parts, were designated with either additional or new letter suffices.  There was not much of a systematic pattern for this, other than that they generally increased in the series.

In the case of the 634 and 684 desk set boxes, both are anti-sidetone instruments.  This is designated by the leading "6", which was incremented from a leading "5" of the corresponding earlier sidetone instruments.  The difference in 634 and 684, is the form factor of the box basically. The 684 is lower in profile, because of the new ringer design in which the gongs were rotated to a position parallel to the base frame.   Along with the technological upgrade of the anti-sidetone circuits, started in the early 1930s, came the standardization to using ringers with high coil impedances, although high-impedance ringers were certainly available earlier.   But when a basic 634 or 684 subset contained a high-impedance ringer, it was designated a 634BA or 684BA, with low-impedance ringers these were just a 634A, and 684A, respectively.

A high-impedance ringer was generally any ringer with a D.C. resistance of at least 2500 Ω.  Use of high-impedance ringers had two principle advantages: On party lines with multiple stations they presented less load on the line, and therefore the loop could be extended farther or the loop could be constructed from smaller gauge wire. In addition, a party line using divided ringing, i.e. where ringers operated via a ground connection, was less susceptible to inductive noise problems with higher impedances against ground.

[Fennec]

OK, thank you, this is much clearer now. Then basically for use on my VoIP home network "BA" would be preferred (there is enough juice to run GPO 232 and AE40, so voltage does not seem to be an issue).

Now, based on your experience, can one take an "A" low-Z subset, and make a "poor man's" high-Z one by adding a couple of resistors in series with the ringer? Granted, it will only affect the DC component, as I would be increasing only R, and not L of the circuit...

[unbeldi]

OK, thank you, this is much clearer now. Then basically for use on my VoIP home network "BA" would be preferred (there is enough juice to run GPO 232 and AE40, so voltage does not seem to be an issue).

Now, based on your experience, can one take an "A" low-Z subset, and make a "poor man's" high-Z one by adding a couple of resistors in series with the ringer? Granted, it will only affect the DC component, as I would be increasing only R, and not L of the circuit...

That depends highly on the type of ringer, and perhaps on its history of aging.  Even 'permanent' magnets can weaken over time, especially when exposed to shock.   It takes only about 10 mA of current, often even less, for a well tuned ringer to sound in full strength.  You can't cut that down at will.

You will have to experiment to see at which values you still get satisfactory performance.

The other aspect to consider is that a low-impedance ringer is used with a larger capacitor, typically 1 µF, while the high-impedance ringers cut that down to 1/2 µF. At the 20 Hz, the latter represents an impedance of 16 kΩ.   The ringing bridge represents an LRC (inductance, resistance, capacitance) resonator.  This functional dependency (overall impedance vs frequency) has a minimum at the resonance frequency, which for a straight-line ringer is ca. 20 to 25 Hz, and which coincides with the resonance conditions given by the mechanical design of the ringer, so that it operates most efficiently.

When adding an extra dissipative element, a resistor, this resonance conditions are thrown off, and efficiency may erode very quickly.

A well working high-impedance bell ringer may have a REN value between perhaps 0.8 REN and 1.5 REN, while the low-impedance ringers start at 2 and higher.  Most central office land lines provide power for 5 REN, and VoIP ATAs often only provide 3 REN.