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634a vs 634ba

Started by extremecinema, January 31, 2015, 08:13:09 PM

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extremecinema

What is the difference between these subsets?

unbeldi

#1
The 634BA employed a high-impedance ringer while the 634A had only a low-impedance version.
Along with ringer type, the condenser was changed from a 1 µF ringing condenser (194A) in the 634A to a 194B (0.5 µF) in the 634BA. Both condensers also contain a 2 µF audio condenser.

subset      condenser                      ringer
=======================================
634A        194A  (1µF + 2µF)         1000 Ω or 1400Ω:  68A(?), or earlier 8-type (most were converted from 534A subsets)
634BA      194B  (0.5µF + 2 µF)     4300 Ω: 68J, or later B1A (4600 Ω)

The details about the actual ringer found depend a great deal on the history of the 634A, because they were typically upgraded from a sidetone subsets.

Many 634A subsets also have different capacitors, based on their history, often they have two separate caps instead of one, for example a 21D (2 µF) and a 149A (1µF)

The 684A subset can also a have a variety of different induction coils, a 120, 146, or 101, but this can be said about the 634BA as well, I think, but since it is a later model, most likely they all have 101A induction coils.

But the bottom line is low vs high ringing impedance.  In general, the border line between low and high impedance ringers would be a DC resistivity of around 2500 Ω.

extremecinema

Is one more desirable than the other in certain configurations?

unbeldi

#3
The trend of the time was toward higher-impedance ringers, which became the standard in the 500-type telephone set, which had a total ringing impedance of ~8000 Ω at ringing frequency.

Lower impedance ringers draw more current from the line when ringing.  Telephone companies are required to provide support on lines for at least a ringer load of REN 5, meaning that up to 5 500-type phones can be connected to a line at the same time.

A low impedance ringer such as in a 534A subset and some 634A subset present a REN value of 2.5, so only two of these can be connected to a line.

Higher impedance ringers became necessary as lines got longer and more stations were installed on party lines. A high impedance ringer also causes a lower level of inductive noise on lines, as it unbalances the line less in divided ringing designs.

So, yes, from an electrical point of view a BA is preferable.