Anti-Sidetone Subset for both AS and ST deskset use

[tekuhn]

I've been looking at schematics and reading through old discussions on ways to wire sidetone desksets to anti-sidetone subsets for improved performance. Here's my problem. I have a 634A subset and both a 20AL and a D1/202. I was planning to wire a 4-conductor RJ-11 jack on the 634A for the deskset connection to allow me to easily swap between the 20AL and D1/202. If I do this, I will not have the option of utilizing one of the alternative circuits, such as connecting tip to C instead of L1 on the 101A.

What is my best option for sharing this subset between these two phones in a plug and play manner without having to rewire each time?

[unbeldi]

I have posted these diagram somewhere else before on the forum, but couldn't find the post just now.

The first diagram shows a 534A subscriber set, which would be used with a sidetone (ST) telephone set, such your 20AL.

The second shows a 634A subscriber set, which is for an anti-sidetone (AST) telephone, such as a 202.  In red color is highlighted the difference to the ST circuit of the 534A. As you see it adds an extra induction coil winding across the receiver, primarily. This really is the only electrical difference when the set is in the transmission state (off-hook).

If this were the only difference, the on-hook state would pose a problem in that now a permanent circuit exists between tip and ring that would present a high load to the ringing bride, since it has much lower impedance that the ringer.  In the AST circuit the line side (point C) of the secondary induction coil is therefore routed back through an extra, fourth conductor (black) in the mounting cord and connected behind the hook switch cut off. It is thus only connected to the line when the set is actually off-hook.

A 20AL does not have the extra hook switch contact to do this.   In the 534A subset, the point C is connected directly with K, actually being the same screw C.

So this means that you would have to provide a switch between C and K of you want to use the same subset for both instruments.

When looking at the 534A subset diagram, you should notice that one can connect as many desk stands in parallel as you wish, because none modifies the circuit when on-hook.   This is not possible for the AST circuit.  However, Western Electric did have special types of D-type handset mounting and the G-type wall hangup set that had extra switches to make this also possible for AST subset.

During World War II WECO faced this problem just the same, as they had warehoused ca. 750,000 candlesticks for emergencies, and placed them into service again. But they didn't have enough sidetone subscriber sets available anymore, and therefore devised a new, but electrically very unfavorable wiring method, for this configuration with restrictions on use.  We had a discussion about that here a few years ago too and user Poplar1 introduced this circuit from a war time Bell Practice. [PS: I think you might be referring to some of that in your post.]

On the first diagram, please note, that it does not show a 20AL connected to the 534A, but also a D1 handset mounting, as in the second circuit.  The connection of a 20AL is by the same colors of conductors having the identical function, only the switches are different.  Let me look if I have another diagram that show that difference too.

[unbeldi]

Ok, I do have another circuit, but it is not for the 20AL, but for a 51AL, which is similar but adds a dial.

The first two circuits, connecting a D1 (for example) to a 534A and a 634A show that a D1 can be used as both an ST telephone, as well as an AST telephone, and the difference is only an extra conductor in the cord.  The D1 was designed with the AST circuit in mind of course, despite being used initially only in sidetone circuits as a 102 telephone.
This was not possible with the 20AL, 40AL, 50AL, or 51AL desk stands.  Those had to be converted with an added switch on the hook switch rack, and had a '1' prefixed to the model number, e.g. to a 151AL, for example.

[unbeldi]

What is my best option for sharing this subset between these two phones in a plug and play manner without having to rewire each time?

Finally, to address this question directly, You could introduce a switch at the junction box that you use for the modular connector. It should connect BK and Y for the 20AL, but this switch has to be operated in conjunction with the hook switch, not exactly user-friendly.  Perhaps a relay circuit could be design for automation, but the effort of all that is more than just getting a second subset.

Alternately, you might just try simply only connecting the red, green, and yellow conductors from your 20AL, without use of the black connection point.   This doesn't create a proper sidetone circuit, as originally conceived with the components provides, as it doesn't use the secondary coil, but instead uses the tertiary winding in place.  I have used that for testing ST instruments with a test subset dedicated to those tests, and found it sufficient for the task.

[poplar1]

When looking at the 534A subset diagram, you should notice that one can connect as many desk stands in parallel as you wish, because none modifies the circuit when on-hook.   This is not possible for the AST circuit.  However, Western Electric did have special types of D-type handset mounting and the G-type wall hangup set that had extra switches to make this also possible for AST subset.

Unless you have these extra contacts (215 (D-8 mounting), 151-R desk stand, or 213), then trying to connect two AST phones to one AST subset will result in both transmitters being connected whenever either phone is off-hook. Even if you were using a Sidetone set such as 20-AL, which does open both the transmitter and receiver to the common red lead from the subset, and one AST telephone, the red and black of the 202 -- which go to its transmitter -- would make the 202 transmitter connected when using the 20-AL (if you were using the 202 with all 4 conductors).

During World War II WECO faced this problem just the same, as they had warehoused ca. 750,000 candlesticks for emergencies, and placed them into service again. But they didn't have enough sidetone subscriber sets available anymore, and therefore devised a new, but electrically very unfavorable wiring method, for this configuration with restrictions on use.  We had a discussion about that here a few years ago too and user Poplar1 introduced this circuit from a war time Bell Practice. [PS: I think you might be referring to some of that in your post.]

The restrictions are only for the circuit using the subset with the black condenser wire connected directly to L2-Y. This setup allows only one telephone for the line and a limit on the number of ringers. This was sometimes used for pay phones. (Shown in Figure 2 of WR-63.373). This is using the AST subset as an AST circuit.

The more obscure Wartime Restrictions circuit that I shared, on the other hand, has no limitations on the number of telephones connected to one 634- or 684-type subset. However, the phones will be connected in a sidetone circuit. This is shown in BSP WR-63.373, Figure 1 (attached).
http://www.classicrotaryphones.com/forum/index.php?topic=4670.msg155990#msg155990

[poplar1]

As originally designed, one sidetone subset (295A, 534A, 584A, etc) could be installed in a central location, where the ringer could be most easily heard, then triple (3-conductor) inside wire could be extended to multiple locations on the same premises for additional desk stands (candlesticks) or hand telephone sets.

When anti-sidetone subsets were introduced, it became necessary to install one subset for each hand telephone set (202, 211, etc.) Had it  been possible to have only one centrally located AST subset, it would have been necessary anyway to replace the triple inside wire with 4-conductor wire. On the other hand, the triple wire could be repurposed (for Tip, Ring and Ground rather than to GN, R and L2-Y) to extend the line to the newly added anti-sidetone subsets at locations that formerly had only a connecting block or 392A jack.

If an additional ringer was not desired in a room that now required a subset, a 684C subset could be used, which is the same as a 684A but without ringer.

The desk stands and hand telephone sets with the extra contacts to open the red lead appeared later, in order to allow multiple phones on one AST subset. However, these must not have been popular, because very few exist today.

[tekuhn]

Wow - thanks both of you for the detailed responses. I will try the 20AL with the black not used and see how it sounds. Outside of that it seems the best solution would be to simply find a ST subset for use with the 20AL. Which subset would have most likely been installed with a 20AL back in the late teens to early 20's? I'm thinking a wooden one would be a nice looking addition to my small but growing collection. 

[poplar1]

Wow - thanks both of you for the detailed responses. I will try the 20AL with the black not used and see how it sounds. Outside of that it seems the best solution would be to simply find a ST subset for use with the 20AL. Which subset would have most likely been installed with a 20AL back in the late teens to early 20's? I'm thinking a wooden one would be a nice looking addition to my small but growing collection. 

295A (wood) or 334A (metal) or 534A (metal)