Author Topic: Subset hybrid?  (Read 1287 times)

Offline dsk

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Re: Subset hybrid?
« Reply #15 on: August 09, 2016, 04:31:51 PM »
The subset mentioned in the first post in this thread; seems to be a well made piece of woodwork. 
Using a 425 network as a subset, and only 4 wires to the desk stand will not be possible if you modify the circuit, e.g. by using only one half of the splitted primary winding. 
This has been done, and works, but how well? 

If you modify the desk stand, and use 5 cords, it will be good as a 500 telephone.
Look at this thread: http://tinyurl.com/he7v95w

dsk

I have even got a regular New York number :-) 646 570 1796

unbeldi

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Re: Subset hybrid?
« Reply #16 on: August 09, 2016, 07:16:35 PM »
The subset mentioned in the first post in this thread; seems to be a well made piece of woodwork. 
Using a 425 network as a subset, and only 4 wires to the desk stand will not be possible if you modify the circuit, e.g. by using only one half of the splitted primary winding. 
This has been done, and works, but how well? 

If you modify the desk stand, and use 5 cords, it will be good as a 500 telephone.
Look at this thread: http://tinyurl.com/he7v95w

dsk

Half of the gain of the 500 set came from mechanical aspects of the handset, so without that nothing can be as 'good'.  'Good' is relative anyhow.
Furthermore, the next major improvement in the 500 set was the electrical efficiency and flat response curve of the T1 and U1 transducers, being far superior to all older elements.
The electrical balance of the 500 circuit is also affected by not having the exact impedance characteristic of the U1 receiver when using other transducers.
So, with mismatched components, there isn't much benefit left from the 500 network over the predecessor.
What is left is the line balancing from the two-stage varistor damping. The second stage is probably thrown off slightly with the receiver mismatch.

Indeed, one half of the primary side of the induction coil is unused in a four-conductor mounting arrangement with a 425 network.
Turns out this is actually beneficial when connecting old, less efficient transducers, as it slightly increases audio volume. Since human hearing has a logarithmic response, the electrical power needs to be doubled to achieve a noticeable change in volume.  This is in fact achieved when the transformation ratio is increased, i.e. the primary winding reduced to one half.

So, it works very well for old instruments, and it has the benefit that one can maintain originality by using the original mounting cords !


Bell Labs and WECo invested much effort into matching operating conditions with instruments variations, because it was difficult to find the right balance of electrical conditions on a line to be good for signaling (dialing) as well as for transmission (audio). The 500 sets did not make it any easier, actually.  It greatly extended the range for signaling, eliminated local batteries on long lines, but the increased performance of the handset had to be compensated on short loops with auto-balancing to avoid excessive sidetone.

It appears that they in general preferred to pair old elements with old induction coil hybrids, while using T1 and U1 with 425-type network.  However, for intermediate conditions, U1 and/or T1 elements could be used with old subsets (> F4 handset).  We're not finding 202-type sets with four-conductor cords to 425 networks.
« Last Edit: August 09, 2016, 07:30:08 PM by unbeldi »

Offline dsk

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Re: Subset hybrid?
« Reply #17 on: August 10, 2016, 05:30:23 PM »
I guess you are right all the way.  I did modify an AE mini network (more job than expected) and got the primary at one side.
Ill guess that would be a way to do it.  Ended up with this schematics:


dsk

I have even got a regular New York number :-) 646 570 1796