Extremely Rare WWII 1944 General-Purpose Intercom System from England, made from Multi-Colored Swirled Catalin, in Mint Original, Complete, and Working Condition

In a Nutshell
It can be used in an Office, at Home, and in a Nursery - or just be looked at in Admiration

Introduction:
Imagine the situation in the last year of WWII in England, as in most other European countries - wartime economy, destruction, starvation, chaos - D-Day on June 6. What a surprise that under those conditions something so beautiful and unique could have been made - and with what purpose? I found a couple of references, referring to a "Catalin Radio Speaker" with unknown maker and model number (ref.1) for the Control Unit, and an "early 30's catalin speaker" for one of the little speakers (ref.2). On page 51 of Robert Hawes' book "Radio Art" a 1930's yellow speaker with unknown maker is shown (pict.65), that is definitely larger than mine. Since my set is complete (I mean complete, with all four original back planes, three knobs, resistive power cord (see techies), two of the three original tubes and every single screw needed) I was able to pin down its schematics (pict.60) and its purpose as a general purpose Home, Nursery or Office Intercom. According to Carl Glover, the editor and an author of the Bulletin of the British Vintage Wireless Society (BVWS, see also ref.4), the cabinet of the control unit has been used also to house an 8" speaker, which explains the presence of 4 screws not used for the intercom chassis (pict.s 29,41). The cabinets of the little speakers apparently have been used over a long time span by other companies to house various electronic devices like crystal radios and a "Mini-Twin" 1-tube 2-stage battery receiver, to be assembled as a kit (pict.66). As Carl Glover says, the catalin cabinets have been produced by Catalin Ltd. at Waltham Abbey, Hertfordshire, Essex (see pict.67 and for more information ref.5). Recently another control unit has surfaced, incorrectly declared as "axillary and amplified external speaker for a table top radio", and having a price tag of $1095 (ref.6).

Additional information:
1. http://www.goldenhue.net/Radios/Catalin%20Speaker/Catspeakl.htm
2. http://www.prins.net/radios/index.html
3. "Radio Art" by Robert Hawes, The Green Wood Publishing Co., Ltd. (1991), p.51
4. http://www.bvws.org.uk/download/bvws-sample-bulletin.pdf, p.22
5. http://www.bakelite-labteam.com/download/free/information_that_you_need_about_bakelite.pdf
6. http://oldradiocrazy.com/?p=3636
7. http://www.radio-antiks.com/IndexRadio-Antiks_Radios_for_Sale_10.htm

About my intercom:
The intercom was bought from England, and after restoration and documentation is for sale (item 53 in ref.7). The set was made in 1944/45 in England, as can be seen from two dated capacitors used in the chassis (Mar 43 and Jun 44, pict.57) and the parts manufacturers (capacitor by A.H. Hunt Ltd. London, speakers by Celestion Ltd., London), including 2 BRIMAR tubes (3^rd tube was missing and is now an Arturus 25L6). Here are some features:

•  Control unit listens to one out of up to 4 or all speakers, used as microphones, or decides to be deaf
•  Control unit speaks to one out of up to 4 or all speakers, or is quiet
•  Deaf and quiet mode save only 4 Watt, but heaters stay hot (no warm-up delay)
•  Control unit controls mode, where "Speak" is a spring-action switch (see techies)
•  All 3" Celestion speakers double as microphones
•  Can be used as an office or conference intercom
•  Can be used as a baby monitor for up to 4 babies
•  Can be used as a home intercom for a large family, saving from climbing stairs
•  Can be used as a music (acoustic, CD, mp3, iPod, ...) distribution center serving up to 5 speakers (see techies)
•  Can be used as multiple guitar amplifier playing through up to 5 famous Celestion speakers
•  Large cabinet, resistive power cord and 120V operation protect catalin even when continuously operated

For the techies only:
A resistive line cord (curtain burner) or a ballast tube was used in many transformerless radios from the 1930-40's, in order to match the tube's heater strings (in series) to the (higher) line voltage. Compared to a ballast tube a resistive power line cord has the advantage that the extra heat is generated outside the radio's cabinet. It is a highly complex and dynamic object, and it is a part, that in most cases has deteriorated, is defect or cut and therefore has to be replaced. The cord (pict.58) of this intercom is state-of-the-art (pict.59) and in perfect condition, flexible, not brittle, with high-temperature rubber-insulated wires, resistive wire made from a rare negative temperature coefficient alloy, expensively spiraling inside a separate asbestos strand, and with perfect outer cotton braid cover. The set came from Great Britain and likely was operated at 240V~. The plug was cut, and the original resistive line cord had an open in the resistive wire. Since conversion to 120V~ was intended, the cord was arbitrarily cut by about half and luckily enough the 36" remaining part connected to the control unit was ok with 490Ω (cold-) resistance.
I measured (using our stove) the wire to have a (negative !) temperature coefficient of -0.00086/°C, the reason behind being to protect the radio (especially the pilot lamp) from high surge currents when still cold. As the heater string warms up, its resistance increases and the cord's resistance drops, partly removing it from the unit and allowing normal circuit operation. Operation of the set at 240V~ must have heated the resistive wire itself to 345°C and the cord to more than hand-warm. For 120V~ operation a dropping resistor of 270Ω is needed. The (cut) cord if operated at a much more comfortable wire temperature of about 100°C, would provide for 450Ω, so a parallel resistor of 680Ω is needed, which can be lower, when rectifying its current. An empirical approach determined the value of this resistor to be 170Ω (see pict.60). In order to protect the pilot lamp from surges it has been paralleled with 6 cheap 1N4005 diodes in both directions. All modifications can be undone, when returning to 240V~ operation, but a dropper resistor of about 480Ω has to be added to compensate the lost part of the cord.
Finally a word about using the set as a music distribution center. The Speak switch is a spring-action switch, so in order to continuously send from the control unit, a permanent Speak action is required. The best way is to install one more switch (S4' in pict.60), switching the unit from the present intercom to permanent Speak. With minimal modifications you can also connect other devices (CD, mp3, iPod, etc.) or make it a guitar amplifier, by using the high impedance secondary winding of T1 (see pict.60), in which case you may even want to rewire the unused internal speaker.

Here are the specifications:

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