[Boatanchors] WTB: Marconi low impedience headphones for magnetic detector?

[Bob Cutter]

Not antique but I found a pair through a crystal radio group. 
72, Bob KI0G


Sent from Yahoo Mail for iPhone


On Tuesday, June 1, 2021, 1:54 PM, Larry Dighera <LDighera at att.net> wrote:


Hello,

I'm hoping you may be able to assist me in completing my Marconi
magnetic detector display with a set of low-impedance headphones.  As
the Marconi's magnetic detector was part of the standard shipboard
wireless room for many years, I would presume that there may be some
still to be had.  

Some early Marconi wavemeters uses a crystal detector which is a
voltage operated device, and thus high-impedance headphones are
appropriate for them.

The Marconi magnetic Detector is a current operated detector so,
low-impedance phones are best for it.  At least that is what I gleaned
from page 54 in Wireless Telegraphy (1914), by, A. B. Rolfe-Martin,
where he states:

    "In expert hands, a crystal detector forms the most sensitive
    device available.  All these contact detectors are 
    potential-operated and have a high resistance -- two facts which
    have to be kept clearly in mind when designing a receiving
    circuit.  Thus a crystal detector must never be put in series in
    any circuit which is required to have oscillatory properties,
    since the introduction of such a high resistance renders a
    circuit aperiodic.

    The Fleming valve also is potential-operated and has a high
    resistance.

    The magnetic and electrolytic detectors are current-operated, and
    having a low resistance can be directly inserted into an
    oscillatory circuit. 

    In the magnetic detector, the chief factor is the magnitude of the
    first current impulse, and as far as the detector itself is
    concerned, damping is comparatively unimportant.  With such a
    detector, therefore, steps should be taken to obtain the
    advantages of syntonic working before the current reaches the
    detector."

I've attached a copy of Marconi's Wireless 1910 catalog (available
on-line here:
http://www.tubecollectors.org/archives/1910%20Marconi%20Catalog.pdf )
in which on page four it mentions:

  "Telephones Required. --Low  resistance (Ref. No. 128R)."

And, on page 42 the headphones are pictured with this caption:

  "Reference No. 128R. Code Word Atemyrtle.

  Telephone No. 128R is the standard 150 ohm telephone used by the
  Marconi Company, and is suitable for all Receivers requiring a low
  resistance telephone."

Alternatively, I'd be happy with the 8,000 ohm resistance 129 R phones
and the 130R or 131R telephone transformer. 

Would your archive happen to contain an "extra" set of Marconi Low
resistance type 128R headphones that you may be willing to part with?
If not, perhaps you know someone likely to have a pair to whom you
might refer me.

Best regards,
Larry Dighera
WB6BBB
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