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

Larry Dighera LDighera at att.net
Fri Jun 4 12:22:18 EDT 2021

Thank you for your response, Bob.  

I understand that a typical telephone earpiece is low resistance.

But, I'm looking for the real-thing to complement the magnetic detector.
Apparently they are rare today.

Best regards,

On Tue, 1 Jun 2021 20:35:38 +0000 (UTC), Bob Cutter <ki0g at yahoo.com> wrote:

>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:
>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
>Boatanchors mailing list
>Boatanchors at puck.nether.net

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