[Boatanchors] WTB: Marconi low impedience headphones for magnetic detector?
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.
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
> 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
>I've attached a copy of Marconi's Wireless 1910 catalog (available
>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.
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