[Milsurplus] BC-348-R

[mac]

Yes, a stock 348 will drive a speaker but in my experience hardly loud  
enough to drive a listener out of the room unless tuned into something  
like a local 50Kw BC station.  But then maybe Dave's hearing is more  
acute than mine.  :^)  The "problem" is that the set doesn't have the  
usual audio amplifier stage (typically a triode) between the second  
detector and the output AF power amp.  I have successfully used a  
second detector modification which substitutes a two-diode voltage  
doubler arrangement for the original (conventional) single diode  
circuit.  Essentially doubles the audio level out of the second  
detector and provides a useable improvement to the overall audio  
performance of the set.  See ER #103 for details.  Circuit is out of  
the RSGB handbook and works FB in the command set receivers too.

Dennis D. W7QHO
Glendale, CA



On Mar 21, 2010, at 10:51 AM, Mike Morrow wrote:

> Dave wrote:
>
>> I use my BC-348s with 600-ohm matched speakers and
>> the volume level is plenty enough to run me out of the room.
>> If a properly matched speaker is low-volume, there is
>> some other problem.
>
> Indeed, there must have been a fair amount of AF power
> available from the AF output stages of the BC-348 liason
> receiver, the SCR-274-N or SCR-522-A command receivers,
> and the SCR-269 ADF receiver to support all the headsets
> which could be aligned to them simultaneously through the
> aircraft interphone system, such as the common RC-36.
>
> The WWII-era aircraft interphone systems did NOT amplify
> the AF output of the receivers that were connected to the
> interphone.  In the case of aircraft like the B-17 or B-24,
> there were typically at least ten interphone stations as
> part of the RC-36.  At worst case all ten of the stations
> could select the same receiver, resulting in ten HS-33-type
> headset coils connected in parallel to the 600 ohm output
> of a receiver.  But usually, the ten stations would be
> connected in parallel across the output of the single-tube
> (VT-99/6F8) interphone mic amp of the BC-347 interphone
> amplifier.  It's amazing that the AF output of a typical
> receiver or the interphone amp itself was sufficient to
> adequately supply enough AF power to supply all those
> multiple loads in noisy aircraft conditions, especially
> as multiple connected stations created greater and
> greater AF impedance mismatch to the AF output stages.
>
> WWII-era USN interphones also did not amplify the AF of the
> receivers connected.
>
> Apparently, it worked well enough.  So, it would be
> surprising if a properly working receiver could not supply
> enough AF output to supply a reasonably matched speaker.
>
> Mike / KK5F
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