[ARC5] Added 6F6 audio stage...Aircraft Interphones

[Richard Knoppow]

----- Original Message ----- 
From: "Mike Morrow" <kk5f at earthlink.net>
To: "ARC-5 List" <arc5 at mailman.qth.net>
Sent: Friday, October 19, 2012 6:49 PM
Subject: Re: [ARC5] Added 6F6 audio stage...Aircraft 
Interphones


> Dave wrote:
>
>> I've never understood this issue of adding audio stages 
>> to Command Set
>> receivers.  I've been playing these sets stock using 
>> matched speakers
>> for decades.  They have all the audio I could want, and 
>> I'm half-deaf...
>
> The single-tube AF output stages of the MF/HF command set, 
> VHF command set,
> liaison set, radio compass, and interphone microphone amp 
> could be fed to
> approximately ten interphone stations, each capable of 
> selecting any of
> these AF sources and hearing it with headphones in a very 
> noisy environment.
> The interphone amp in all common WWII-era aircraft 
> interphones (until the
> late-war AM-26/AIC, part of AN/AIC-2) did not provide 
> further amplification
> of any receiver's AF output, even one that is connected to 
> ten headphones.
> The weak little VT-99 (6F8) of the common RC-36/BC-347-* 
> single-tube
> interphone amp was always connected simultaneously to all 
> headsets.
>
> So Dave's statement seems necessarily 100 percent 
> accurate, even when a
> speaker in a quiet area is substituted for those ten 
> headphones working
> against aircraft ambient noise.
>
> 73,
> Mike / KK5F

     I am not very familiar with the ARC-5 equipment.  What 
sort of output impedance does the amp have?  Perhaps the low 
level into a speaker is due to a mis-match. Most 
communications receivers have only a watt or two of power 
and a decent speaker needs only about a watt to be very 
loud.
     FWIW, the 6F6 output stage in a Super-Pro is run Class 
AB2 with a transformer coupled driver stage. It will put out 
about 14 watts. There are a few other receivers with 
push-pull output but most run Class-A with about half the 
power.
     Most of the "driver" stages for speaker amplifiers will 
drive headphones quite well with no additional help.  Some 
receivers, for instance the Hallicrafters S-20R or S-40 
series have the headphone jack connected this way. I think 
the National NC-100 and maybe NC-240 have a similar 
connection. The headphones are supposed to be high impedance 
types. A normal speaker-impedance output stage will drive 
dozens if not a hundred high-impedance headphones.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com