[Milsurplus] BC-348-C Modifications

[Glen Zook]

Just put in a matching transformer between the audio output of the BC-348 and the speaker.  I get enough audio to drive someone from the room with this.  That is, the audio in my BC-348 has not been modified at all.

A lowly filament transformer works fine.  Filament transformers with like 450 mA are available from Radio Shack for well under $10 and they work very well.  Higher current transformers are also available for around $10 from Radio Shack.  Just get a 12.6 VAC transformer and hook the primary to the audio output of the BC-348 and the 12.6 VAC winding to the speaker.  This works very well with an 8-ohm speaker and well with a 3.2-ohm speaker.  If the secondary is center tapped for 6.3 VAC, you can use one side of the secondary and the center tap to get a closer match for a 3.2-ohm speaker.

One can also use a 6.3 VAC filament transformer and that works very well with a 3.2-ohm speaker and well with an 8-ohm speaker.

Filament transformers are not "hi fi".  However, they definitely work well with frequencies up to at least 8 kHz and many make it to over 10 kHz.
 
Glen, K9STH


Website:  http://k9sth.com


________________________________
 From: Al Klase <ark at ar88.net>
To: milsurplus at mailman.qth.net 
Sent: Monday, October 22, 2012 1:50 PM
Subject: Re: [Milsurplus] BC-348-C Modifications
 
Hi Mike,

A couple of comments on the mods in that set:

The 6V6 is udoubbtedly an attempt to get more audio to properly drive a 
speaker.  The radio was intended to drive a whole B-17 full of headsets, 
but not a speaker.  The problem is lack of gain more than lack of 
driving power.  The real solution is to add another triode to drive the 
41, but lets not go there at this late date.  As the socket has been 
changed, I'd replace the 6V6 with a 6K6, the octal equivalent of the 
original 41, but it probably won't make a big difference.

The 6AC7 was a misguided attempt to achieve better noise performance.  
The problem is, it's a sharp-cutoff pentode, and will mess up the AVC 
action.  I'd surely replace it with a 6SK7, the single-ended (no grid 
cap) version of the 6K7.  Also, check that they didn't mess with the 
screen grid resistor value.