[Milsurplus] BC-348's

[Mike Hanz]

telegrapher at att.net wrote:

>My newly found BC-348J came with a DM-24-B dyno.  I
>was surprised that this one had 6AC7 tubes in place of the 6SK7 RF amps
>,1st and 2nd IF.  No wiring nor component changes had been done as the
>unit is like brand new.  I put 6SK7's back in after changing the
>filaments around but didn't notice any difference including comparing it
>with my 348Q which is a 28V Dyno model.  Anyone else have that change? 
>Is there any advantage to leaving the 6AC7 tubes in?  
>

Nope, and there are a lot of disadvantages. They aren't remote cutoff 
tubes, so you forfeit much, if not all, of your AVC envelope.  The 6AC7 
is but one kind of tube that hopeful hams tried after the war in already 
well designed sets without understanding that you have to treat the 
receiver, antenna, and external noise environment as a *system*.  They 
heard the 6AC7 was a low noise, high gain tube, so what could be a 
better plug-n-play substitute?  Not entirely their fault...it's an 
American trait to look for a magic bullet, but the original designers of 
these sets were superb engineers.  They understood that in the frequency 
range of the BC-224/348, atmospheric noise would significantly 
overshadow tube noise.  They understood that AVC was a good thing for 
most conditions, and when it wasn't, provided an effective MVC.  They 
understood the need to balance stage gains to eliminate oscillation.  
They understood a lot of things that we keep on rediscovering today in 
the old texts, and that's good - we need more articles by folks like 
K9EUI that have careful comparisons, with well documented data.  Nothing 
wrong with experimentation if you have some sense of what you are 
looking for, and understand what the tradeoffs might be.  All of this 
takes some test equipment or you'll be in the realm of our audio 
brethren, with talk about transparent highs and other nuances that can't 
be felt by mere mortals.

The 6AC7 is an unusual tube - it was used by the thousands in 30MHz IFs 
for radar and countermeasures applications.  Considering the limitations 
of the metal octal format, it is remarkable in its noise figure and 
gain.  It works pretty well as a low noise mixer but requires additional 
active circuit elements for isolation in that application, so you really 
have to wanna use it to go to the trouble.  It's a pity it's not remote 
cutoff - it would have been more useful in a wide variety of homebrew 
circuits.

>fun project and the radio runs well from a 35A MFJ supply.  Even starts
>the Dyno without any grunts.  Something one of my other 25 amp supplies
>wouldn't do.  
>

Switching and series transistor supplies are notorious for this 
problem.  My 10 amp bench supply does just fine, but it's a brute force 
type with an internal Sola transformer for regulation.  It's tough to 
start dynos with a current limiting power supply...