Even with the tubes running at their rated max B+ and screen voltages, the BC-348 is not a spectacular receiver. But if the RF input tube is replaced with a 6SG7, the receiver gets terrific sensitivity down to the 0.4 to 5.2uV range (6dB above noise floor). This particular BC-348 had a transformer that produced a B+ of 360v; I was able to reduce that B+ with the usual resistors as well as a few more volts for the B- supply to the audio stage grid. All of the tubes ended with their rated 250 or 300vDC plate and 125VDC screen. The AM stations now come nicely through the crystal filter.

Pete AI2V

I never could understand the fascination with the BC-348 (except possibly for its military history). It's such an ugly receiver, and a poor performer when compared with others.

.4uV is impressive, but 5.2 ain't so hot. Could you document all the changes you made? Others may want to enhance their 348s. I don't have one; sold it years ago.

I don't think that the reason that most of us like the BC-348 is because of its sterling performance. Twenty years ago, my primary AM recvr was a mostly stock 348, including the dynamotor. with only those &^^((&) leaky caps replaced!!! I mated the 348 with a rebuilt LS-3 spkr (with a much better spkr) and a Viking I xmtr. It was a "kick in the pants" using that rcvr!! True, adjacent SSB QRM could make the 348 unuseable, but that didn't diminish the "fun" of using it. Everyone that came into the shack just plain didn't believe that it was dynamotor-powered. You couldn't hear it with normal audio levels from the spkr. Turning down the audio, you could just hear the dynamotor running in the background. Much of what I've done in this hobby has been for the plain fun and enjoyment of the hobby - not the technical satisfaction of owning the best gear on the planet.
Dave - WA6VVL

The BC-348 was the receiver I learned CW on, in the summer of 53. After spending several tens of hours across the summer setting in front of it, and realizing that I could literally talk to the world, I came to really like the old girl. I bought my own in the early 70s and worked alot of CW using ARC-5 transmitters with it.

The OPS and Maint book of Dec 15, 1942 states the receiver max limit at 7 micro-volts with typical sens at 1.2uV. Haven't taken any data for a while but I don't remember it being exceptionally deaf so I'd guess the sens to be in the 2-4 uV range. Mine is an early version with the double ended tubes. I sometimes still use it on the morning 3885 nets.

I kind of remember they were very expensive for my teenagers budget in 1950s. I wanted one so bad but couldn't afford it. I also thought they were the coolest looking and still do. I've gone thru many and still have a few and always regret not owning a new one in the box! They came pretty well packed/
I'd like to see more on the 6SG7 mod.
Thanks
Hank

I have one of the "Q" versions, and the previous owner subbed a 6AC7 for the 6SK7 in the RF stage; it's pretty hot.

Another old trick was to replace the 6SK7 with a 717 to get more gain; I never tried that one.

The 6AC7 and 717 are sharp cutoff and destroyed the AGC on AM and made the AGC-Off RF gain almost useless. Typical Hammy Hambone mod when used with strong signals.

I tried a 348 at a friends house and it was a real dog to use....I wouldnt call it fun having to fight to hear thru the QRM, almost no tuning range, and useless crystal filter on 40 and 20CW. At the time I had a SX-25 which was a also a dog but I didnt know about leaky caps yet in 1956.
I went home and got a used HQ-129X and never regretted it. A 6SG7 in the RF and first IF helped a lot and a 717A sometimes was used on 15 and 10 for the RF amp when signals were weak.

Carl

Agreed, Carl; the 6AB7 would be a better choice. I'll have to find one and try it in there someday. That rig was a real basket case when I got it, with wires hanging out and parts disconnected. I used it for a year or so, before moving up to an R-390A.

This is my first BC-348-Q and hence the first one that I played with. Replacing the front end 6SK7 with a 6SG7 was like night and day. One great feature that I found with the BC-348 was that the distortion in the recovered AM audio signal from 50Hz to 6KHz was so low that I didn't even break out the distortion analyzer; I was very impressed. Maybe I impress easily too.

True it's no R-390 but it's a whole lot lighter and easy to move; and for general purpose SW listening it's perfect and nearly impossible to damage. The slow tuning isn't really a bad feature- you get to slow-scan the whole band and you never know what will show up.

Pete AI2V

In many cases its a poor choice as its much lower input impedance loads down the coils, reducing voltage gain as frequency increases, and also decreases image rejection. QST had an article on RF stage octal tubes with curves, and pros/cons in a pre WW2 issue. The solution was to tap down on the coil for the highest band.

Ive often wondered if the SX-28 front end coils were changed when they went from a 6SK7 to 6AB7. Later they recommended a 6AB7 in the 2nd RF also but the effect there would be much less. I used 6SG7's in both of mine along with a 6SB7Y and 10M is as hot as needed. Not sure if the 6SG7Y base would make any difference; didnt have any NOS at the time.

Carl

I'll probably dump mine at Dayton. Think it can bring 75$?

A lot more if the Japanese are out in force and its very clean.

Carl

I agree. I've had an unmolested "N" model, except it's less the dynamotor, since 1974. Not a super hot radio on the high band but still fun.

Mike