[Milsurplus] BC-348 and the 6AC7

[Ray Fantini]

Finally finished the stack of BC-348Q receivers, got four out of the stack of six working. The last was the worst being it was heavily modified and had to replace the front panel and gear drive for the tuning. Someone had also removed the first and second RF amplifier and replaced the first with a 6AK5 and the second with a 6AC7. Ok, so had to remove a socket and parts from a donor chassis and remove all the junk for the 6AK5 and also had to do some work to the cathode bias set up for both tubes. Put everything back for using 6SK7 tubes but recall seeing that the 6AC7 was alleged as a swap for the 6SK7 with improved performance so with nothing else to do did a series of test looking at noise and sensitivity 6SK7 and 6AC7 and found that especially up on twenty meters saw about two dB of improvement in gain for around the same signal to noise. For 10 dB or so signal above noise at twenty meters sensitivity improved from about 8 to 10 micro volts down to 6 micro volts with the 6AC7 This was with using a smaller cathode resistor for the two RF amps. The sensitivity on forty and eighty meters is about the same with both the 6SK7 and the 6AC7. Replacing both RF amplifiers with 6AC7 worked but started to have issues with AVC action on strong signals but with a 6AC7 as 1st RF and a 6SK7 as 2nd RF AVC action was ok, the 6SK7 was designed to work with AVC but think the 6AC7 is just a sharp cut off tube and not good for AM receivers with a AVC system.
Yet another reason to like the J, Q and N just try doing that with the old design version with its grid cap 6K7 bottles!

Ray F/KA3EKH



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