[Milsurplus] BC-348 and the 6AC7

[kgordon2006]

Well done, Ray. Thank you for that valuable information.The 6AC7 is a sharp-cutoff tube and is not suitable for AGC control.However, I have always insisted that THE important parameter for tubes in all RF amp, oscillator, and mixer functions is plate transconductance, the higher, the better, as far as noise is concerned.The plate transconductance parameter of the 6AC7 is quite high, over 9K micromohs as I remember it.When used as ECOs, screen transconductance becomes important, but I have never seen that parameter listed for any tube.I know that the 6AG7, which is very commonly chosen as an oscillator, has a high plate transconductance, but in an ECO where the screen is the "plate" of the oscillator, its transconductance is not exceptional Ken W7EKBSent via the Samsung Galaxy S21 5G, an AT&T 5G smartphone
-------- Original message --------From: Ray Fantini via Milsurplus <milsurplus at mailman.qth.net> Date: 8/23/25  13:35  (GMT-08:00) To: List Milsurplus <milsurplus at mailman.qth.net> Subject: [Milsurplus] BC-348 and the 6AC7 

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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