[Milsurplus] BC-348's and 6AC7s...

[Kenneth G. Gordon]

> Someone else suggested the 6SG7.
> 
> Larry

Here's the deal: generally, the higher the transconductance which a 
tube is rated for, the more sensitive the tube is, and the less noise it 
produces.

So, the transconductances of the three tubes in question are:

6SK7 about 2200
6SG7 about 4500
6AC7 about 9000

so the 6SG7 could be a good sub. Heck, in some cases, the 6AC7 
would be a good sub, for that matter.

As I said, the only thing you really need to worry about is that if the 
transconductance is "high", there is MUCH more likelihood that the tube 
and circuit will oscillate undesireably, whereas, all things being equal, 
the tube with the lower transconductance is less likely to oscillate.

Then there is another problem which can come up: the 6SK7 is a 
remote-cutoff tube, the 6SG7 is a semi-remote-cutoff tube, and the 
6AC7 is a sharp cut-off tube.

Remote cut-off tubes are normally used in circuits with AVC since they 
have a much more gentle grid characteristic than a sharp cut-off tube. 
Sharp-cut-off tubes don't generally work very well with AVC, gain 
suddenly dropping at some point in the AVC curve rather than more 
evenly, slowly dropping as the signal level rises. Semi-remote cut-off 
tubes are somewhere in between and seem to do all right with AVC.

However, if you are primarily going to use whatever receiver you have 
the 6AC7 in for CW, then you probaboy won't be using AVC anyway, so 
it probably wouldn't matter.

I guess it depends, in the final analysis what you intend to use the 
receiver for. If you are going to use it for hamming on ham bands, and 
are going to use it mainly for CW, then the 6AC7 might be your RF amp 
tube of choice. I would at least try the 6SG7.

If you were sure the receiver didn't oscillate with the 6AC7 in it, and 
didn't plan to use it for SW BC listening, or didn't care about the AVC 
characteristic, then that is what I would use.

If you are going to use it as the BC-348 was originally intended, then I 
wouldn't do it.

In either case, I would TRY the 6SG7.

Now, the way you test to see if these make any real difference is to first 
test the completely and correctly aligned receiver on, let's say, 40 
meters when the QRN is quiet and you can find a weak DX station.

Peak everything, then listen to the weak station for a bit. Then 
substitute one of the others, re-peak everything and listen some more. 
Is there any noticeable change for the better? Etc.

Then do the same with the last tube.

If you decide to use the 6AC7, then after listening on 40, try all the 
other bands, including the highest ones, listening carefully for sudden, 
or more than usual, background noise. This last would indicate that the 
thing is oscillating.

Then, if it passes all these tests, do the same with the 2nd RF amp.

Now the last thing to keep in mind is that the interelectrode 
capacitances of the three tubes in question are different. Although there 
is very little difference between the 6SK7 and the 6SG7 (the cathode-
to-grid capacitance is 2.5 pfd higher in the 6SG7), all of those are 
different in the 6AC7, so the RF amp stages might have to be re-
aligned for best sensitivity if you use the 6AC7. THEN when you re-
align, that might cause oscillation.

Anyway, unless you just like to experiment, and I certainly do, it might 
not be worth your effort to do this.

Ken W7EKB