Discussion: does the number of IF tubes REALLY matter?

[miniman82]

I've often thought that the reason for the number of IF amplifier tubes in a given set was related to it's build quality, I.E. lower cost sets generally have less tubes in the strip. It is often pointed out that famous chassis such as the CTC-2/2B had 5 IF tubes, the reasoning being that more tubes equals better reception especially in weak areas. I've always accepted this to be true, since the logic makes sense- until last week.


I was getting some help from my brother ordering peaking coils for my 2B chassis (I hate Digikey's website), when I pointed out that it had 5 IF's. We agreed that it likely does have great reception, but we looked into it a little more. Turns out there's more to the story than first meets the eye, and I'll explain. Later chassis like the CTC-10 have less IF tubes, but there is a caveat: transconductance.

We started looking at tube data sheets for the tubes used in different chassis, when we realized that the early tubes had much less transconductance than ones used in later chassis. For example, the 6DC6 used in the CTC-2 has 5500 micro ohms, and there were 5 IF's. The CTC-10 has only 3 IF's, but the tubes employed have nearly triple the transconductance (like 6GM6 with 13,000 micro ohms).

I'm thinking that at the time chassis like CTC-2 were built, they were simply working with what parts they had available. Later on down the line, better tubes such as the 6GM6 became available. Because of their high transconductance, less of them were required because the gain was so much better. So less IF tubes should not automatically mean a 'worse' reception or poorer picture quality, I think it simply means that better parts to do the same job eventually became available and so they were used.

Thought?