Just want to throw this out there and see what the
Experts think! Which radio is generally more sensitive
And selective, one that has an RF amp and only one IF
Stage or one with no RF amp but has two IF stages?
Ron
My answer would be, more sensitive with an RF stage, more selective with a 2nd IF, but you might not be able to tell the difference all other things being equal.
Now having both, that is good.
Better image rejection with the RF stage - if it is tuned.
Is it a tuned RF amplifier stage or just a straight RF amp? I think that when you say two IF stages you mean two IF amplifier stages? Again there are two kinds with that, some use an extra IF amp tube but only two IF transformers and others use three IF transformers having an interstage IF transformer coupling the two IF amp tubes. Philco liked to use three IF cans with two IF amps in their sets, they also liked used twin triodes for mixer-oscillators with it especially in the 1940s. I have quite a few sets that used two IF amplifier tubes but with only two IF transformers, especially battery set built by Electrohome, I haven't really examined the circuit in each but I suspect that the IF amp tubes are capacitive coupled together.
I have two Philco hippo model sets with two different front ends, one uses the twin triode XXD/14AF7, but has two IF amps and three IF transformers, the other uses a tuned RF amplifier stage. I plan to get both sets restored and see which design works better, it may not be as easy to judges as I think.
Regards
Arran
Thanks guys for your input. I have a 41-250 that will be my winter project ( to nice now and other outdoor projects to be in at the bench ) and it has two I F amp tubes and I am anxious to complete it and see how it performs compared to common AA5 sets. Also missed out on a 40-190 today, it just went for too much, oh well I will keep looking.
Ron
Later superhets utilized tubes that eliminated the requirement for an RF amp tube/xfrmr. Older sets had an RF amp both to increase sensitivity and to reject image. Of course there were sets that still utilized an RF amp, usually in the form of a 58, 78, 6K7 or 6L7 tube.
That being said, sets that employ two (or more, like Scott) stages of IF amplification tend to be better receivers.
I just completed restoration of a Philco 16B, which has 2 stages of tuned IF amplification but NO RF amplifier. I am not impressed with its sensitivity - nothing to write home about. This is subjective, of course, since I have NOT tested the radio side by side with other radios using the same antenna, etc. But I feel my Zenith 8S661 and 9S367 both outperform it. Both Zeniths use external antennas and have a tuned RF amplifier and only one stage of IF amplification.
This may not be a fair comparison though, since the Zeniths are later technology.
Personally I'm a IF amp guy as it makes it easier to separate out the signal that I do hear. Too much RF gain can be a problem too. There needs to be some balance between the amplification of all the stages.
Terry
Generally, after they knew what thery wer doing, and pricing was not such an issue, RF amps blew the socks off sets without them, more or less without regard to IF stages as I know of. Always easy enough to control RF gain if the set allowed.
Tons of opinions here!
If a 16B is being outperformed by a brand Z set with fewer tubes then there is something still wrong with the 16B that has been overlooked, period.
Regards
Arran
A lot depends on what type of mixer/osc the radio uses for the converter. A pentagrid tube like a 6A7, 6A8 or 6L7 is very noisy internally and a radio using them benefits greatly from an RF stage since it increases the signal level into the mixer, overcoming the noise.
The Philco dual triode converters of the 40's are much quieter and there is little to be gained sensititivity wise with the addition of an RF stage. In this case the second IF is really useful as it adds gain and selectivity while preserving the low noise and high overload characteristics of the triode front end.
Oo the other hand, the tuned RF stage increases image rejection and is very helpful in this regard, especially in the short wave bands where images are always a problem in single conversion receivers.
Well said. Alas soon to be moot.
A lot of the higher end sets, and some mid end set, used separate mixer and oscillator tubes to get around the noise created by a pentagrid converter. My CGE E-81 (same chassis as a an RCA T-7/T-8 did this, and they did not use a tuned RF stage. Though I should point out that Philco sets of the 40s often used another type called an octode converter, a type 7A8, what difference the extra grid made I don't know, but in a single band five tube set it doesn't really matter. RCA liked using another type called a 6J8, it was a triode-heptode converter, the triode was the oscillator section and the heptode was the mixer, this was what my Philco 40-180 used in the front end, in effect you are using two tubes but in one envelope.
Regards
Arran
The 16B is a better receiver than the Z 8-S and 9-S sets. In fact, it is superior to most Z ten and twelve tube sets.
If otherwise I would suspect component or alignment issues.
That was a great question. I really appreciate the information I got from reading this thread. You all are so knowledgeable.
Tom