[etech]

At Brian's suggestion I decided to do a comparison between the Philco 90 and the 650B. Since the 90 has only the broadcast band, I used that for the comparison. Let me say that I suppose that this is more so of qualitative comparison than a quantitative type, since I don't really have the necessary equipment or the inclination to do a quantitative analysis. I spent a couple of hours this morning with the two chassis side by side on the bench. I first started using my long wire antenna, switching it back and forth between receivers. What I first noticed was that it appeared that the 90 had a definite advantage in selectivity. That is, it has a tighter bandpass and it tunes sharper. Now, I suppose that how the I.F.'s are aligned may have a bearing on this.Both radios are aligned per Philco's alignment procedures. It is also likely that that the four gang tuner with the double tuned antenna coil may have some bearing on this too.

I also noticed that the 90 appeared to have a slight edge in sensitivity across the band. It was hard to tell for sure with that large antenna. So, I decide to reconfigure the test. I installed a four foot long wire antenna on each radio draped in parallel fashion from the ceiling. You will see them in the picture below. I then tuned each receiver to the weakest stations I could find, comparing one against the other. In each case except for band center, the 90 had better quieting than the 650B. At one point, I believe it was WWJ 950 AM Detroit, the signals were about equal. I suspect this has one has more to do with the overall tracking of each receiver than anything else.

Here is a pic of the test setup.
[Image: http://i135.photobucket.com/albums/q136/...vs650B.jpg]

We all know that the 650B is a darned good receiver with 'hot' sensitivity. But I think little is known about the Philco 90's. I have two of the 90 chassis with the four gang tuner and the single 47 output tube and I noticed almost immediately after restoring them what impressive performance they had. They seemed to be extremely sensitive and quite selective, but I really had no basis for comparison other than the sense about these kind of things that you get after using lots of receivers over many years. So for me, this test was worthwhile in terms of defining (at least somewhat) what that difference really is.

It would be interesting to get some other opinions about this.

Regards,

Ed

[Chuck Schwark]

One suggestion for your test.

If BOTH sets are to be on at the same time for the test, a large grounded steel or aluminum plate needs to be between the two chassis' to shield interference (local oscillators especially) between the sets so your results will not be skewed with images, birdies, beats, etc.

Your results will be interesting.

Chuck

[etech]

Chuck,

Thanks for the suggestion. I did think about the possibility of cross-talk between the two radios in my original test set-up. So, I plugged the 650B into the isolated variac and operated the 90 directly from the AC line. That turned out to be a bad choice. I realize now that with such a short antenna, each receiver was using the AC line as the other half of the antenna.

So in my next test, I plugged both receivers into the AC line and I would tune in the same weak station on both. Then I unplugged one while the other was playing to eliminate the possibility of cross-talk or interference from the other, and vice-versa. So, operating both receivers one at a time both plugged directly into the AC line, the 650B was improved. With this set-up, I would say the 650B was at best on par with the 90, maybe just slightly less sensitive, but so close that one could easily say that could have been a difference in alignment or tracking across the band. Overall, you would have to say the sensitivity was about equal.

However, the 90 still had the 650B beat on selectivity. It is considerably sharper. You could easily tell when tuning in a weak station nearby a strong station. There was considerably less splatter on the 90 from the strong station.

Regards,

Ed

[Chuck Schwark]

The better selectivity on the Model 90 is due to the lower I.F. freq of 175 kHz., as opposed to the 460 kHz. on the 650.

Chuck

[Ron Ramirez]

Also consider the fact that the Model 90 has Philco's "double tuned input curcuit"; i.e. two tuned circuits ahead of the RF amplifier tube, which is designed to provide sharper selectivity.

[etech]

The better selectivity on the Model 90 is due to the lower I.F. freq of 175 kHz., as opposed to the 460 kHz. on the 650.

Yes, I hadn't thought about that, Chuck. But , if all other factors were equal, than the 175kc I.F. ought to be a little better than 2.5 times more narrow.

Also consider the fact that the Model 90 has Philco's "double tuned input curcuit"; i.e. two tuned circuits ahead of the RF amplifier tube, which is designed to provide sharper selectivity.

Ron, yes I am still very impressed by this fact. That four gang tuner with the double tuned antenna coil was quite an advanced circuit for its day. Philco spared no expense on that.

I have to say that I'm still very surprised by the performance of the model 90. I guess the use of those #24 screen grid tubes made quite a difference. After all, it is quite advanced for a 1932 set.

Ed

[BDM]

Very interesting Ed. I've often wondered how the sensitivity of a 90 or 70 is. This is the most information I've seen on a 90s ability at reception & selectivity on the NET. Now, how does it perform on SW compared to the 650

[Chuck Schwark]

Model 90s do not have a SW band.

So.....I'd say rather poorly.


Chuck

[BDM]

I know. Don't let me secret out 8)