[morzh]

Mondial,

Thanks for the link, it is a very good paper.

[morzh]

Here's another theory:

On that station (and this is all my music) when tuned in I hear 60Hz (or it looks lie it) hum.
I think when mixed with music it actually makes that distortion. This is that choppiness I hear.
The hum comes through on every radio I got, and it is not there all the time, depends on the time of the day.
But on other radios it is just being a low hum and so when the music is there I cannot hear it, especially when the volume is medium and up.

On this radio and on the 20 this hum sounds more like a buzz and it does not mix well with the music. Not all other stations have it. But they do not play music. So I cannot check.

[BrendaAnnD]

Ah.. I may have an explanation for this behavior. When these radios were made, broadcasting standards were far different than they are now. Mostly, modulation peaks were limited to 100% on both positive and negative excursions. Also, program material was not processed at either the recording studio nor the radio studio in order to maintain a high average level (volume wars). Radios could be designed with a wider range of detectors and get relatively low distortion.

On the other hand, modern broadcasting is different. Regulations now allow positive excursions of 125% while keeping negative excursions to 100%. Compression/expansion allows stations to keep the average level close to 100% for the majority of the time (some stations process so much that their dynamic range is reduced to around 6dB.)

With these combined differences, the radio may process the signal outside of the linear part of the curve of the detector. This would cause distortion which on a scope would look like flat topping of the detected waveform. This will show up more on music than on speech, with some kinds of music (higher bass or sharp peaks) causing worse distortion than others.

[morzh]

Could be it, Brenda.

This is why I am trying to see if Scotts 90 which is the same I have has similar problem.

[morzh]

Tonight I was doing the switcheroos.

First of, I noticed that the radio sounded unusually well tonight, nd sure enough there was very little of that hum coming through when tuned in the station. I noticed the him changes during the day with that station. They played "I'm a believer" and it sounded very nice. Nothing close to 37-116 but that was not ever expected.

Then when the program changed and they started playing something with a bit more distortion, I switched the detector tube with another 24. The sound became worse, at least subjectively, and became better again when the tube was replaced. I tried various tubes in teh detector socket and the one that was there sounded better than others. Strangely enough this is the tube with the lowest mutual conductance.
Then I placed the last tube thart was in the radio I did not try, the original Philco globe 24, and subjectively it sounded a bit softer and less raspy, so I left it there. Other tubes' swapping did not matter.

The second one was using the 90's speaker in the console 20.
The 20 also played unusually well tonight.
Although I stated before that the distortions are similar when I transferred the speaker in the 20 the distortion seemed to migrate with it to a good degree.

All this being highly subjective and a bit confused, I think the problem might be many-fold, including possible output transformer impedance mismatch, the detector being what it is, the mentioned before different depth of modulation, all of it combined.

Well, I realized I am yet to change the tuning cap grommets which are totally destroyed.....plan a trip to Lowes tomorrow.

[morzh]

Unexpectedly I got the transformer I ordered this morning.
So After I took my daughter to her daycare and got the grommets (that is combination of grommets and washers - nothing exact) from Lowes, I put the grommets in (took some patience and ingenuity, considering I not only had to put the grommets in but also put the washer on top and then screw the cap, and the cap was not unsoldered either), turned it on, listened to the station...sounded nicer than usual (not due to grommets though), and then I connected the new transformer.

Well, this is not the one that will go in permanently - it is too large, I only bought it to use as a testing transformer with variable impedances' combinations - but it does sound nicer with the one I connected which is 8kOhms / 8 Ohms. This is the closest to the required load, as 7 to 8 Ohms of my coil will present 7 to 8 kOhm ipedance and this is what the tubes want.

The quality might not be up too much as right now it is OK the way it was, but I certainly can go up in volume much more and keep the acceptable quality whereas before it would start distorting at much lower volume level.

So, will probably by a smaller transformer 8k/8 Ohm from a used supply (do not want to spend 40 bucks for Hammond), install it and assemble all back.

It still sounding nice enough while I am writing this.....upped the volume, still nice and very very loud.

Will see what the evening brings.

[Scott Fletcher]

Morzh

Just saw your remarks about distortion on your 90. I noticed some mild distortion on my radio also. I attributed it to the 45 output tubes which have tested very weak, and since they are quite expensive I have not replaced them. I also have not done an alignment yet nor have I measured the plate voltages on the output tubes,or any other voltages for that matter. I want to install a bucking transformer in the cabinet as my line voltage runs about 125V. I have put the chassis aside for now until I get the cabinet refinished. Too cold now to tackle that-is -13F as I write this. I guess I was so relieved to get the radio working considering all the unknowns with the sch. ect. that the slight distortion I heard didn't alarm me very much.

[morzh]

Thanks Scott. BTW the 90 s 105-125VAC device, so I think you are OK the way it is.

[morzh]

Well, this is evening, and it sounds better with the new transformer.

there is that hum again, it seems to appear in the evenings, but it does not seem to behave as bad with the new transformer.

Well, this is my plan of action:

1) for whatever reason the Philco tube behaves just a smidge better in the detector place. Gonna leave it there.
2) The new transformer behaves noticeably better. Gonna try to fit it (it is a 25Watter but is still cheaper than the Hammond P-P). if it does not - I will buy a 8-kOhm-to 6.6-kOhm/8-Ohm transformer and put it in.

This will be it for this baby.

[Scott Fletcher]

Has anyone measured the wattage drawn by these early model 90's? Mine measures about 95 watts. I wonder if this is OK as I have not seen a watt rating on any literature or on the chassis name plate on my radio.

[BrendaAnnD]

The evening hum may just be because of a power or directional change of the station during nighttime hours. Lower signal level allows 'tunable hum' to be more pronounced. Tunable hum is what you get when your home wiring is acting as a part of your antenna system.

[morzh]

This is an evening (mostly) hum. I also have an early morning hum. In all radios. it disappears midday and then reappears some time after 7 or 8.

I also suspect the "evening power factor" plays a part, when all folks come home and turn on their TVs and what not, with poor power factor correction, and all those filter caps charging create some abomination of a waveform in the MAINs.

[BrendaAnnD]

You want something that creates an abomination of a waveform in the mains? Here, use of fluorescent lights is almost universal. Not just CFL's, those are relatively quiet. Also 2' tubes, 4' tubes, and those 'bent' lamps are used extensively in kitchen and bedroom lights. Millions of fluorescent lamps all operating at the same time creates RFI to the tune of S7 to 20dB over S9, depending upon time of day and meter band.

[morzh]

I don't use them. I use CCFLs since we are running out of alternatives here, but one long fluorescent I have was inherited from the previous owner and it is in the basement where I only go for a few minutes maybe twice a week. If that often.

[TA Forbes]

No CFLs here. Although I like the brightness, especially over my "bench," they made too much noise on my old radios.