[morzh]

OK, it seems like an X-cap improved the sound a bit.

I thought about the MAINs line contributing to the radio reception (have to yet understand how it is....since it is a pair it's gotta be a common mode interference and should be rejected...but I do not claim to understand the whole mechanics).
Anyways I have several X-caps and I put 0.68uF across the L-N, and the sound improved. Or maybe it is my imagination.
And while at that, I installed 100K 1W resistor across the AC line to discharge that cap that gave a pretty menacing spark when I decided to see how much it stored.
There is a convenient place right across the bakelite housing the two Y-caps.

I am now tempted to try the same in 37-116, 9S-262 and the rest, but they unlike 90 sound good even with that interference, so I only hear the hum when the music stops, whereas in 90 it causes the distortion. The latter is not fully gone, but seems to subside quite a bit. I am unwilling to start installing micro-farads worth of caps as the inrush will be large and the sparks will be pulled when plugging-unplugging.

[morzh]

Well, now that I own a fully working oscillator.....this evening a ghost was howling in my house.
Long story short, I hooked it to the pre-amp 27 tube and watche excellent undistorted output on all frequencies up to 7-8V peak at the speaker voice coil using the scope. This makes it at least 5V RMS which makes it at least 4 Watt output on the speaker.
Quite good.

So the residual distortion I am having when the buzz no longer an issue comes from RF, I think from the detector.
Well, the audio amp and the speaker and transformer are exhonerated.

[morzh]

I just defeated a major part of that distortion.

Funny that I tried thaty before but did not think it worked.
Probably because there were two reasons - the tuned hum and the detector distortion.

So, tuned hum right now is not present.

What I did, I removed the 0.5uF capacitor #25 across the cathode resistor #21 in the detector. The large capacitor is there to increase gain in the sound frequencies. This indeed increases the gain (you can hear it immediately) but also increases distortion.
The capacitor there should be on the order of a few hundred pF (uuF) to only bypass RF. BTW it works almost the same without that cap, the difference is little.

But the difference in sound quality is very noticeable.


As a post-note, the voltage across the plate-cathode of the detector in this and other schematics is shown (I think ) wrong. They show it in the renge of 40-60V in plate detectors of 20, 70 or 90, and the voltage applied to the load is the 250V or so. The problem with all this is the very principle of the plate detector which should be at the border of the tube not conducting, which means the plate voltage should be almost the same as the B+ and the whole voltage drops across the tube. If the voltage is 40V it means the tube is conducting close to 0.5mA.
Unless they mean RMS voltage and the volume is very high (still don't quite get how it can go that low).

And lastly this voltage is very tube-dependent. One tube goves you 209V where another will be 160V.

[Mondial]

I think that the detector plate voltage measurement is inaccurate on the Philco schematic.

Keep in mind that the Philco tester was 1000 ohms per volt. So if they were measuring on the 100 V range and measured 40-60 V, the plate circuit was loaded by the meter's 100K resistance to ground, which formed a voltage divider with the high value plate resistor. So it was not the tube conducting current but the meter loading causing the low reading.

As you said, the detector tube should be biased to cutoff, so the true plate operating point should be very near supply voltage.

[morzh]

Another thing I noticed while investigating: in my set the screen grid of the second 24 tube (they call it 1st Detector - there is also confusion there in Riders how they call what) goes to "J" voltage, and it seems factory - there is one wire running through all tubes and the soldering was clearly original.

It is supposed to go to "H" voltage, same as 2nd Detector's screen grid. But the 2nd det. SG is the only thing going to "H".
And, in fact, before I moved the 250K resistor to it, it ALSO did go to "J" and nothing was going to "H". And the soldering was original. (we spoke of it in the very beginning).

So....I tried to move 1st detector's SG to "H". The sensitivity went way down. And then something started happening - reception started fading. So I decided to leave it where it was - on "J".

[morzh]

OK, I am closing it.
I am listening to it, sound good. Not your 37-116, but good. No bad distortion anymore.
And although the tuned hum right now is present, it does not cause bad distortion like it was before and is heard as what it is - a hum. Before it mixed with music and created very bad effect.

The gain dropped a bit but was expected. Still very loud, but the local station no longer tries to rip out the cone at maximum on "normal" gain setting. And this is still without any antenna. With antenna it is very-very loud.
I am satisfied. Short of redesigning the detector I don't want the gain at the expense of bad music.

Will keep it on the desk 'till the weekend and then I will assemble it.

[Arran]

This is an interesting thread, this is the first time I have seen an electronic engineer actually go through one of these. One thing that I have found with many early AC sets is that they often need a good ground connected to them, both for noise reduction but also performance.
Regards
Arran

[morzh]

Good ground may help but in teh first place one does not necessarily have it. I certainly don't - I have a regular Earth that is probably a good protection ground but noise-wise.....don't know.
On some radios I saw it helping, on this one it did didley-squat when I connected it.

And this tuned hum - nothing rerally helps except turning that light I spoke of before ON. Andf as I said before I probably know why - the hot and neutral come probably from different cables and form the loop.
It's a guess but I think it is indeed that way (there was an episode that made me suspect it).

Well, I spent this day on line with first Dell Indian team, and then with Cable Retention department. Both somewhat successful.

Now is the time to assemble the speaker back and put teh chassis on.

[Radioroslyn]

I wonder if some snap-on ferrites on the line cord might help.
Terry

[morzh]

You could try. They are common mode devices so should suppress anything that the power wire carries inside as the common mode signal, that is acting as an antenna. The effect of it entering the chassis and the part of it capacitively coupling into some RF sensitive circuitry might be attenuated.

How effective it will be - I am not sure. In essence Y-caps do the same job. I did not see them being that effective for the radios.

For cleaning the power line of the CM RF they (and ferrites) are effective. That is when you protect MAINs from your device spewing interference in The Grid.

[morzh]

One last thing: when a speaker is reconed and gets a new modern cone with a new modern spider, it, as we know, improves the sound.
However part of the improvement is a very strong deepening of the bass to the point of over powering the other frequencies so even at the mas treble the sound is too deep in the bass.

It almost calls for some artificial low frequencies reduction, which would be another slight change in teh schematic I haven't done so but some of you might want it if this is gonna be the case. Possibly this capacitor from detector to the 1st audio could be played with....dunno.

It sounds well right now but a bit too booming. And once I assembled the speaker in the cabinet, it is really....booming
When I connected the 95's speaker the booming went away and the treble became very pronounced so it had to be dealt with by trurning the tone control one notch down.