[morzh]

It could be, this is what I was trying to ask in post #39 - if the detectors have something to do with it, I remember Ron told something to that effect when I complained about the 20's performance.
I am not sure whether the distortion is smaller at the high volume though I think it is. But then high volume makes one less susceptible to the distortion.
Also my daughter was asleep so I did not have a chance to really rev it up.

[morzh]

OK, yes the distortion is more pronounced at low volume (I cannot make it too high - I am afraid the cone will fly out) and then someone (read - my wife) will come and strongly disapprove).

But: there is that choppy distortion at very low frequencies - low base etc. Voice and higher instruments are acceptable.
And I do realize we are not talking Hi-Fi here. All in all, I think it is acceptable overall. I'm just interested how and why. I have to prove to myself that, OK, this is the plate detector and you did what you could, this is it, unless you are willing to mod the chassis (which I am not).

Keep listening..no, it's not bad. It is playing Rod Steward "Maggie", and it is not bad, especially considering Rod's voice is raspy a bit by itself

PS. I wish I had a large PM speaker (I have one in Stromberg Carlson I just fixed but it would require another disassembly plus it is not quite large a speaker) - I could then quickly eliminate the speaker itself.
Maybe I should by a new modern PM speaker.


PPS. Just remembered I have two Yamaha 4 Ohm speakers that I toptally forgot about - unused rear speakers from my stereo.
Well, disconnected the Philco speaker's voice and connected the Yamaha to it.
Well, in short, it is not the speaker. The sound is the same, only the Philco sounds better (well, it was reconed with a new modern cone, so...)

[morzh]

Well, using my Variac I found that each input winding's half of my output transformer has the ratio of 30:1 to the output winding. Which makes the impedance transfer as 900. So, 6 Ohm of my speaker look like 5.4kOhm to each tube and the optimum is 3.9kOhm which is not too bad.
Plus I tried the Yamaha and it is declared 4 Ohm which makes it 3.6kOhm which is right where it needs to be and it still distorts.

So. It is not the speaker OR the output transformer.
Nice to know.


PS. The programming on the station has suddenly changed and they have that doo-wop and the distortion went way up.
Well, I turned on my 37-116....and it also kinda distorted, so at least that worsening was not due to the radio itself.

[codefox1]

Well, try feeding a know good audio signal into the volume control, (groan) IPOD OK. You could than rule out everything (east) of this part. These old sets are not that fussy about exact impedence match, so long as there are not shorted windings in the primary of the output transformer. Actually you can remove one output tube at a time and run for a short time to confirm this. Might just be a little hum, but if the distortion disappears, do a swapperoo of the output tubes, remove one and run again. Then you can find out if you have a bad tube or possibly some problems with the output transformer that were not immediately apparent. Disregard if you already tried all this, your work looks great. Best.

[morzh]

Codefox

If you run on one tube won't you have distortion by design? They run either B or AB which means you will get half the wave.

I have no sound source, or rather no usable one. I need to buy an audio generator.That's the best test. And I already have a scope.
Well, in fact I do have a Western Electric oscillator but I never fired it up. It has weird plug. I need to solder wires to it I guess. And it is old tube from 60s or 50s, a colleague gave it to me about 3 years ago. It was either that or he would toss it. It came from Bell Labs.

[morzh]

Well, I used a Heathkit RF osc modulating signal for audio scoping, looking at the audiotransformer out, looks good.
Then from my RF genertor I fed modulated signal , both 400 and 1000 Hz, to antenna input, and the output is a pretty clean sinewave.
When it plays the station the output is such that one could not figure out if it is good or not especially with a slow digital scope. But the sound gets distorted. Although one song was almost OK, the next one sounded distorted.

The sinewave output signal gets distorted when the amplitude gets to just above 2V, which although sounds loud is about 1/3 Watt. This is why I suspect transformer, it may not be optimal. Still not sure.

[BrendaAnnD]

Are you playing the radio out of the cabinet for these tests? If so, remember that if there is a significant low frequency component to the audio, the speaker will rattle or bottom out on bass notes with no baffle. This will affect the amount of at least perceived distortion. Try cupping your hands around the edges of the speaker and see if some of the distortion goes away. Alternatively, you could cut a speaker sized hole in a piece of cardboard and set this in front of the speaker.

[morzh]

Brenda


yes I play it out of the cabinet, and I will try what you suggest, but I remember I did the same with 37-116 and it was playing beautifully.

[BrendaAnnD]

FWIW, the 37-116 is a much more complex radio with a better audio stage/AVC/etc. I would expect it to perform better than the 90. Though the 90 is no slouch, I have a 90 console that sounds pretty good, though it, too, does have some distortion at low volume levels. Especially on my local 1.5MW station.

[morzh]

Well, in fact even my measly Stromber Carlson out of the cabinet did not do it.

Whatever AVC and complexity a radio might have, if a distortion is due to acoustics this is then out of electronics hands short of introducing some acoustic negtive feedback, negative output resistance damping and such...if a speaker vibrates out of the box due to resonances not being damped, then the class of radio is no longer important.
Which is not to say that this is not the case....maybe the sheer speaker in 37-116 is better.


I also found that the 60Hz hum gets reduced quite a bit by installing a .2uF or so cap across the filter choke. I was also hoping this was partly responsible for the distortion as the scope shows well modulated 1kHz by 60Hz. But not much effect other than hum attenuation.
They in fact did introduce this change (a smaller cap for 60Hz) in later revisions, plus changed the choke.

[morzh]

Brenda

1. Did the experiment, simply fully assembled the radio in the cabinet with the grill cloth and all.
The distortion persists.

2. While still out of the cabinet, some songs sound better than others.
For instance, "Hello Marie-Lou" by Rick Nelson sounded quite OK.
"Pretty Woman" - so-so.
Some of the Credence C. R. sound OK.

3. Now, I also soldered that 0.25uF cap across the choke in the filter and the hum got reduced significantly, at least on the scope 400Hz signal when applied to the 37 preamp tube's grid via a cap, does not look 60Hz- modulated so much anymore. Again, this did not help distortion much.

What I need is an audio gen, so I could check the distortion across the range and not just at 400Hz or 1000Hz.

4. The undistorted sine wave comes out to about 5V peak which makes it about 1.7Watt out, which is close to what the datasheet lists as the UPO for the 45 tube (1.6W) at 250V plate Voltage.

The last thought is, Philco 20 (2x71A), and 90 (2x45) use the same 2766 transformer for push-pull which has 100:1 ratio and it wants to see app. 7kOhm of resistance (6700 Ohm from the table) which is what 0.7 Ohm time 10,000 will get you;
The tubes at the max voltage want to see 4.8kOhms each which is 9.6kOhm for CLASS A (not sure but probably this is what we have).

Mine is 60:1 across the push-pull (30:1 each half) which makes the expected impedance for CLASS A = 3600*6 Ohm*1.25=27kOhm. The output expects ideally 7.8kOhm (each 45 expects 3.9kOhm load), and with the original speaker voice coil and transformer they would get the same 7kOhm which is close.

SO I am quite a bit away from optimum.

I might try a different transformer.
Just for the heck, to see if this is my problem. I should probably keep a spare 125 Hammond pushpull at home.

[morzh]

Here's what I've just tried - I plugged my 20 lowboy's speaker right into the 90, they are exactly the same compatible pliugs and, at least when original, the same speaker and the same transformer.

My 20 has re-coned speaker and hammond 125C-CT pushpull transformer whoe taps were chosen to closely match to what was used before, which should be close enough to both 71A and 45 tubes' requiremenrts.

Well, I hear very much the same distortion.
What's more, my 20 sounds very close to the 90, the distortions of the 20 (which I complained of before a while back) being very similar to 90's.
The 20 sound has way less bass but otherwise that choppinness during some sounds is the same in both the 20 and the 90. Which makes me think of the detector.

Which will not sway me from buying a proper transformer - I still want to hear the sound with correct loading of the push-pull.

PS. I just realized that I have another 20 delux cathedral which I have just bought recently.
But when I checked the speaker the field coil was open (though the transformer seems OK). I am not sure but somehow I think maybe the cone in that one is original also. Well, so much for this one.

[morzh]

Heard weird chirping on higher volume. After investigation (not sure what the exact mechanism of it was) found that right at the AC plug one wire was broken about an inch from the prong.
The cord is original and I tightened the screws since it was loose, but now I had to cut it and strip etc. Those cords are bare copper and they blacken some all the way inside that insulation.

[morzh]

Scott - wanted to ask you, when you listen to your 90, do you hear that distortion we have been talking about here? Or any? How would you describe them if you are having them at all? Or is it just me? I have very similar distortions on both my 20 lowboy (though it lacks bass but otherwise similar) and the 90, and those are the only two having a plate detector (80 has grid leak).

[Mondial]

As a reference to the details of detection in general and specifically the plate detector, you might want to take a look at this:

http://www.ax84.com/static/rdh4/chapte27.pdf

It is a chapter from the Radiotron Designer's Handbook, the definitive reference for radio design in the 30's and 40's.

Plate detector is covered starting on page 13 of the PDF.