Philco 570 - Printable Version
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RE: Philco 570 - Ed Locker - 05-15-2019
I found that the Plate detector performs very good with a low modulation source. Using my AM transmitter with the audio input low, there was no distortion with music or speech using the Plate detector.
Possibly when these radios were new, the transmitted modulation level was lower than what it is today?
Ed
RE: Philco 570 - morzh - 05-15-2019
That can be explained.
The plate detector works essentially as a tuned C-class amplifier.
While fairly linear at the top, it curves at the bottom and so with low amplitude signal the result is distorted as it is at the bottom of the curve.
However the whole range is not exactly linear and so the lower the modulation, the less it gets from the particular point at the curve so it gathers less distortion, as the smaller part of any curve one cuts out, the closer it is to a line.
RE: Philco 570 - Ed Locker - 05-15-2019
OK,that explains the function of a Plate detector but it doesn't answer my question.
Were modulation levels lower in the 30's compared to what they are today?
Why do Plate detectors sound normal using a AM transmitter with low audio input?
RE: Philco 570 - Ron Ramirez - 05-15-2019
Seems I read somewhere once that modulation levels were lower back then. Certainly, back then you didn't have stations trying to peak all of their programming at 100%+ modulation as has been the practice for the last several years (last few decades?) in broadcasting.
RE: Philco 570 - morzh - 05-15-2019
That could also explain some of it.
RE: Philco 570 - rfeenstra - 05-15-2019
I was just reading a bit of information on WLW's flame thrower transmitter back in the 30's. The indication was given that they could achieve 100% modulation and the implication was that that was unusual. So maybe 100% was rare. I was also looking at the 35 tube data from Cunningham. They gave a sample superhet schematic using 35's. Low and behold, there was the plate detector/amplifier. I guess I can't blame Philco engineers if everyone was doing it!
RE: Ron. cancer post - Jake Blake - 05-15-2019
I have been studying that modification to the 70 that you posted. That was one of your best posts to me so far. I do watch for yours.
RE: Ron. cancer post - Ron Ramirez - 05-15-2019
Jake
Please, do not give me credit for that. Let's give credit where credit is due - to Ed Locker for the mod and drawing. I merely posted the drawing for Ed.
RE: Ron. cancer post - Jake Blake - 05-15-2019
Yes sir.
We should at least give you credit for posting it....it's the first I've heard of it, and it inspires all sorts of inferences.
RE: Ron. cancer post - morzh - 05-15-2019
It's an extensive one. The volume regulator stopped being an antenna attenuator and cathode bias changer and became a regular part of the detector circuit, and the plate detector became 1st AF.
Good few hours at the workbench.
RE: Ron. cancer post - Ed Locker - 05-15-2019
At the time I came up with that circuit, I didn't know there was a Triode with diode plates and a 2.5V filament.
I later heard of the 2A6 after I drew the mod. My head was hurting after it all, so I said the heck with it and stayed with the 1N34 diode.
Shhh.......no one will ever know.
Ed
And yes, we do miss you, Ron!
RE: Ron. cancer post - Jake Blake - 05-15-2019
"Shhh.......no one will ever know." [Image: https://philcoradio.com/phorum/images/smilies/icon_shh.gif]
<chuckle>
RE: Ron. cancer post - morzh - 05-15-2019
Well ... a small diode is inconspicuous, could be easily hidden and allows to keep the tube count which does preserve the authentic look. And with one diode vs many tubes that still leaves it at being a tube radio.
Speaking of which, a semiconductor detector is as old as the tube one. Actually older. It's just it did not assume the shape of a small diode for a while and was made of lead and sulfur and not Germanium.
RE: Ron. cancer post - morzh - 05-15-2019
As was noted privately, we seem to be hijacking this thread and probably should ask the moderator to move this part into the proper thread.
RE: Philco 570 - Jake Blake - 05-15-2019
"...The volume regulator stopped being an antenna attenuator..."
From my grasp of the circuit , ( which isn't good), that seemed a little silly, trying to insert resistance between the antenna and the first stages. The majority of attenuation ( or perhaps I should say 'gain'), appears to be accomplished by the grounding of the cathode of that 3rd 24. But like you, I would have thought our interests in "authenticity" would lead to a substitute of a triode tube with those twin diode sections,. . . NOT (GASP!), a semi-conductor in a 90-year-old radio! [Image: https://philcoradio.com/phorum/images/smilies/icon_eek.gif]