01-04-2014, 09:32 PM
I have tons of time on my hands (well, I am on vacation now and will be for another week or so) so I've decided a couple of hours ago to see if anything that I could do to that plate detector, that would improve its performance.
As I have written in the main 90 topic, I found out that the buzz (hum) I get when tuned in to some stations seems to misbehave more in this radio and my 20, this I think is one of the reasons for the distorted sound.
Now, I thought maybe some optimization could at least improve something. I could give you heads up before you read it all, that nothing I did really mattered, at least to my ear.
Here's what I tried:
1. I tried to replace the cathode cap in the detector with a smaller capacitance. It is in fact recommended that it be about 100 - 500pF and not the microfarad range caps that are usually there. This would reduce the gain but according to that article is supposed to improve the results.
Well, it did reduce the gain all right, but did nothing to improve the output quality.
2. I shorted that inductance in the plate filter. Heard some hum. No improvements.
3. Eventually I tried to decrease that DC blocking cap down to 1000pF as it was initially in the sch. To my surprise the ouput did not even change. So one thing I can tell, a 1,000pF cap works the same way (to the ear) as 0.01uF (10,000pF) one does.
4. Last thing is, whatevert literature I read so far, including looking at the schematics with a very similar plate detectors in one of the 70 series and in 20 (exacftly the same) the RF cap in the plate is in hundreds....220pF or so. In the literature it says from 150 or 200pF and up to 1000pF. So 35pF is not enough, but it is a good question again, whether or not it would noticeably change the sound.
Ifv I do anything else I will post it here.
As I have written in the main 90 topic, I found out that the buzz (hum) I get when tuned in to some stations seems to misbehave more in this radio and my 20, this I think is one of the reasons for the distorted sound.
Now, I thought maybe some optimization could at least improve something. I could give you heads up before you read it all, that nothing I did really mattered, at least to my ear.
Here's what I tried:
1. I tried to replace the cathode cap in the detector with a smaller capacitance. It is in fact recommended that it be about 100 - 500pF and not the microfarad range caps that are usually there. This would reduce the gain but according to that article is supposed to improve the results.
Well, it did reduce the gain all right, but did nothing to improve the output quality.
2. I shorted that inductance in the plate filter. Heard some hum. No improvements.
3. Eventually I tried to decrease that DC blocking cap down to 1000pF as it was initially in the sch. To my surprise the ouput did not even change. So one thing I can tell, a 1,000pF cap works the same way (to the ear) as 0.01uF (10,000pF) one does.
4. Last thing is, whatevert literature I read so far, including looking at the schematics with a very similar plate detectors in one of the 70 series and in 20 (exacftly the same) the RF cap in the plate is in hundreds....220pF or so. In the literature it says from 150 or 200pF and up to 1000pF. So 35pF is not enough, but it is a good question again, whether or not it would noticeably change the sound.
Ifv I do anything else I will post it here.
