Anyone else with this problem? It is annoying. I don't like modifying radios, but the Majestic curtain burner TRF I'm working on right now, I'm about to throw out the window. All caps have been changed, and all resistors are on value. I've experimented with various schemes and tricks, and nothing solves the problem. I'm very tempted to change the volume control to a 1 meg unit, throw it in the audio circuit, and then give the RF an automatic volume control circuit, which would make the radio more practical, anyway....I just didn't really want to modify it. Another problem with this radio is hum in the power circuit. It's strange because just about every other AC/DC that I've come across has the output connected directly to the rectifier. In this particular radio, though, hum results. I can reduce the hum by really piling on the electrolytics (100 MFD). I doubt that this value was used originally. Another method is to put everything in series with the field (the field is normally across the B circuit). This method reduces the power of the radio significantly, though, since the field is designed to drop a full 120 volts. A solution to that would be to wire the 25Z5 as a voltage doubler. A voltage doubler has two benefits--both the field and the radio would get their fair share of voltage, and a voltage doubler circuit produces a full-wave rectified DC, which is cleaner and easier to filter.
This radio is cheap in every way. I'm beginning to not like it. I guess since I can't figure out who it's made by, and I can't find a schematic for it (not that I need one...the radio's dirt simple), I shouldn't care about changing things around. The circuit is so simple, I could memorize it for life, and I could always change it back to its original design. The radio is amazingly sensitive and selective (with only 4 functional tubes). That is about all that's good about it.
...Again, if anyone else has trouble with RF volume control distortion (with the volume turned down low), or has a solution, please let me know.
T.
I have a Radiola 33 TRF with a variable resistor in the antenna circuit and have not experienced this problem. The volume smoothly drops to zero when the resistance is turned up.Perhaps the distortion is elsewhere, like in the speaker. Some early speaker designs may not like having too low an input drive, due to the stiffness of the cone material.
Just musing.
Lou
:Regarding anyone with a TRF, or possibly superheterodyne, that uses RF bias for the volume control....do you get distortion when the volume is turned down low? I have this problem with all sets I own where the volume control is in the antenna/cathode circuit. The only solution I could come up with was what I did to my Majestic 180. I gave the circuit AVC, and then put a volume control in the audio circuit instead. What that does is it allows the RF circuitry to operate at a lower bias (if a manual volume control was used, it would be turned up, making the cathodes less positive with respect to the grids). When the RF tubes are operated in this manner, and are kept away from the heavy bias area, there is no distortion. Still, I can turn the volume down low, since there is a volume control in the AF circuit.
:
:Anyone else with this problem? It is annoying. I don't like modifying radios, but the Majestic curtain burner TRF I'm working on right now, I'm about to throw out the window. All caps have been changed, and all resistors are on value. I've experimented with various schemes and tricks, and nothing solves the problem. I'm very tempted to change the volume control to a 1 meg unit, throw it in the audio circuit, and then give the RF an automatic volume control circuit, which would make the radio more practical, anyway....I just didn't really want to modify it. Another problem with this radio is hum in the power circuit. It's strange because just about every other AC/DC that I've come across has the output connected directly to the rectifier. In this particular radio, though, hum results. I can reduce the hum by really piling on the electrolytics (100 MFD). I doubt that this value was used originally. Another method is to put everything in series with the field (the field is normally across the B circuit). This method reduces the power of the radio significantly, though, since the field is designed to drop a full 120 volts. A solution to that would be to wire the 25Z5 as a voltage doubler. A voltage doubler has two benefits--both the field and the radio would get their fair share of voltage, and a voltage doubler circuit produces a full-wave rectified DC, which is cleaner and easier to filter.
:
:This radio is cheap in every way. I'm beginning to not like it. I guess since I can't figure out who it's made by, and I can't find a schematic for it (not that I need one...the radio's dirt simple), I shouldn't care about changing things around. The circuit is so simple, I could memorize it for life, and I could always change it back to its original design. The radio is amazingly sensitive and selective (with only 4 functional tubes). That is about all that's good about it.
:
:...Again, if anyone else has trouble with RF volume control distortion (with the volume turned down low), or has a solution, please let me know.
:
:T.
For good analysis, your volume control must control cathode bias of the RF circuit. If it merely shorts out the antenna, and does nothing else, it is not performing in the same way as my volume control(s).
Ah.....who knows. I'm going to sleep. ...Such a neat looking little radio (nothing special, but nice just the same). Too bad it's such a headache. I did discover one thing, though. By reducing the value of the potentiometer, I somewhat reduce the distortion at low volume. I have noticed that in many circuits that the potentiometer's value is around 7,500 ohms. In this particular radio, whether it is supposed to be or not (I have no schematic), it is at 20,000 ohms. Reducing the value of the potentiometer has this benefit: as the wiper shorts out the antenna circuit, the resistance between it and the cathode circuit (wiper goes to B-) is not as great, so not as great a bias develops for the low volume level. I like it! There always is the problem of having the RF circuit perform too much, being unshielded. It is possible for it to receive signals even if the antenna is shorted out. This did not happen to me, though. If I don't modify the circuit completely, I may simply change the volume control and live with the small amount of distortion that still results. The distortion I am talking about is a garbled sound. Variable mu tubes aren't supposed to have a steep cut-off point, and so when high bias is used, they are simply supposed to reduce gain, not act like a detector. However, it sounds like the signal is getting detected somewhat in the 1st RF circuit, which isn't supposed to happen until the 2nd circuit. Rectification in the 1st cicircuit will get rectified again (whatever is created will turn back into AC as it passes through the 2nd RF transformer). 2nd rectification is rectifying the audio at that point, not the RF, if rectification is occurring already in the 1st RF stage.
Although variable mu tubes do a pretty good job of regulating volume, I believe that they do have their limits. In more modern receivers, the AVC never biases them quite so heavily (unless you're right next to a transmitting tower) as when you turn a manual volume control down really far, so it is rare to get such a distortion (unless the AVC capacitor is shorted, and the RF circuits are simply being overloaded).
T.
marv
:Well, in my case, the distortion definitely isn't in the speaker. It only occurs when a lot of bias is added to the RF circuitry. If I reduce the volume by any other means (using a separate potentiometer to short out the antenna only, or to reduce the size of the antenna, or to shunt in the audio section) I get no distortion.
:
:For good analysis, your volume control must control cathode bias of the RF circuit. If it merely shorts out the antenna, and does nothing else, it is not performing in the same way as my volume control(s).
:
:Ah.....who knows. I'm going to sleep. ...Such a neat looking little radio (nothing special, but nice just the same). Too bad it's such a headache. I did discover one thing, though. By reducing the value of the potentiometer, I somewhat reduce the distortion at low volume. I have noticed that in many circuits that the potentiometer's value is around 7,500 ohms. In this particular radio, whether it is supposed to be or not (I have no schematic), it is at 20,000 ohms. Reducing the value of the potentiometer has this benefit: as the wiper shorts out the antenna circuit, the resistance between it and the cathode circuit (wiper goes to B-) is not as great, so not as great a bias develops for the low volume level. I like it! There always is the problem of having the RF circuit perform too much, being unshielded. It is possible for it to receive signals even if the antenna is shorted out. This did not happen to me, though. If I don't modify the circuit completely, I may simply change the volume control and live with the small amount of distortion that still results. The distortion I am talking about is a garbled sound. Variable mu tubes aren't supposed to have a steep cut-off point, and so when high bias is used, they are simply supposed to reduce gain, not act like a detector. However, it sounds like the signal is getting detected somewhat in the 1st RF circuit, which isn't supposed to happen until the 2nd circuit. Rectification in the 1st cicircuit will get rectified again (whatever is created will turn back into AC as it passes through the 2nd RF transformer). 2nd rectification is rectifying the audio at that point, not the RF, if rectification is occurring already in the 1st RF stage.
:
:Although variable mu tubes do a pretty good job of regulating volume, I believe that they do have their limits. In more modern receivers, the AVC never biases them quite so heavily (unless you're right next to a transmitting tower) as when you turn a manual volume control down really far, so it is rare to get such a distortion (unless the AVC capacitor is shorted, and the RF circuits are simply being overloaded).
:
:
:T.
T.