I was wondering if anyone has ever spent any time thinking about modifying the volume circuit in a Philco Jr. Model 81 radio (schematic attached)? The current one simply limits the antenna input. This design leaves you caught between keeping the sensitivity high enough to pull in weak stations and low enough to be able to control the volume on strong stations. I am thinking about moving the volume control to some where in front of the Audio amplifier (42 Output tube). I know this changes it from being stock, but I would also like it to be usable and sound good.
Because the set has no AVC, and it has a regenerative 2nd detector, you'll probably need to keep a control at the antenna input. The 2nd detector will give low distortion only for a very limited range of input signal. But, if the set is too loud when it sounds best, you could make the grid resistor of the 42 a pot too. I'm sure there is something wrong on the schematic. The 4 meg resistor at the grid of the 2nd detector makes no sense as shown.
Perhaps I'm misreading the schematic. It looks like the 4 meg resistor is across a coil. If so, it can't be a grid leak. Maybe that coil is just forming a cap to the winding below it. I wonder what the switchable caps on the antenna coil do.
Yes, the "coil" in question is a capacity winding, and the 4 meg. resistor is indeed a grid leak and is correct as the set was designed.
Bear in mind that the 2nd detector is regenerative.
I found in my own experiments that a 2 meg resistor works much better in this position, and I authored the service tip in question at the link to Chuck's site.
Well actually this set IS a superhet. It's a superhet with a regenerative detector. The 4 M resistor is the grid leak for the detector. By varying that value it will vary the selectivity of the detector. The high the value of resistor and the lower value of grid leak capacitor will give the max selectivity (sharpest). Average values are 1M and 100pf which is good for AM reception.
As Bill mentions there is no AVC. Regenerative detectors are very sensitive they don't like strong signals as they tend to overload easily. That's why the volume control is across the antenna to keep the signal low before reaching the detector stage. When it overloads the station it is tuned to will seem very wide. It will take up a lot more space on the dial than a weak one.
Regens make great one or two tube receivers, the detector with a stage of audio after it. Have built a few over the years and I'm always amazed at how well it works for just a tube or two. http://www.flickr.com/photos/42774321@N0...hotostream
Terry
After some playing around, I have made a few minor changes to the Model 81. The changes are shown in the attached drawing (highlighted in green). The changes helped normalize the volume adjustment quite a bit. It is also quite a bit more linear. Prior to this, there was very little volume control range, especially with strong stations. I have adjusted the sensitivity, based on the strength of the stations in my area and don't appear to have any overloading issues. The size of the resister across the Antenna coil may need adjusted based on the strength of the stations in your area.
All of this talk about volume controls in the Model 81 made me think of the Philco Model 600 - second to the last of the cigar box pee-wee Philcos to use only four tubes and a regenerative 2nd detector (the last was the 37-600).
The 600 uses an interesting volume control circuit that I've long though would be ideal for adapting to the 80, 81, 84 or 37-84.
Notice how the 15K volume control is not connected between antenna and ground, but between antenna and the 6A7 cathode (through a 200 ohm resistor). And the center tap of the control is connected to ground. When the volume is at maximum, the 15K of the volume control is across the antenna coil primary, the 200 ohm cathode resistor goes direct to ground, maximum volume from the set. When the volume control is at minimum, antenna is shunted to ground plus there is an additional 15K of resistance in the 6A7 cathode circuit, reducing its gain. So as the volume control is turned up, the 6A7 gain is also increased. This feature, on paper anyway, looks like it would overcome the lack of a good ground. A good ground is necessary for proper operation of the 81, 81, 84 and 37-84 for without a ground, the volume control circuit in these sets simply won't work as designed.
I've never owned a 600 nor have I (yet) tried this volume control circuit, but I may do so in the near future.
Well I messed around a bit trying to adapt the 600's volume control circuit, but didn't have much luck. I basically connected the antenna and volume control like the 600's schematic shows. After doing this, the volume would peak about in the middle of the potentiometer and go down when I moved it in either direction. I tried different size resistors for the 200 ohm one, with not much difference. I did not disconnect the suppressor grid (pin 4 on the 77 tube) or the feedback capacitor (3 turn wrap) from the cathode. I'm not that familiar with the intricacies of the Detector/Oscillator circuit, so was hesitant to do so. Any thoughts are appreciated?
(03-09-2013, 10:53 PM)Bill Bacco Wrote: Beautiful work Terry!
After some playing around, I have made a few minor changes to the Model 81. The changes are shown in the attached drawing (highlighted in green). The changes helped normalize the volume adjustment quite a bit. It is also quite a bit more linear. Prior to this, there was very little volume control range, especially with strong stations. I have adjusted the sensitivity, based on the strength of the stations in my area and don't appear to have any overloading issues. The size of the resister across the Antenna coil may need adjusted based on the strength of the stations in your area.
Hi Bill:
Just completed the changes to my volume control you recommended. I am completely satisfied! I can now control the volume on all strong stations and actually receive more stations than I did before making the changes. Your drawing is simple to follow. I was real disappointed when I had the radio restored, bakelite caps stuffed, etc., only to have stations so loud that I could not listen to it. Grounding was not an option, as there was no way to get to a ground rod.
