06-01-2026, 03:37 PM
I did some searching around and I realized I have a Zenith 10-S-155 set in my collection that needed work. It had a double peak on the strongest stations which turned out to be the 6A8 tube. Anyway, back to the matter at hand.
My radio uses the same basic tube lineup. 6H6 Detector, 6K7 IF, 6A8 Osc/Conv, but it also has an 6K7 RF amp, which yours does not. Regardless, the 6H6 detector and AVC circuit has the same -2.5V feed from the candohm and dropping resistors before it gets to the preceding stages for AVC. Yes, the resistors are different values, but the AVC takeoff point is the same. I took some readings that could be used for reference which gives you a good starting point on how it operates.
I used a 1mv (-47dbm) modulated signal at the antenna jack. My AVC test point is the junction between R5&7 in my set. See attached schematic for test point. I also measured voltage on the grid caps of the 6K7 IF and the 6A8. Here is what I found.
AVC test point -7.7V w digital meter, 6K7 cap -4.6V, and 6A8 cap -4.6V.
Keep in mind anytime you touch a meter probe to a grid cap you are de-tuning the stage, but the voltages are still reasonable.
I also tested the voltages in the chart and found a big difference on the grid caps when I used my Simpson 20K/volt meter. This is obviously expected. I think your -2.5V on the grid caps with no signal would be closer to -0.5V as in the manual if you used an analog meter like what was called out in the voltage chart.
I also experimented with 6H6 tubes from my stash. A couple of them started out good, but either dropped down low or went erratic as they warmed up. Tube testers are a good ball park but should not necessarily be taken as gospel. Testing in a radio while monitoring voltages is the best method.
I went further and measured signals off the air from my longwire and was a little surprised. My strongest local was -30dbm or 7.5mv on the analyzer. Weaker stations were in the 80-100uv ranges. So yes, you can conceivably receive stations in the mv signal range.
10-S-155 Schematic
My radio uses the same basic tube lineup. 6H6 Detector, 6K7 IF, 6A8 Osc/Conv, but it also has an 6K7 RF amp, which yours does not. Regardless, the 6H6 detector and AVC circuit has the same -2.5V feed from the candohm and dropping resistors before it gets to the preceding stages for AVC. Yes, the resistors are different values, but the AVC takeoff point is the same. I took some readings that could be used for reference which gives you a good starting point on how it operates.
I used a 1mv (-47dbm) modulated signal at the antenna jack. My AVC test point is the junction between R5&7 in my set. See attached schematic for test point. I also measured voltage on the grid caps of the 6K7 IF and the 6A8. Here is what I found.
AVC test point -7.7V w digital meter, 6K7 cap -4.6V, and 6A8 cap -4.6V.
Keep in mind anytime you touch a meter probe to a grid cap you are de-tuning the stage, but the voltages are still reasonable.
I also tested the voltages in the chart and found a big difference on the grid caps when I used my Simpson 20K/volt meter. This is obviously expected. I think your -2.5V on the grid caps with no signal would be closer to -0.5V as in the manual if you used an analog meter like what was called out in the voltage chart.
I also experimented with 6H6 tubes from my stash. A couple of them started out good, but either dropped down low or went erratic as they warmed up. Tube testers are a good ball park but should not necessarily be taken as gospel. Testing in a radio while monitoring voltages is the best method.
I went further and measured signals off the air from my longwire and was a little surprised. My strongest local was -30dbm or 7.5mv on the analyzer. Weaker stations were in the 80-100uv ranges. So yes, you can conceivably receive stations in the mv signal range.
10-S-155 Schematic
Tony
“People may not remember how fast you did a job, but they will remember how well you did it”
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