10-21-2013, 04:05 PM
Buzz
What sch tells you is probably indeed the resistance which is 140 Ohm. Yours is 49 Ohm. This is OK. In this case the sch does not rely on your DC primary resistance to set the working point.
You should worry about the total impedance of the primary, as Terry suggested. The impedance is a complex thing but in first approximation you could consider it to be your voice coil impedance reflected via the transformer which is simply multiplying it by the square of the turn ratio.
For example if your voice coil is 0.7 Ohm and your turn ratio is 100 then your reflected impedance is 0.7x10,000 which is 7kOhm.
Potentially (not necessarily in your case) the impedance mismatch may cause:
1. Loss of power (the impedance should be matched for the optimal transfer of power from the tube to the voice coil, this is in fact why we use the output transformers - to match impedance).
2. If the impedance is badly out of range the working zone of the tube may slide to the non-linear zone, hence distortion. Again, may or may not. There is range where the device is linear and it is not that narrow that a step left or right results in distortion.
3. And of course, there are inductive part that coupled with other elements (parasitics or intentionally used capacitances etc, plus the output impedance of the tube) affects the gain on various frequencies, so you may have less of some and more of some others. This is in small radios is not that important as the quality of the sound in the first place is not, well, exactly Hi-Fi, plus it can be corrected somewhat with tone control.
the easiest thing is first to look at your audio signal quality using an audio gen and a scope (or simply look at the received sound with the scope - see if it is distorted - clipped, clipped unevenly, etc), and see where (at what stage) it gets the distortion.
What sch tells you is probably indeed the resistance which is 140 Ohm. Yours is 49 Ohm. This is OK. In this case the sch does not rely on your DC primary resistance to set the working point.
You should worry about the total impedance of the primary, as Terry suggested. The impedance is a complex thing but in first approximation you could consider it to be your voice coil impedance reflected via the transformer which is simply multiplying it by the square of the turn ratio.
For example if your voice coil is 0.7 Ohm and your turn ratio is 100 then your reflected impedance is 0.7x10,000 which is 7kOhm.
Potentially (not necessarily in your case) the impedance mismatch may cause:
1. Loss of power (the impedance should be matched for the optimal transfer of power from the tube to the voice coil, this is in fact why we use the output transformers - to match impedance).
2. If the impedance is badly out of range the working zone of the tube may slide to the non-linear zone, hence distortion. Again, may or may not. There is range where the device is linear and it is not that narrow that a step left or right results in distortion.
3. And of course, there are inductive part that coupled with other elements (parasitics or intentionally used capacitances etc, plus the output impedance of the tube) affects the gain on various frequencies, so you may have less of some and more of some others. This is in small radios is not that important as the quality of the sound in the first place is not, well, exactly Hi-Fi, plus it can be corrected somewhat with tone control.
the easiest thing is first to look at your audio signal quality using an audio gen and a scope (or simply look at the received sound with the scope - see if it is distorted - clipped, clipped unevenly, etc), and see where (at what stage) it gets the distortion.
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