Re: SX-28A Hum
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Richard Comments inserted in the text below. Regards, Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
On Monday, February 17, 2025 at 03:09:24 AM CST, Richard Knoppow via groups.io <1oldlens1@...> wrote:
Ch2 and C-43 form a parallel resonant circuit, a tank, if you will. It is in the plate lead of the pre amplifier half of the 6SC7. When the switch is in the position marked IN on the diagram it is shorted out by the switch. The plate load of the tube is then R-37. No, the plate load for the 1st triode section is R37 and R38.? The voltage gain of the 1st audio stage is approximately the plate load resistance divided by the cathode resistance or 147k / 1k? or 147 when SW10 is in the IN position.??All of those bypass capacitors that are sprinkled throughout the audio amplifier cause a fast roll off of the midrange audio so that the bass frequencies are emphasized because of the higher gain. When the switch is in the OUT condition, the tank forms the plate load and R-37 is shorted out. What about R38?? In the out position, the voltage gain of the 1st triode is approximately 47k / 1k or 47.? Figure 11 shows this gain reduction when SW10 is in the OUT position so Figure 11 actually agrees with the schematic. Note that C-44 along with R-48 is a bypass filter for the B+ going to both 6SC7 plates. Since the plate load with the switch in the IN position is resistive there should be no frequency discrimination. In OUT the plate load is a resonant choke (about 1100 Hz). Again, what about R38?? It is still in series with the parallel combination of CH2 and C43.? In the OUT position, the frequency response of the 1st Audio is now relatively flat so there is no base boost compared to what you see in the IN position curve.?? I was surprised the frequency is not lower but calculated it a couple of times. I have not looked up the plate resistance of the tube. R-36 and R-37 are 100K. C-44 is 10uF and R-48 is also 100K so there should be no audio at that point even down to quite low frequencies. You are considering the electrolytic capacitors to be ideal.? The components in the late 1930s and early 1940s were far from that.? The ESL and ESR were not controlled back then.? Modern components are much closer to the ideal but still aluminum electrolytic capacitors start to become inductive at midrange audio frequencies.? This is why they are not recommended as coupling & bypass capacitors for high end audio.? They cause distortion. As far as fidelity the SX-28 was intended to be a good fidelity receiver for AM broadcasts. Like the Super-Pro, it is designed to have a wide IF and, for the time, a relatively high quality output amplifier. This begs the question, what is the speaker that is used with this SX-28?? Could it simply be the lower gain of the 1st audio amplifier when SW10 is in the OUT position that causes the normal 120 cycle hum to disappear into the mud?? If so, then there is actually no problem in the SX-28 audio amplifier.? We are chasing our tail. While broadcast stations in the old days were required to have good performance to about 10Khz (double the standard now) few receivers could recover much beyond perhaps 4 or 5 Khz. Hallicrafters offered a "High Fidelity" speaker for use with the SX-28, a bass-reflex made by Jensen. The bass boost offered is, IMO extreme according to the response chart. There is a crude high frequency control, all roll off, probably to reduce the effect of static and other noise. A HF boost would have required another stage of amplification. They knew how to do it, see the old (third edition) of the RDH for some circuits. I think the labeling of the BASS control in the handbook is an error even though it got continued in the 28A. If you consider that BASS really means BASS BOOST then what is happening will make more sense.? Someone with an actual receiver can determine it for us, I am just guessing from what's on the schematic. I am strongly suspicious of the tube, easy to prove by substituting?it. On 2/16/2025 11:11 PM, Jim Whartenby via groups.io wrote:
So the hum in question might just be 60 cycle leakage instead of the --
Richard Knoppow Los Angeles WB6KBL SKCC 19998 |
Jim Whartenby