10-21-2016, 07:24 AM
Arran;
Before the advent of the superhet design, there were lots of tuned radio frequency radios (TRF). Those had multiple RF amplifiers, each with its own tuned circuitry in order to have adequate selectivity. Selectivity is more of an issue when tuning a weak station next to a strong station. The selectivity helps to keep the strong station from wiping out the weak signal station. Philco had some high end radios in the middle to late 1930s that even had IF circuits with adjustable selectivity. RF sensitivity is more of an issue on shortwave bands, especially the higher frequencies. On the AM broadcast band not much sensitivity is really needed. I have a Hallicrafters SX-25 which has two 6SK7 RF amplifier circuits. On the AM broadcast band one RF amplifier stage is not used. A 4-gang tuning capacitor is used along with a 4-gang fine tuning capacitor. It has two IF amplifier stages with switchable extra coils in the IF transformers plus a crystal filter for CW and voice only reception. It does do a better job of receiving shortwave stations than the Philco 40-201, but then it is not a broad-band radio with the best audio frequency response. The 40-201 has limited use on shortwave bands as its tuning is coarse and very touchy on the highest shortwave band.
It is interesting to see the variations in design offered by various companies during the 1930s and 1940s. A significant number of people were keeping up with foreign news due to events leading up to WWII. After WWII interest in shortwave broadcasting dropped off. Consequently the number of sets offering shortwave reception declined.
Joe
Before the advent of the superhet design, there were lots of tuned radio frequency radios (TRF). Those had multiple RF amplifiers, each with its own tuned circuitry in order to have adequate selectivity. Selectivity is more of an issue when tuning a weak station next to a strong station. The selectivity helps to keep the strong station from wiping out the weak signal station. Philco had some high end radios in the middle to late 1930s that even had IF circuits with adjustable selectivity. RF sensitivity is more of an issue on shortwave bands, especially the higher frequencies. On the AM broadcast band not much sensitivity is really needed. I have a Hallicrafters SX-25 which has two 6SK7 RF amplifier circuits. On the AM broadcast band one RF amplifier stage is not used. A 4-gang tuning capacitor is used along with a 4-gang fine tuning capacitor. It has two IF amplifier stages with switchable extra coils in the IF transformers plus a crystal filter for CW and voice only reception. It does do a better job of receiving shortwave stations than the Philco 40-201, but then it is not a broad-band radio with the best audio frequency response. The 40-201 has limited use on shortwave bands as its tuning is coarse and very touchy on the highest shortwave band.
It is interesting to see the variations in design offered by various companies during the 1930s and 1940s. A significant number of people were keeping up with foreign news due to events leading up to WWII. After WWII interest in shortwave broadcasting dropped off. Consequently the number of sets offering shortwave reception declined.
Joe
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