Checking the schematic of the above radio - it looks as though the volume control has ( depending how it is pugged to mains) either a negative or live connection to the grid of Tube 12SQ7GT. Is this correct. Looks funny and I am wary of using the set after recent repair.
Frank Goddard. doncaster England.
The radio that you cite is a very common design by many manufacturers called the "All American Five by many Radio Buffs.The frid of the 1st AF Amp (12SQ7 in your set) ties to the wiper of the volume control. One leg of the volume control ties to signal ground (Earth)and the other leg ties to the output of the diode detector (also a part of the 12SQ&). However, the Schematic shows the load side pf the power switch (also part of the volume control)t to be tied to the same ground. The chassis is also tied to the same ground through the 0.25mfd cap. Although disconcerting, this is normal (and dangerous).
In the United States, normal house current (Mains) is 120 V. The 2 poles are known as "Line" and Neutral" The Neutral is tied to ground (Earth) at the Fuse Box. Therefore the chassis of these radios are tied to one leg of the Mains Power, either through a capacitor or directly. As safety standards in the USA improved, all exposed metal parts of the radio had to be made to be insulated from the Mains, and a power interlock scheme was employed. If the radio is from the 1940s or early 1950s (very likely based on the tubes used) any exposed metal parts such as screws, shafts (if knobs are removed), etc (but not pointers, speaker grille, etc) are connected to the chassis and through the above cap, to power.
If you are using a step down ISOLATION transformer to power your radio from Great Britain's 220V Mains Power, then you are safe. in the USA, many hobbyists advocate modifying theradio by using a polarized plug to ensure that the leg of the power supply containing the ground is connected to the Neutral leg, and placing the power switch in the Line (Hot) leg.
Hope that helps.
:
:Checking the schematic of the above radio - it looks as though the volume control has ( depending how it is pugged to mains) either a negative or live connection to the grid of Tube 12SQ7GT. Is this correct. Looks funny and I am wary of using the set after recent repair.
:
:Frank Goddard. doncaster England.
:
The radio that you cite is a very common design by many manufacturers called the "All American Five" by many Radio Buffs.The grid of the 1st AF Amp (12SQ7 in your set) ties to the wiper of the volume control. One leg of the volume control ties to signal ground (Earth)and the other leg ties to the output of the diode detector (also a part of the 12SQ7). However, the sochematic shows the load side pf the power switch (also part of the volume control)t to be tied to the same ground. The chassis is also tied to the same ground through the 0.25mfd cap. Although disconcerting, this is normal (and dangerous).
In the United States, normal house current (Mains) is 120 V. The 2 poles are known as "Line" and "Neutral" The Neutral is tied to ground (Earth) at the Fuse Box. Therefore the chassis of these radios are tied to one leg of the Mains Power, either through a capacitor or directly. As safety standards in the USA improved, all exposed metal parts of the radio had to be made to be insulated from the Mains, and a power interlock scheme was employed. If the radio is from the 1940s or early 1950s (very likely based on the tubes used) any exposed metal parts such as screws, shafts (if knobs are removed), etc (but not pointers, speaker grille, etc) are connected to the chassis and through the above cap, to power.
If you are using a step down ISOLATION transformer to power your radio from Great Britain's 220V Mains Power, then you are safe. in the USA, many hobbyists advocate modifying the radio by using a polarized plug to ensure that the leg of the power supply containing the ground is connected to the Neutral leg, and placing the power switch in the Line (Hot) leg. Lead dress and twisting the pair of conductors to the power switch becomes important toprevent hum.
Hope that helps.
:
:Checking the schematic of the above radio - it looks as though the volume control has ( depending how it is pugged to mains) either a negative or live connection to the grid of Tube 12SQ7GT. Is this correct. Looks funny and I am wary of using the set after recent repair.
:
:Frank Goddard. doncaster England.
:
Many thanks John. You explained the circuit perfectly.
Frank