06-13-2022, 08:56 PM
Hi, Phil27,
If the center tap of the HV Section of the transformer goes to the 245 Ohm section, then yes, there will be a big issue with DC voltages. If open and the HV center tap was still connected to the open leg, then there would be no "B-" and very little current.
If the HV Center tap is connected to the center tap of the resistor (chassis ground), then the grid bias of the '42 Audio Output would be 0V instead of the usual -16.5V on the grid. This would cause the tube to draw much current, causing a drop in B+ and likely causing damage to the tube or output transformer if the condition continues.
The standard tolerance for most resistors is 20%, therefore replacing a 245 Ohm Resistor with a 250 Ohm resistor is fine (this is an error of about 2%. The issue is that the 1 Watt resistor would probably smoke in time. you can do it temporarily, just to continue testing the radio, but measure the voltage drop and order a higher wattage resistor.
You might get away with a 2 Watt unit, but I would use a 5 Watt resistor at minimum. since this resistor is between the B Voltage Return and the rest of the radio (Chassis), the vast majority of High Voltage current passes through this resistor. Granted, if one applies Ohm's Law to determine the current through that resistor, and therefore the wattage needed, the 1 watt may pass the math, but there is a reason that they used wire wound resistors for that one.
Hope this helps.
Best Regards,
John "MrFixr55"
If the center tap of the HV Section of the transformer goes to the 245 Ohm section, then yes, there will be a big issue with DC voltages. If open and the HV center tap was still connected to the open leg, then there would be no "B-" and very little current.
If the HV Center tap is connected to the center tap of the resistor (chassis ground), then the grid bias of the '42 Audio Output would be 0V instead of the usual -16.5V on the grid. This would cause the tube to draw much current, causing a drop in B+ and likely causing damage to the tube or output transformer if the condition continues.
The standard tolerance for most resistors is 20%, therefore replacing a 245 Ohm Resistor with a 250 Ohm resistor is fine (this is an error of about 2%. The issue is that the 1 Watt resistor would probably smoke in time. you can do it temporarily, just to continue testing the radio, but measure the voltage drop and order a higher wattage resistor.
You might get away with a 2 Watt unit, but I would use a 5 Watt resistor at minimum. since this resistor is between the B Voltage Return and the rest of the radio (Chassis), the vast majority of High Voltage current passes through this resistor. Granted, if one applies Ohm's Law to determine the current through that resistor, and therefore the wattage needed, the 1 watt may pass the math, but there is a reason that they used wire wound resistors for that one.
Hope this helps.
Best Regards,
John "MrFixr55"
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
Best Regards,
MrFixr55
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