Resistors - what to check with power off
Posted: Thu Sep 28, 2023 10:44 pm
Tube equipment affords us the freedom to get accurate ohm readings on a modern DMM (digital multimeter) from transformers, pots, wirewound power sandblocks and the lowly carbon resistors. I check all this on a unit BEFORE diving in on a recapping and ordering parts.
Checking resistors has proved that minor issues add up through the stages to bigger issues such as a weak channel, after finding one 12AX7s plate resistor at 560K and the other right on 470K!
The plate resistors determine the amplitude of the output passed to the next stage via capacitor and being selected along with cathode resistance for biasing, determine the tubes critical operating point and quiescent voltages we use for reference. Measuring from tube grid and cathode pins to chassis ground can clear the resistors there, usually with no part disconnection required.
Buying a mass selection of 1/2 watt carbon resistors is better than trying to upgrade to alternate materials 1 x 1 that may introduce inductance and even more importantly, arent rated for voltages up to 500 volts! The phantom capacitance of metal film resistors adversely affects tuner and IF circuits, all as duly noted by Shang066.
Make sure when you order them, research what voltage they are rated for. Double all your quantities, if 5 then get 10.
Sand blocks are 3-5-7-10 watts and wirewound in most all cases, only fail completely and measure very close to face value. I have found them very reasonable online. The sand bars and metal-clad power resistors used in older Zeniths are always suspect due to exposed dissimilar metal connections that get heated. They fail often.
A word about re-using old resistors. Carbon resistors drift high just sitting in a parts drawer, test any that you intend to use first. I need to separate some Don't assume they are GOOD if shiny and clean with bright colors if you can see that as well.
I have a bunch of NOS dogbones, carbon sticks with wrapped wires at both ends used in early 30s radios, all separated based on resistance range, to use on various projects. They actually don't fail but are rarely close to their marked values
Checking resistors has proved that minor issues add up through the stages to bigger issues such as a weak channel, after finding one 12AX7s plate resistor at 560K and the other right on 470K!
The plate resistors determine the amplitude of the output passed to the next stage via capacitor and being selected along with cathode resistance for biasing, determine the tubes critical operating point and quiescent voltages we use for reference. Measuring from tube grid and cathode pins to chassis ground can clear the resistors there, usually with no part disconnection required.
Buying a mass selection of 1/2 watt carbon resistors is better than trying to upgrade to alternate materials 1 x 1 that may introduce inductance and even more importantly, arent rated for voltages up to 500 volts! The phantom capacitance of metal film resistors adversely affects tuner and IF circuits, all as duly noted by Shang066.
Make sure when you order them, research what voltage they are rated for. Double all your quantities, if 5 then get 10.
Sand blocks are 3-5-7-10 watts and wirewound in most all cases, only fail completely and measure very close to face value. I have found them very reasonable online. The sand bars and metal-clad power resistors used in older Zeniths are always suspect due to exposed dissimilar metal connections that get heated. They fail often.
A word about re-using old resistors. Carbon resistors drift high just sitting in a parts drawer, test any that you intend to use first. I need to separate some Don't assume they are GOOD if shiny and clean with bright colors if you can see that as well.
I have a bunch of NOS dogbones, carbon sticks with wrapped wires at both ends used in early 30s radios, all separated based on resistance range, to use on various projects. They actually don't fail but are rarely close to their marked values