Hi
Some resistors in my tube amplifier are mounted with the numbers looking toward the PCB, so I can't read them. I did measure them already but not sure if the results are correct because I did it while they are in the circuit.
Do I need to desolder the resistor on one end or can I measure them in circuit with a digital multimeter?
I did some googling and I am getting contradictory answers.
thanks a lot
Some resistors in my tube amplifier are mounted with the numbers looking toward the PCB, so I can't read them. I did measure them already but not sure if the results are correct because I did it while they are in the circuit.
Do I need to desolder the resistor on one end or can I measure them in circuit with a digital multimeter?
I did some googling and I am getting contradictory answers.
thanks a lot
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That depends on the circuit. When there is nothing that conducts at the meter's test voltage in parallel with the resistor, then you can measure them in the circuit.
@Yoshimitsu
Generally, you need to remove the resistor.
Trying to measure in situ you will get the effective DC resistance for the entire network from the two nodes in question.
Kind regards,
Drew
Generally, you need to remove the resistor.
Trying to measure in situ you will get the effective DC resistance for the entire network from the two nodes in question.
Kind regards,
Drew
This is where hard-core practice in resistor-network problems is useful. There use to be lesson books full of such problems.
A and C are tough and probably unlikely in amplifiers. (Some bias networks look like A.)
For your purpose, you can probably limit your study to "Parallel Resistors", since your meter probes can get past any series resistor. (And of course, "digital" is no better here.)
B is a very likely circuit. And it hinges on another concept. What is "V"? Perhaps a battery which you disconnect (it would fool your meter). Maybe a wall-power supply and you surely must unplug from the wall. But is the "dead" power source an equivalent resistor? What value?
And some parallel mysteries can't be solved in situ. Two parallel resistors read 100 together. No visible marks. Is it two 200 Ohm parts, a 220 and a 183, a 200 and a 1Meg?
A and C are tough and probably unlikely in amplifiers. (Some bias networks look like A.)
For your purpose, you can probably limit your study to "Parallel Resistors", since your meter probes can get past any series resistor. (And of course, "digital" is no better here.)
B is a very likely circuit. And it hinges on another concept. What is "V"? Perhaps a battery which you disconnect (it would fool your meter). Maybe a wall-power supply and you surely must unplug from the wall. But is the "dead" power source an equivalent resistor? What value?
And some parallel mysteries can't be solved in situ. Two parallel resistors read 100 together. No visible marks. Is it two 200 Ohm parts, a 220 and a 183, a 200 and a 1Meg?
In many tube circuits the plate, cathode, and grid resistors will measure right, because there is no DC path around the resistor.
In a tube power amp, the feedback resistors will not measure right, since the output transformer secondary provides a path
around them. Just look at the schematic and see. Of course, wait until the power supply is fully discharged first.
Who cares, they're in parallel and it's 200 ohms. 😉 Modern DVMs will be confused by any residual circuit voltage on charged caps. Even a little can skew readings by a lot. If you discharge caps and keep them shorted you'll get more predictable results. Just remember to remove your shorts. 😆 Here's an important point- you can't put anything in parallel with a resistor to increase its value. Thus, if you don't have residual voltage and see a higher than expected reading, the resistor is probably bad. That's how to spot a lot of carbon composition resistor problems, as they always drift high, not low. Another trick to avoid calculating things- check both channels. If they match, odds are lower that there's a problem. Not zero, but lower.
@MarcelvdG I don't understand. What schematic is that for? OP said he was trying to determine what the values are.
@Yoshimitsu without some pictures, (or better yet an attempt at a schematic) it is very hard to say whether the resistors in question can be measured without removal.
Kind regards,
Drew
@Yoshimitsu without some pictures, (or better yet an attempt at a schematic) it is very hard to say whether the resistors in question can be measured without removal.
Kind regards,
Drew
Yoshimitsu sent me some things by PM, the schematic of post #8 has been derived from that. Not all resistor values are filled in.