Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jordan86 on August 10, 2023, 08:08:06 pm
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I’m troubleshooting a very noisy amp that I recently modded (detailed here https://el34world.com/Forum/index.php?topic=30601.0 ) and not sure if this is normal behavior…
I am unable to measure the resistance of my power tubes’ cathode resistor with the amp on. I don’t think that is normal? In standby, I can measure it right at 325 ohms. But when I turn the amp on my MM quits reading any resistance. As I flip the standby off, the meter begins to read up in the 1500 ohm range and then slowly drops down to 325 ohms over about a 10 second timeframe. Does this imply a short somewhere? I currently have:
348v plate to cathode
26v cathode to ground
Temporarily have a variable resistance arrangement wired in. Cathodes go to a 1R resistor>5W wirewound pot>10W 150 ohm resistor to ground. 150uf bypass cap.
The amp also pops quite loud when I flip the standby off (kill B+) which it did not used to do. Not sure if that points to something specific or conclusive. I’ve only ever had that happen with an old amp when turning the standby ON.
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:think1: I wonder what your meter owner's manual has to say about measuring resistance when voltage is present.
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:think1: I wonder what your meter owner's manual has to say about measuring resistance when voltage is present.
You know…something tells me you don’t actually wonder what that manual says :icon_biggrin: But I have my answer nonetheless. Thanks Sluckey.
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From my manual...
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Yes mine said the same :)
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A meter measures resistance by sending a precise known current through the probes, measures the resulting voltage between the probes, and then calculates the resistance from the voltage / current. The current will be small to miniscule, depending on the resistance range.
If you’ve got another meter, you can check that for yourself by setting one meter to resistance, the other to current, and connecting the probes appropriately.
Now consider that if attempting to measure the resistance of something in which a current is already flowing, there’s going to be a voltage across it, so the meter can’t provide a valid reading.
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I’m troubleshooting a very noisy amp that I recently modded… and not sure if this is normal behavior…
Seems like normal behaviour around here. :-)