Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mindcrazedbanjo on January 04, 2012, 07:51:16 am
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I've made 4 of these probes, when they are all in place and I'm checking one valve with my DMM, what do I do with the unconnected leads from the probes on the other valves? do I connect them together to complete the circuit? or is it ok to just let them hang loose unconnected?
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Don't connect them, only pay attention to the terminals
a little box with a rotary switch to commutate one at a time to the DMM will be of some practical use
Kagliostro
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thank you!
"only pay attention to the terminals"
do you mean don't let the touch the chasis or something?
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I've made 4 of these probes, when they are all in place and I'm checking one valve with my DMM, what do I do with the unconnected leads from the probes on the other valves?
You must short the two banana plug terminals for each bias checker that doesn't have a DMM connected. Otherwise the cathodes of the unused tubes will be open and the tubes will be switched off. See this pic...
(http://www.el34world.com/charts/images/biasprobe3.gif)
A better solution would be to permanently put a 1 ohm resistor across the terminals of each banana plug (4 resistors total). Now set your DMM to measure millivolts instead of measuring milliamps.
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Always thought about this... Why not install the one ohm resistor inside the probe. I find on occasion the meter fuse will blow when the amp is switched from Standby.
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Someone sells an octal bias checker with the 1 ohm resistor mounted between the sockets. But it would be a lot of work to dissassemble the sockets to add the resistor. Much easier to put the resistor directly on the banana plug at this point.
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Thx again sluckey for having the patience, that about nails it for me. :worthy1:
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I made mine from Dougs design. I'm going to build another with the one ohm resister.
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do you mean don't let the touch the chasis or something?
yes, I was meaning that
but I remembered badly the schematic, I remembered the resistor was in, not out of the probe
SORRY
Kagliostro