• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Measuring milliamp with Fluke 771 Clamp Meter Question?

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At up to 5A you can use something like a LEM LA25NP with pretty good accuracy and a bandwidth well beyond audio - so good for isolated connection to an oscilloscope, or to a simple meter. The 771 looks great for rapid idle DC current level, but simple datasheet doesn't indicate AC performance, and no good for scope interface. Can't say I've looked at what's commercially available nowadays, but TEK used to have a great 20A wand for oscilloscope use.
If you put the Fluke clamp over a section of insulated wire then you should achieve a suitable insulation level (the wire insulation would need to be rated suitably). But for general use on any amp you may have a problem, although their is insulation on the wires going to many output transformers.

Sorry about the LEM suggestion - which requires you to provide a +/-15V supply and a load resistance (the current signal is measured across the load resistance as a voltage). The measured anode circuit is connected to the LEM input windings (which are isolated from the load resistor side), and the LEM windings need to be set up for the 5A current range - so it is not a 'clamp' type device and requires the opening of the circuit.

Ciao, Tim
Joined 2009
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One solution, that does require breaking the circuit, is to install a 10 ohm resistor in series with the OPT primary. Then measure the voltage across the resistor. Even a $5 meter can measure voltage with enough accuracy to get the tube bias dialed in. 10 mV across the resistor will equal 1 mA of plate current.

You can leave the resistor in the circuit after you're done. That way you only have to break the circuit once.

alternative measurement technique

For general purpose current measurement without breaking the ciruit you will need to follow one of the previous suggestions. But if what you want is specifically the DC, no signal, plate current of an output tube loaded by a transformer, then you may want to try the following technique I learned from an old radio repair manual.

Just use an ordinary current meter connect across the transformer primary. One lead to the B+ terminal of the transformer, the other to the plate connection. Since the meter resistance is milliohms and the transformer winding resistance is tens or hundreds of ohms, the meter is shorting out the transformer and drawing all the plate current. Yes, the total resistance in the plate circuit is now lower, but the net effect on the current is quite small.

Obviously this is no good for ac measurements, but it is perfectly good for setting bias etc.

As already pointed out by someone, be aware of the meter voltage rating. Also be careful when poking around the B+ with meter probes and alligator clips etc in what may be cramped area of the chassis. You may want to set it all up with the power off first.
The safest way is with permanent test points on the outside of the chassis, assuming it is a diy amp, or one you can mod.
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Yeah I want to avoid poking the B+. My 845 amp has 1098VDC. Permanent test points and bias adjustment on the outside of the amp is a good idea.

Be really, really, really careful with that. IF you want external test points make sure you use connectors that are rated for the voltage and that require the removal of some sort of grounded cap to get to the connector. You really don't want an unsuspecting child to approach those connections with a paper clip. As I'm sure you're already aware of, 1.1 kV does not need a fixed connection to find you. It'll be happy to take a shortcut through the air to kill you... You only get one chance.

Just sayin'...

I have used an olde 5-pin DIN (eg. tape deck in/out) connector for rear panel sensing of output valve bias current levels and HT rail levels.

The 1 ohm cathode current sense resistor has the equivalent mV signal on it relative to 0V/ground, and so that sense pin is not a danger. If the fuse in series with the cathode fails, then there is no mV signal level at the panel connector, although the cathode itself is cold-biased to whatever the cold-bias resistor allows (eg. say 70V for a KT88, and is meant to keep the heater-cathode voltage to within limits).

For HT rails it is worth doing a say 100:1 divider (as part of a typical cap discharge resistor), which would keep sensed voltage for even a 1kV HT rail to just 10V.

Ciao, Tim
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