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A Solution for Measuring Bias Current

11/21/1997 11:44 PM
John Kos
A Solution for Measuring Bias Current
How would you like to measure bias current without connecting probes to dangerous high voltages (transformer shunt method) or inserting a one ohm resistor in the cathode? I have found a solution that makes both of these unnecessary. It is called a Clip-On DC Milliameter. It has a special probe which can be clipped around any insulated wire and measures the DC current in that wire. That means that measuring bias current is as simple as clipping a safe, insulated probe on a transformer lead.  

Granted, the three I have are old hp model 428B's. They use vacuum tubes themselves. Newer versions must exist, though. This seems to be a safer, simpler, more accurate way of measuring bias current. It would even work with pcb based amps since there are still leads to the output transformer.  

Does anyone else who reads this BBS use such a device? Does anyone know of a newer source for such an instrument?  

Just thought I would share my new found knowledge.  

John Kos

11/22/1997 1:44 PM
Used them all the time back in my power supply design days. They work by Hall effect. The ones I remember are newer than the tube ones you mention, but I suspect that they suffer from the last, greatest flaw of good instrumentation - they are EXPENSIVE. Other than that, they're perfect. I've never mentioned because they're even too expensive for me to keep in my garage, and I have an equipment habit that I hate to admit to (I'm the only amateur that I know of that has TWO distortion analyzers!).  
HP and Tek made some desktop modular plugin Hall probes, but they were well over $1K back in the mid 70's.  
I actually prefer the bias probe approach in most cases. My bias probe is a box with two octal sockets and two DPM's on it, and two cables leading to octal plugs. You can unplug the tubes, plug in the probes, mount the tubes in the bias probe, and then read current directly from both DPM's at the same time, all without opening up the amp at all. Circuit Specialists has digital panel meters with 200mv sensitivity for $10 each, and so the box contains a 9V batter, two 1 ohm resistors, and the two DPM's. It reads directly in ma.
11/22/1997 4:47 PM

You can unplug the tubes, plug in the probes, mount the tubes in the  
bias probe, and then read current directly from both DPM's at the same  
time, all without opening up the amp at all.>>>  
What's the point in that? You have to open the amp to adjust the bias anyway.
11/22/1997 7:02 PM
Steve Morrison

The point is that many amps do have bias adjustments that are accessible without removing the chassis.
11/22/1997 8:07 PM
ken Lewellen

Also, not every amp that has the bias "checked" needs to have the bias "set". Saves time still.  
11/25/1997 10:59 AM
Dave Charneski

I use a bias probe myself and have  
gotten around the "chassis removal"  
problem by drilling a hole in the  
bottom of the amp head right above  
the bias pot. I purchased a LONG  
screwdriver at Sears (guaranteed for  
life!) and I'm in business!  
However, I'd really like to be able  
to externally measure and adjust the  
bias on all four tubes without  
removing them from their sockets or  
removing the back tube-protector  
panel. My tech and I have discussed  
various techniques, including the  
series one ohm cathode resistor  
method, but I haven't been able to  
bring myself to cutting into this  
expensive amp! In addition, no  
permanent bias-measuring technique  
is completely roadworthy and  
foolproof. My tech tells me that  
even his 2W, one ohm series cathode  
resistors in his stereo occasionally  
burn out and need replacing, so, for  
the time-being, it looks like I'm  
stuck with the bias probe...unless  
anyone has a suggestion for a better  
solution! Thanks.  
Dave Charneski  
Dave Charneski
11/26/1997 11:49 AM
Karl Zwengel

If you burn up a 2 watt cathode resistor there is definetly something wrong!!! You should only see a couple of volts across the resistor.  
If you use a half watt resistor and the tube shorts you'll probably take out the resistor. To alleviate this you can shunt a diode from the cathode to ground to save the resistor.

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