Tube Amps / Music Electronics
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|11/21/1997 11:44 PM|
||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.
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|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|
The point is that many amps do have bias adjustments that are accessible without removing the chassis.
|11/22/1997 8:07 PM|
Also, not every amp that has the bias "checked" needs to have the bias "set". Saves time still.
|11/25/1997 10:59 AM|
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
|11/26/1997 11:49 AM|
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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