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Re: A Solution for Measuring Bias Current

11/26/1997 7:27 PM
Re: A Solution for Measuring Bias Current
I was thinking the same thing earlier this morning Karl, but I had to get to work!  
That's like close to 2 amps!  
Maybe it is a 200- 300 watt stereo.  
Probably more like a blown tube that has gone dead shorted,  
11/22/1997 5:37 PM

I manufacture probes for use with your existing milliammeter.  
They're on the "Test Gear for Sale" classifieds, here.  
(I hope the shamless plug wasn't against the rules, but it seemed to fit the context...)
11/24/1997 7:57 AM
J Epstein

>I'm the only amateur I know of that has TWO distortion analyzers!<  
Milchedich & Fleischedich?  
Happy Holidays,  
11/24/1997 1:11 PM

RG, is there a schematic available for this somewhere?
11/24/1997 2:12 PM

Hand scratched only, right now. I guess I could gen one up if there's interest. But:  
There's not much to it. Just the raw mechanicals, box and cables, a plug/socket combination with the 1 ohm resistors to read current. Then two digital panel meters, Circuit Specialists part number PM128A ($9.90 each in ones in their last catalog) and a 9V battery clip. The DPM reads +/- 199.9 mV directly as it comes, so it ties directly across the 1 ohm resistors. This is properly an mechano-electrical rather than an electronic project.  
A purist might think that using two DPMs with two different 1 ohm resistors could lead to different scale factors - and it could. I find that I prefer the ease of use of having two meters simultaneously to the precision and lower cost of using the same 1 ohm and meter and switching between the two tubes. This lets me match currents on amps that do that, and see how matched output tubes are (and how well they stay!). They're only going to be a count or two different from "perfect" anyway. It's possible to trim the higher value "1 ohm" resistor down to match the lower value "1 ohm" resistor with a parallel resistance to any desired degree of accuracy, of course.  
It was a modest pain to get two octal plugs removed from two dead output tubes to make the cables, but I did get that to work. I recently encountered pre-made octal plug (apparently these were common in the Golden Age) that are ideal.  
You have to pay some attention to insulating the wires, as the plate pins carry full B+ over to this box. Use 600V rated wire minimum, better rated wire insulation if you can get it.  
Not much schematic to it, but a neato device. Your costs are about $20 for the DPM's, maybe $10 for a good looking box if you don't already have something usable, then whatever the plugs, sockets, wire, 1 ohms, and battery clip cost you.
11/25/1997 2:10 AM
Steve Ahola

Pittman's book mentions a Bias Probe available from Groove Tubes that plugs in between the output tubes and the sockets, and evidentally has some jacks to plug in your meter. Do you know anything about them (like are they horrendously over-priced?) For the amp mod I did, I put in high-quality banana jacks to measure voltages and bias current (I looked at the cheaper Radio Shack variety and was sure that they would electrocute either me or the amp...)  
I hadn't studied the amp schematic for quite awhile and recently decided to try measuring the bias current using the transformer shunt method with the tubes set for cathode bias- it immediately shorted out the fuse I added to protect the output transformer. Well, I guess that is better than shorting out the output transformer! (Should have left in those 1 ohm resistors on the cathodes but I was running out of room on the chassis to add more banana jacks...)  
Torres book didn't explain that there is no output to the speakers when you measure bias with the transformer shunt method- I thought I hooked up something wrong so I pulled the lead out of the jack only to get a spark that rivalled many 4th of July fireworks displays!  
After those experiences, I see a LOT of advantages to the box you made up!  
Steve Ahola
11/25/1997 6:33 AM

In a push-pull amp, you should still hear sound coming through one output tube & half the transformer. Forgive me for asking, but did you shunt the meter from plate to plate, rather than one plate at a time to center tap?  
I can't see how the fuse could have blown. Since you mentioned "no output to the speakers", were you trying to set idling current while driving the amp?  

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