Bias panel meter problem.

[maynard999]

I built a set of KT88 PP mono blocks with Poseidon boards recently. I installed a 100mA panel meter in each as well as test points (have a look by going to the Poseidon forum). The test points make it really easy to insert DMM probes and bias in seconds. The panel meter was meant mainly for aesthetics. I never planned to use that to bias the tubes. It's not accurate and would involve a switch to back and forth between tubes.

So the problem is I can't figure out how to wire it in without affecting the measurements from the test points. If I put it in parallel on one tube, then the test points on that tube are way off. If I put it in series with one of the tubes then I'd have to have some sort of jumper at the test points and the internal resistance of the panel meter would throw things off.

A DPDT switch (it's backlit) to turn the panel meter off when wired in parallel would work, but I don't want to add another switch if I can help it.

Any ideas about how to make everything work together?

[TomMcNally]

Here's an old thread on the subject.



http://www.diytube.com/phpBB2/viewtopic.php?t=2214&highlight=meter+metre+switch

A voltmeter with a series resistor is a better way to go.

You'd need at least two switches to use the meter you have,

one to switch between tubes, and one to short it out when

not in use. If you are concerned with the series resistance of

the meter, adjust the 10 Ohm cathode resistor to compensate

when the meter is switched in. Instead of the normal 10 Ohms,

use the meter plus 8 Ohms or whatever.



The bottom post in the thread talks about opening the meter

and removing the shunt. Good idea.



... tom

[mesherm]

A milliamp meter is usually a voltmeter in parallel with a low value shunt resistor inside or connected across the back terminals. To convert it into a voltmeter remove the internal shunt resistor. Then using a potentiometer in series with the meter apply exactly 1 volt and adjust the series potentiometer until the meter reads 100 ma. full scale. Now disconnect and measure the potentiometer value and replace it with a 1% resistor. Now you have a voltmeter. Now just add 10 ohm 1% resistors from each power tube cathode to ground. Then get a SPDT (on-off-on) switch with center position off and connect one pole of each end position to the cathode of each power tube. Connect the center pole to the positive side of the voltmeter and ground the negative side. The center position will be the null position when not being used.
Now when you bias your tubes you can switch to the power tube being adjusted and the meter should read out in 0-100 ma.

[maynard999]

Had an either/or setup going on already. The wishlist was the panel meter or the test jacks would all play nice.

I ordered some on-off-on DPDT switches last night. Must have read the mind of the author of the last post. It'll be a clean setup. It'll also give me an option to kill the light on the panel meter.