DC panel meter - anyone try this?

Has anyone here installed a DC mA meter for tube biasing?

This is what I was planning on using: 100mA or 150mA DC Panel Meter
I can find similar items from online retailers, but this site has decent application information.

With four tubes, I either have the option of a very obtrusive (and expensive) row of meters or creating a circuit design that would allow for switching between the four bias circuits. I imagine five positions with the fifth having the meter completely out of the circuits. It's the switching circuit that required the brain-work. Maybe a four plate, five position switch with jumpers that short four of each of the five position on each plate? Silver contacts, of course...its oxide is still quite conductive. Or, a meter-on / meter-off 4pdt switch to totally cut the meter out of the circuit.

Before I start working this out, I thought someone else has done this already. Searching forums has yielded little so far.

EDIT:
Added application diagram.

I plan this for a tester but not using current sensing meters. If a tube flashes your meter is gone.

http://www.ebay.com/itm/0-36-Red-LED-Voltage-Monitor-Meter-DC-0-3-300V-4-Digit-High-Precision-Voltmeter-/391085114372?


Instead i will use 4 digits Voltmeters ranging 2.999V .
In series with tube cathode will be 10ohm resistor, and the voltmeter will be connected with 1kohm
in series AND 2 1n4007 diodes in series from the meter input to ground. These diodes will start
conducting when the voltage across them > 1V thus protecting the meter.
One could have a cap of 0.5 uF decoupling the meters also. The 1k resistor is big enough to
not interfere with the cathode ( and sound)
In your case it's a simple selector that chooses to connect the meter to one of the cathode resistors,
no current interruptions on the tubes, no affection on bias.
Reading is easy, 0.5V equals 50mA ( i don't think i can move the decimal point on my meters)
If one uses 1ohm cathode resistors the meters will show mA . It's a matter of taste .

Your method is much closer to the technique a wiser man than me explained earlier this morning. Measuring the applied voltage seems to be the preferred technique.

Using mA meters for bias ... well, that's just CRAZY talk!



I went a few rounds with getting it right ... final version simply piggy backed the meters onto the bias test points. You have to clip the internal shunt resistors so the meters read as mV, and add a 100 ohm VR to each test point connection to compensate for the additional resistance added by the meters themselves.



Circuit was mostly stolen from a Citation II.



I also added double throw center off switches for each bank. That allows me to use one meter for each pair, and turn the meters off when not actually in use. I still use a VOM for fine tuning, but the meters are great for quickie spot checks when warming up.

Calibration is simple ... I set the bias to 55mV with the meters switched out of circuit, and then tweaked the VRs to get the same reading with the meter on. That's pretty much a one shot deal.

Oh boy... Game on. Again though, I need to build the rig first. But have no doubt, instrumentation is my gig...
Until then, if you're not making X-rays, your voltage is too low.

Main issues I see between Ma and Mv. metering are; one is wired in series and can affect the bias reading. The other is wired in parallel and is passive. I can't see one giving more accurate readings than the other. IMHO there are better reasons to use Mv.
You should also make sure it can be removed from the circuit when not used. I did this with a simple rotary selector switch.The switch made only one meter and one switch necessary. I follow the less things in the signal chain the better.

This reminded me of seeing a review of the Rogue Audio Cronus integrated amp. It has a self-contained biasing system that's pretty ingenious.

baddog1946 wrote:Main issues I see between Ma and Mv. metering are; one is wired in series and can affect the bias reading.  

Main issue I saw with mA vs mV is the mA meters were purtier ...  ;-}

Surprising the limited selection available. Especially anything that has back lighting. The ones I used are surface mount and consistent.



Good deal too, at $10 each. Also easy enough to get inside to snip the dropping resistor. Clear cover pops off, two screws to remove the faceplate, and snip the resistor that bridges the two terminals. I just bent mine out of the way so I could reconnect if things went horribly wrong.



Same process should apply to any such conversion.

Accuracy? As mentioned, that's controlled with the matching pot on each bias point. As is, my "55" reading is arbitrarily set to the tick just below 60, as that's easy to read. Technically, it should be set somewhat below that as each tick represents 2 points on the scale. So shoot me already ...

I still use the VOM to double check every now and then, but feel comfortable setting for minor deviations just using the meters. Readings are well in the ballpark to get a good balance on the amp. If nothing else, they certainly take the work out of tube swapping, as adjusting one tube affects all the rest, and it's a lot easier to just flip a switch and turn a pot then it is to move the probes until you get a good match all around. Once that happens, THEN I'll use the VOM to dial it in perfect.

PS ... these meters are quite large, but that's what I wanted and designed the chassis for. IMHO, the larger the scale, the more accurate the set.

That is exactly what I am going to do. I have a lovely little Simpson round faced job that will do the trick nicely. Current through any of the four 270kohm is what it will be metering.
Why? Because we can.
(Same with the wattmeters calibrated at 8ohm in parallel with the loudspeaker outputs. Since they are RMS, I'm putting in one LED 'warning' light set at 40w peak.)
But first, I've got to build the amp.