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Old 2nd April 2012, 04:24 PM   #1
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Default Help hooking up panel ammeter for bias adjust

Hi,
I am in the process of reconstructing a GTA SE-40 point to point. I found a nice little panel ammeter that measures from 0-500mA and wanted to use something like this for my bias adjustment on the output tubes. The bias measurement taps are before each of the three cathode resistors and ground and i was wondering how one would implement something like this...i.e. how do you connect an ammeter in a circuit like this to measure the cathode current. I was going to use one per side with a three pos rotary switch to switch between each tube. I have neve use oneof these ammeters before...it has a voltage supply for the meter and a simple in and out...what do I hook the in and out to? Surely all the current is not meant to pass through these little meters is it? Currently on this amp i believe the bias was intended to be measured by checking the voltage across the 11ohm cathode resitor...i could put a voltmeter across the resistor but i wanted to use an actual ammeter....
May sound like a silly question but i have never used an ammeter before, i always have used a voltmeter across a resistor to check current....
Thanks!
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Old 2nd April 2012, 04:25 PM   #2
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Here is the meter i found....
Digital DC 500MA Current Panel Meter Ammeter (5V Power) | eBay
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Old 2nd April 2012, 04:46 PM   #3
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You need to wire it in series with the load you are trying to measure. IMHO a good voltmeter across a convenient resistance value like 1 or 10 ohms will be much easier to implement.

You can use this meter but must arrange it to insert the meter in series with each cathode resistor independently of the others. The simplest route would be to use a 4P4T rotary - you would just ignore one of the decks. You could for example wire position 1 of all three "decks" to gnd the cathode connections of all three tubes, position 2 / deck 2 would insert the meter in series with the first tube, position 3 / deck 3 would insert the meter in series with the second tube, and position 4 / deck 4 would insert the meter in series with the third tube. All other contacts to gnd. Wipers go to cold end of each cathode resistor which will ground through the meter switch. Use a make before break rotary switch to avoid pops as you switch it.

Switches like the one mentioned above sell for just a couple of bucks at digi-key and mouser. (And probably eBay)
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Old 2nd April 2012, 05:02 PM   #4
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This meter will measure current flow through it. For that reason, you will have to place it in SERIES with the resistor, for each of the tubes. If the meter is rated at 500 ma, that implies the meter has an internal shunt in it, that will handle the 1/2 amp (500 ma) of current.

You will have to switch the meter into the path of each resistor. A 4P4T rotary switch will do it, but I'd suspect you'd spend a lot more on the switch than on this meter.

You'd also have to find a 5v DC power supply, internal to your amp, to drive the meter. Since you are talking a vacuum tube amp, you might be able to tap of the filament supply (typically 6.3v or 12.6v AC), rectify it, filter it, and drop it down to provide 5v DC. But candidly, if you can't figure out how to wire a milliameter, do you really want to dink with your amp, and modify it for the 5v dc requirement?

Might be best to stick with a voltmeter, to set bias.
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Old 2nd April 2012, 05:56 PM   #5
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rather just find a 200MV panel meter

put 1 ohm resistors in series whit the cathode network . and switch connections whit the 4p4t switch earlier mentioned.

Be cautious ! the panel meter will likely need its own separate supply get a 3Va epoxy'd tranny .


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Old 2nd April 2012, 08:02 PM   #6
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Quote:
Originally Posted by CanAm Man View Post
But candidly, if you can't figure out how to wire a milliameter, do you really want to dink with your amp, and modify it for the 5v dc requirement?

Might be best to stick with a voltmeter, to set bias.
i knew this was coming...just because i have never used one of these digital ammeters doesnt mean I dont know a lick about building tube amps....
Yeah, i know how to hook up its power supply, i was just feeing a little dubious about passing the current through one of these....i thought they may actually work similar in fashion to measuring voltage across a resistor....which is probably exactly what it does inside the contraption....

Actually, putting a voltemeter across each of the cathode resistors is what I had originally envisioned but I habent been able to find a panel voltemeter that works in the range of milivolts....
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Old 2nd April 2012, 08:09 PM   #7
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Found it....didnt't think to search for 200mv, kept searching for milivolt and othing would come up....
3 1/2 LCD Blue Digital VOLT Panel Meter DC 200mV | eBay
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