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Old 8th March 2009, 03:54 AM   #1
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Default Tube bias testers, your opinions, please.

Looking to buy a "plug-in" tube bias tester. Any brands you are familiar with or can recommend ?

It seems most brands on Ebay measure the cathode current. Is this equivalent to measuring the Plate/Anode current ? Seems as though it should be pretty close.

http://cgi.ebay.com/AMP-HEAD-Bias-Te...3A1%7C294%3A50

Thanks for your input.



..................Blake
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Old 8th March 2009, 12:33 PM   #2
SY is offline SY  United States
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Cathode current is plate plus screen current. That's dominated by plate and you can use it to a good approximation. However... all it is is a 1R resistor. You could just solder one in permanently (though most amps already have a current sensing resistor) and save $28.85.
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Old 8th March 2009, 03:08 PM   #3
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This brings up a question I have about these adaptors.
I just built one of these myself, and I'm going to add the 1R resistor externally instead of having it inside the adapter. My question is, these types of bias measuring devices can't really be used on SE amps, can they? The reason I ask is that typically the SE amps I have run high wattage power resistors as the cathode bias resistor (10-25w). They don't make 10W 1R 1% power resistors, do they? I'm afraid that I'll cook the 1R resistor on the adapter if I just use an average 1/2W resistor. Any suggestions?
Sorry for the hijack, but this question is kind of relevant.

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Old 8th March 2009, 03:22 PM   #4
SY is offline SY  United States
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You don't need high power for that resistor. Suppose that your tube is run at 100mA (which is pretty stout). The wattage dissipated in a 1R is P = I2R = (.01)(1) = 10mW. The resistor doesn't care if it's a push pull or single ended amp.
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Old 8th March 2009, 03:38 PM   #5
Trout is offline Trout  United States
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I just mount a couple Tip Jacks from mouser on the rear of the chassis, add a couple of 1 ohm 1 watt resistors, very simple setup.
It's plenty accurate enough and easy access.

Heres one I have put hundreds of hours on. Just plug the cheapo meter in on the 200mv scale and your good to go.

Click the image to open in full size.
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Old 8th March 2009, 09:45 PM   #6
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If it is just as simple as measurig the voltage from the Cathode through a precision 1 0hm resistor, and the Mv=Ma , then couldn't I do that to the Plate and get a more accurate reading (no screen draw ) ?

Thanks for the info !




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Old 8th March 2009, 10:00 PM   #7
disco is offline disco  Netherlands
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Indeed, the same priciple goes with the plate current.

If you want to set the bias permanently look at this lil' toy.
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Old 8th March 2009, 10:55 PM   #8
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Quote:
Originally posted by Nihilist
If it is just as simple as measurig the voltage from the Cathode through a precision 1 0hm resistor, and the Mv=Ma , then couldn't I do that to the Plate and get a more accurate reading (no screen draw ) ?

Thanks for the info !




.......................Blake
Yes, you can but you need two terminals and they'll be at deadly dangerous voltage. Measure at the cathode.
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Old 8th March 2009, 11:05 PM   #9
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Ok,

So I'll need to hook up one end of the resistor to the cathode , and the other end floats ? Attach one lead of the VM to each side of the 1 ohm resistor , and Mv=Ma ?

I am running only 1 cathode bias resistor for both tubes, so I would need to divide the Mv/Ma# by two , right ?

Then subtract the cathode bias voltage from the Anode voltage, take that # and multiply it by the Ma's to get total power for the Anode ?

Anode V= 405V

Cathode Bias V= -30V

Actual V= 375V

Ma's/2 x 375V= Total Power across Anode


I am running my output tubes with a single cathode bias resistor without a bypass capacitor (differentially). As far as I understand it, I need to run the outputs in Class A for this to work properly. I am planning on running them at about 90% of rated Anode power. Is this the correct way to go, or should I run them at a higher or lower percent ?


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Old 8th March 2009, 11:17 PM   #10
SY is offline SY  United States
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No, the resistor goes from cathode to ground. The measure the voltage between the cathode end and ground and you're good to go. An ohm will have hardly any effect on the operation of the stage so don't worry too much about running AB.
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