Tube bias testers, your opinions, please.

[Nihilist]

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-T...5|66:2|65:12|39:1|240:1318|301:1|293:1|294:50

Thanks for your input.



..................Blake

 

[SY]

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.

 

[porkchop61]

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.

Glenn

 

[SY]

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.

 

[Trout]

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.

An externally hosted image should be here but it was not working when we last tested it.

 

[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

 

[disco]

Indeed, the same priciple goes with the plate current.

If you want to set the bias permanently look at this lil' toy.

 

[SY]

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.

 

[Nihilist]

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 ?


.................................Blake

 

[SY]

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.

 

[Nihilist]

If I place a 1 ohm resistor between my cathode and ground, this is parallelling the cathode bias resistor, which is NOT what I want to do. What am I missing here ?

I don't want to run A/B. I was under the impression that when running a single cathode bias resistor without a bypass cap (differential) that you MUST run class A , or that the amp will have major distortion when it goes into B.


Would it help to see a schematic ?

http://img101.imageshack.us/img101/5166/myampschematic169aarevizj4.png

........................Blake

 

[Tom Bavis]

If you have cathode bias, no need to add a resistor - just measure across the one that's there. I = V/R

 

[Nihilist]

OK, that's what I previously thought. Sometimes I get a little turned around with all the different options. . . .


So I have 60Ma across my Cathode resistor, according to Ohms law. So that is 30 Ma per tube, or 11.25W give or take.

For a triode tied 6P6S , this is about 85% of Max power at the Anode (13.2W).

For my situation using differential bias on the outputs , should I bias it up higher, closer to the theoretical max ?

What is the practical upper limit , 90% , 95% of Anode Max ?


.......................Blake

 

[Hi_Q]

I made my own unit-

http://homepage.ntlworld.com/lez/YAQIN MC10/Valve bias meter pic_1.JPG

http://homepage.ntlworld.com/lez/YAQIN MC10/Valve amp bias current checker.TIF

Although intended to check out my Yaqin EL34 amplifer, it has been very useful for matching up tubes in guitar amplifiers such as Marshall's.

I have been experimenting, with some success, a 100/120Hz low pass filter for feeding an oscilloscope. It enables one to crank up the scopes gain without noise issues and observe the ripple on the amplifiers output. After setting the correct bias on both PP output valves, although the ripple is so low, the filter enables one to fine tweak the bias on one tube for minimum ripple.