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Post subject: measuring a tube's bias at idle
Posted: Fri Jun 04, 2010 12:11 am
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There seems to be some confusion concerning this subject. I'll offer my view. This applies to the fixed bias scenario used in most push-pull power sections of Fender amps: A negative voltage is applied to the power tube's grid, via the power transformer, to set the operating parameters of these tubes. This voltage can be measured directly at the grid socket connection.(ie -50 V) and confirmed on the schematic. What we're concerned with is how much current(amps) the power tubes are drawing. This is governed by the negative voltage. In some cases this voltage is set by a resistor, in some cases this is set by the use of a variable resistor control(trim pot). The usual way is to measure this current with an ammeter at the plates. But this can be inconvinient and hazardous. So amp builders have added a precision 1ohm resistor between the tube's cathode and ground. Now a reading across this resistor converts the current measure to a voltage measure(ma-mv). Further, some amps combine the tubes measurment to the test point. So we're not reading each individual tube's condition. Thus, there is a need of matched tubes. Bias probes use the same approach, but measure the tubes seperately, and usually provide a plate voltage. IMO This method is reasonably accurate, but much safer and way more convinient. This is a very basic view, but maybe it can clear up our way of dicussing the subject. Art

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Posted: Fri Jun 04, 2010 5:27 am
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Thanks Art, great explanation. I like the Bias Probes much better. It's simple and you will have a bettre idea of what's going on.
For how important biasing is you would think checking it would be more convenient. Some higher end Egnater models have probe jacks and an adjustment pot right on the back of the amp. My Rebel has them inside. You just plug your red probe into the jack and ground off the chassis with the black one. You always need to have respect for the voltages and use common sense to be safe. Even the Eurotubes video show Bob with his hands in the amp while he's wearing jewelry. :shock:

Boys and Girls, always remove all jewelry, rings, bracelets, necklaces etc before working on an amp whether it's plugged in or not..

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Posted: Fri Jun 04, 2010 7:24 pm
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You know, Supro, actually you were the guy that clued me in to the combined tube reading situation on the Hot Rod series, sometime ago. You said the bias was at 85mv(or some thing like that). I thought, "this guy better be at a launching pad, because we got lift-off or melt-down". I didn't realize this was for both tubes. Glad to hear that the Egnator is doin' a good job for you. I'd like to get into a lighter amp myself. Art

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Posted: Fri Jun 04, 2010 10:20 pm
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If you measure at the cathode, aren't you getting a little grid current in addition to plate current?


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Posted: Fri Jun 04, 2010 10:39 pm
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Yeah, for sure RJ, you're spot on. It's a compromise type of method. Art

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Posted: Fri Jun 04, 2010 11:03 pm
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Definitely better than the shunt method.


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Posted: Fri Jun 04, 2010 11:53 pm
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I don't know, RJ. I've used the Ken Fischer shunt method for years,and still do, if the chassis is on the bench. Only cooked a couple of fuses in my Flukes. I'd have to say his method is more accurate. And I've gone back and checked the measurements against the Weber Bias Rite a number of times. Just because you're trusting the components in the probe. But it always came up close. The only reason I use the probe, is my aging eyes, and if the chassis is in the cab. Art

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Posted: Sat Jun 05, 2010 6:51 am
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I hear what you are saying on the age thing, Art. My eyes aren't what they used to be. I shunt biased most Fender amps. Recently, I started measuring the voltage drop on the output transformer after getting the resistance of each side of the primary. May not be as accurate, but alot safer.


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Posted: Sat Jun 05, 2010 1:56 pm
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Very good idea, RJ. I'll have to give that method a try. It sure would be alot safer!!! Actually, I only work on the older amps now a days, for friends and players I've known over the years. Posted the thread over here because there seems to be more questions about biasing. I will try your method. Thanks, Art

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