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Which bias method is the most accurate?

4/23/1999 2:23 AM
Which bias method is the most accurate?
I'm familiar with the voltage drop across the 1 ohm resistor on the cathode method, the ammeter and tap method and the ammeter in series with the B+ methods of biasing an amp, my question is, which of these methods is the most accurate way to bias a tube?  
Thanks for the help.  
4/23/1999 5:00 AM

You have to consider the accuracy of every part of the process. For example, if you're looking at the cathode resistor method you have a percentage error for the resistor, and a percentage error for the voltmeter. You have to add these percentages to get the error.  
The transformer shunt method's reliability depends on the ratio of the transformers resistance to the ammeter resistance. It's very accurate if your ammeter is very low resistance, but of course you have then disturbed the circuit's resistance, so the idle current you read is not real.  
The only truly accurate way of doing this is to put the ammeter in series with the B+ going through the OT winding in question, which isn't terribly quick or convenient (but is as accurate as your ammeter).  
The method I generally use is to check the OT resistance (tubes removed and amp off for safety), and then measure the voltage drop across it with the amp idling. The idle current is ther voltage divided by the resistance. With my meter this gives an error of 3-5%, which is good enough for me.
4/23/1999 7:57 AM

Wouldn't the phase split cause an induction in the primary of the OT? If so there will be a counter EMF won't there? I'm not too clear on how this would affect DC current, seeing as XL is usually an AC thing. I appreciate the response, this will help clear up some questions.  
4/23/1999 8:12 AM

I think that is why Liam said he measured the real DC resistance of the OT, JP.  
Bias is a DC only measurement and the AC is riding on top of it when you are playing.  
4/23/1999 8:20 AM
That was the one, Bruce. If you've got any AC at idle, then you need to fix the amp rather than set the bias (ha ha). At DC all inductors are short circuits, and all caps are open circuits.
4/23/1999 1:12 PM

So what your saying is that it's the amplified AC signal of the guitar that is causing the mutual inductance that induces the secondary to supply power to the speaker. For some reason (I don't know why) I thought it was the phase shift of the DC that was doing that. Sorry for asking so many newbie questions. This is all new to me and I appreciate the input.  
Thanks to all of you who have contributed to this topic  
4/23/1999 1:18 PM
Reid Kneeland

"If you've got any AC at idle, then you need to fix the amp"
It's not unusual to see AC on the B+, at least on a push-pull output stage. Power supply ripple is often easily measurable with an AC meter. It shouldn't be enough to skew the idle current measurement significantly, though.  
"At DC all inductors are short circuits"
This obviously isn't true, or you wouldn't be able to measure the DC resistance of the OT primary as you described earlier. The *ideal* inductor would have zero DC resistance, but doesn't exist.  

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