Bias Measuring points, pros cons

[Zero Cool]

I am curious about the Pro's and cons of various Bias measuring methods.

I am finally finishing the rebuild of an amplifier and i need to set the final bias once it is all done. It appears that the factory method of setting bias is by measuring across the emitter resistors. This is how i set initial bias. But noticed that if i was not very careful, that the amp would oscillate due to my meter leads.

So i am curious how i can better my method. I have a very good Fluke 189 multimeter that i have been using. Would shielded leads be better then the stock leads for the meter?

And what about measuring Bias by putting the meter inline with one of the power supply leads? the amp would be less prone to oscillation that way, but is that a better method? it is definitly harder to set up as this amp does not have B+/- fuses. and using this method i understand i need to add the total of all the devices for the bias measurement. but am i adding ALL devices. for example if i have 4 pairs (8 devices 4N & 4P) and they need to be set to sat 50ma each, would i then be looking for 200ma, or 400ma? I would assume 400 ma correct?

advice please.


Zc

[ChrisA]

Originally Posted by Zero Cool

I am curious about the Pro's and cons of various Bias measuring methods.

A common trick with tube amp building is to place a 1 Ohm resistor in series. Then to measure bias current you measure the voltage across this resistor (and divide by one). You can choose the location or the resistor. I think the best place is such that one of its leads in grounded. Now when you measure, one of your meter's leads is at ground and the other has a 1 ohm pure resistive impedance to ground. You are NOT going to pick up noise. You test leads are in effect shorted by a 1R resistor and one is on the ground rail



In my case, physically the resistor gets soldered across two banana jacks so I can measure the bias current without opening up the amp. But that is because tubes tend to get replaced years after the amp is built and the bias needs to be set again so a screwdriver trim pot get mounted right next to the jacks.

You can skip the externally accessible jack and the trim pot but keep the 1R resistor

[Enzo]

Since he is talking emitter resistors, I have to suspect there are not a lot of tubes in there.

COnsider thoseme meitter resistors are connected directly to the output of the amp, as hot a signal as there is in the amp, and when you conect your meter probes to the resistor, the probe wires become antennas broadcasting that signal to the earlier stages of the amp. Try taking the reading with your meter and probe wires leaving the amp area as quick as possible and as far from the earlier stages as possible.


Try setting bias by watching crossover distortion on a sine wave. Setting by emitter current is at best an approximation.

[Conrad Hoffman]

I'm surprised you're seeing oscillation, since that should be a low impedance point that's immune to such things. Since your meter is high impedance, you might try measuring through a couple 2 kohm resistors, one right at each probe tip. That should isolate the leads and not cause much error.

I've seen amps like the Marantz 250 where they set the bias by looking at the AC current draw. They start with the bias as low as it goes, measure the draw on the mains cord, then increase the bias until the mains draw goes up by a specified amount for each channel. I'm not a fan of the method but it probably works OK.

[Enzo]

COnrad, I don;t think the probes are making the amp unstable by themselves, I think they are coupling the output signal into the earlier stages and causing feedback.