Which bias tool should I get?

[tommythelurker]

As you know, with a class A/B push/pull amp you have one set of tubes driving the top half of the waveform and another set of tubes driving the bottom half. When you bias with a scope, you're trying to smooth out that transition between the two sets of tubes. On the scope you'll see a little kink in the waveform where it transitions between positive and negative. You adjust the bias until this kink just reaches it's minimum. You should still check the current to make sure it's not too hot.

Google it and you'll find more info with pics. You need to connect a dead load (i.e. resistor of the proper impedence) and connect the scope across that, and feed a sine wave input.

[rog951]

 

Originally posted by tommythelurker As you know, with a class A/B push/pull amp you have one set of tubes driving the top half of the waveform and another set of tubes driving the bottom half. When you bias with a scope, you're trying to smooth out that transition between the two sets of tubes. On the scope you'll see a little kink in the waveform where it transitions between positive and negative. You adjust the bias until this kink just reaches it's minimum. You should still check the current to make sure it's not too hot. Google it and you'll find more info with pics. You need to connect a dead load (i.e. resistor of the proper impedence) and connect the scope across that, and feed a sine wave input.

 

 

So, you just send a 1k tone through the amp or whatever, and while viewing the output on the scope, tweak the bias voltage until the "kink" at the zero-crossing is minimized? Sounds easy enough.

[tommythelurker]

Originally posted by rog951 So, you just send a 1k tone through the amp or whatever, and while viewing the output on the scope, tweak the bias voltage until the "kink" at the zero-crossing is minimized? Sounds easy enough.

Easy in theory. In practice it can be a little more tricky to judge.

Dammit, now you've got me wanting a scope again

[SteinbergerHack]

Originally posted by tommythelurker As you know, with a class A/B push/pull amp you have one set of tubes driving the top half of the waveform and another set of tubes driving the bottom half. When you bias with a scope, you're trying to smooth out that transition between the two sets of tubes. On the scope you'll see a little kink in the waveform where it transitions between positive and negative. You adjust the bias until this kink just reaches it's minimum. You should still check the current to make sure it's not too hot. Google it and you'll find more info with pics. You need to connect a dead load (i.e. resistor of the proper impedence) and connect the scope across that, and feed a sine wave input.

 

 

FWIW - that little 'kink' at the crossover is InterModulation Distortion - IMD.

 

It's not the only source of odd-order harmonics, but it definitely makes the amp sound 'harsher' and 'less musical'.

 

This, BTW, is why so many guys like Class A amps - no IMD, because there is no crossover point.

[tommythelurker]

I've always seen it refered to as crossover distortion, but I guess intermodulation works too. I've never paid attention to all this even order/odd order harmonic distortion or whatever so I dunno about that stuff.

[rog951]

Well, from a practical standpoint, the only adjustment you have at your disposal for "biasing" is a pot which raises and lowers the bias voltage by "X" amount, right? So, you still need to check the current draw to make sure it falls into the correct operating range, regardless of using a scope or not, right? So, will using the O-scope just allow you to fine-tune the bias voltage even more?

[tommythelurker]

Ya well, that's one of the dangers of the scope method if you don't know what you're doing - you can end up with the bias too hot. So yes, like I said before it's a good idea to measure the current as well. After you get some experience on a particular amp and are familiar enough with it you can probably bias with just the scope and not have to check the current.

The whole idea behind class AB is that there is some overlap when you switch from the push to the pull (and vice versa) which is supposed to smooth out the kink. The only way to tell if you are at the optimal bias to do that is by looking at the output. The 70% rule of thumb for the bias current is just a ballpark and may not be the ideal spot.

[rog951]

Man, I just wanted to change my tubes! Now I've gotta go finish my EE degree!

[tommythelurker]

Originally posted by rog951 Man, I just wanted to change my tubes! Now I've gotta go finish my EE degree!

Ha ha. Naw, what I do is just set it around 70%, then adjust it by ear without going above that point, then if I've moved it up and down a lot, double check it to make sure it's still ok. I usually like it a little colder than that when it's crankin' anyway.

It may not be techinically perfect, but in the end all the matters is how it sounds