Tube Biasing

[SlagJones]

Can this be a DIY job or do you need to bring it in to a amp tech?

[wiggy]

The answer is actually yes to both.

 

However, if you have to ask then my guess is that you probably don't have the technical know how (though I could be wrong) to do it yourself so probably the answer in this case is take it to an amp tech.

[theshrewz]

I did the bias adjustment on my SoCal 50 myself. Please see the SoCal 50H thread on page 2 of this forum for details.

 

Michael

[Eracer_Team]

Well technically a Valve Junior doesn't need to be biased since it's cathode biased with a resistor and changing tubes should not effect it (there are other reasons why you would change this resistor).

 

For a push pull amp check out this site and ask yourself if you could do the work

 

http://www.eurotubes.com/euro-video.htm

[SlagJones]

I'm asking more for my Crate V18 where the manual says take it to service for biasing. I have seen videos on youtube about biasing and it doesn't look too hard if you can read a meter and know the values you need but I looked up the price for a bias master and it's relatively pricey. I am assuming you need to have some sort of bias meter, correct? Anything cheaper out there?

 

edit:

OK, I looked at the Eurotubes site. The $25 bias probes are sufficient? Any other equipment needed?

[Eracer_Team]

It's not that hard but the voltages are enough to kill you.

 

at best you bais your amp no problems

Small zap; you are partially paralyzed for an indefinite period of time

At worst (or better depending on how you look at it) BIG ZAP you’re dead.

[SlagJones]

It's not that hard but the voltages are enough to kill you.

 

at best you bais your amp no problems

Small zap; you are partially paralyzed for an indefinite period of time

At worst (or better depending on how you look at it) BIG ZAP you’re dead.

 

Well then, maybe I will just pay the tech to take that kind of risk...

[Dave]

I've never biased an amp myself, but wouldn't you really need a tone generator and a scope to do it accurately? Or, at least a tone generator and a voltmeter to make sure that the positive and negative swings are equal? I don't see how you could tell if there was clipping or notching without viewing a driven waveform. You'd have to calculate the current flow in the ouput and match it to known boundaries.

 

As I understand it, the goal is to center the zero crossing point of the signal between zero (ground) and the voltage level of the B+ voltage of the power supply. You can't see clipping with a voltmeter. You'd need some tables created by people who do this stuff frequently.

 

Here's an article that talks about all this.

http://www.lynx.bc.ca/~jc/bias.html

[Eracer_Team]

Dave.. did you watch Eurotubes videos?

[m-theory]

I've never biased an amp myself, but wouldn't you really need a tone generator and a scope to do it accurately?

Yes. Numbers will only get you in the general ballpark. You need a scope and signal generator to really tell you how the amp is performing.

 

I don't see how you could tell if there was clipping or notching without viewing a driven waveform.

You can't, without a scope.

 

You'd need some tables created by people who do this stuff frequently.

Even that would only get you so far, as every tube and amp is different. Sometimes, you can push a lot harder than what you might think, and the amp will sound significantly better as a result.

[m-theory]

Dave.. did you watch Eurotubes videos?

I have. They're great, but again, that's really not the optimum way to set bias on an amp.

[Dave]

Dave.. did you watch Eurotubes videos?

 

I did watch the Eurotubes video and it does show an easy way to set bias, but the process is still a "do it and try it, then do it again until it sounds good to you" type of setup. Granted, bias settings will produce a given level of volume, clean, or dirty sound and it is still up to the user where they like it, no matter what method you use.

 

I like the scope and signal generator method that allows you to see the waveform. At least that way you are viewing the output waveform and can adjust the bias for the maximum output and assure that your clean settings are clean with no clipping, and you are getting maximum clean power without overdriving at normal settings. Anything other than a sinusoidal waveform is not clean, although it might sound good to you it also might burn your tubes up in a year or less.

 

OTOH, the method in the video should get you into the ballpark using accepted, field tested methods. For instance, the video says that Fender recommends about 60mv, but that is for stock tubes and the text states that various tubes need various bias levels and some can't take that. It's still an experimental process, but we are into that sort of thing. We buy stompboxes until we find one that we like and spend a lot of time and money in the process.