Biasing amps, how hard is it?

[cyndicate]

I never learned how to bias an amp before, and probably need to do so soon. Any tips on how to actually do it or links that are helpful? What do I actually need to bias one? Going to try and bias my XTC when I play around with some new power tube swaps

 

[Code001]

Get a Weber Bias-Rite and use their online chart. That should be all you need.

 

[cyndicate]

Wow never seen that before, looks like a cool unit. Sorry I don't know crap about this, what is Octal and 9-Pin?

Also if I have amps with 2 power tube sockets and some with 4 tube sockets, what do I need?

 

[Marshall Law]

I have a bias rite, it's very easy to use

 

[RedB4Black]

Wow never seen that before, looks like a cool unit. Sorry I don't know crap about this, what is Octal and 9-Pin?

Also if I have amps with 2 power tube sockets and some with 4 tube sockets, what do I need?

Get the octal. You really only need 1 tube socket if you have an amp that biases in quads. More than 1 is not really necessary. If you amp biases in pairs, get one with 2. That being said, I have one that has 4. I haven't used it yet though.

 

[cyndicate]

Wow never seen that before, looks like a cool unit. Sorry I don't know crap about this, what is Octal and 9-Pin?

Also if I have amps with 2 power tube sockets and some with 4 tube sockets, what do I need?

Get the octal. You really only need 1 tube socket if you have an amp that biases in quads. More than 1 is not really necessary. If you amp biases in pairs, get one with 2. That being said, I have one that has 4. I haven't used it yet though.

How do I know if my amp biases in Quads? The amps would be the following Splawn QR, Bogner XTC, Mako Mak2

 

[mysticaxe]

The 4 head one is about $20 more expensive than the 2 head one. If you have 4 power tubes, then you can check the bias on each one (make sure the tubes are matched). I did it for the 1st time a few weeks ago and it took about 20 minutes per amp.

 

[MARK1970]

I never learned how to bias an amp before, and probably need to do so soon. Any tips on how to actually do it or links that are helpful? What do I actually need to bias one? Going to try and bias my XTC when I play around with some new power tube swaps

I just purchased one of these this weekend. There tech help is excellent.
http://www.compu-bias.com/

 

[cyndicate]

I never learned how to bias an amp before, and probably need to do so soon. Any tips on how to actually do it or links that are helpful? What do I actually need to bias one? Going to try and bias my XTC when I play around with some new power tube swaps

I just purchased one of these this weekend. There tech help is excellent.
http://www.compu-bias.com/

Cool, how is that compared to the Weber Bias-Rite?

 

[cyndicate]

The 4 head one is about $20 more expensive than the 2 head one. If you have 4 power tubes, then you can check the bias on each one (make sure the tubes are matched). I did it for the 1st time a few weeks ago and it took about 20 minutes per amp.

So if I purchase the 4 head one, can I use just 2 heads out of 4 if I am doing it on a 50watt amp?

 

[Code001]

The 4 head one is about $20 more expensive than the 2 head one. If you have 4 power tubes, then you can check the bias on each one (make sure the tubes are matched). I did it for the 1st time a few weeks ago and it took about 20 minutes per amp.

So if I purchase the 4 head one, can I use just 2 heads out of 4 if I am doing it on a 50watt amp?

Yea. Really, you can just use the 1 powertube model (BR-AH w/ VI Option). It just means you gotta keep turning on and off the amp, checking each tube. Granted, you could always be real lazy and only test one tube when you bias an amp, but that would be a bit risky, IMO.

 

[MARK1970]

I never learned how to bias an amp before, and probably need to do so soon. Any tips on how to actually do it or links that are helpful? What do I actually need to bias one? Going to try and bias my XTC when I play around with some new power tube swaps

I just purchased one of these this weekend. There tech help is excellent.
http://www.compu-bias.com/

Cool, how is that compared to the Weber Bias-Rite?

Off hand, these display the following: current,voltage and tube dissipation. The weber bias rite does not. Here is a link to
show a comparison.
http://www.compu-bias.com/compare.shtml

 

[MARK1970]

cyndicate, I will pm you an e-mail I received from compu-serve. If I can I will try and post it on here. It just explains the push and pull of tubes when biasing. Another thing with the compu-bias, all calculations are done for you. The weber is cool, I owned one of those, but I don't have it anymore. The compu-bias cost more, but is definately easier.

 

[mysticaxe]

The 4 head one is about $20 more expensive than the 2 head one. If you have 4 power tubes, then you can check the bias on each one (make sure the tubes are matched). I did it for the 1st time a few weeks ago and it took about 20 minutes per amp.

So if I purchase the 4 head one, can I use just 2 heads out of 4 if I am doing it on a 50watt amp?

Yes. The toggle switch just changes which head is on the display.

As far as the calculations go. It's a damn easy equation that we're talking about -

P = I * V
V = plate voltage
I = Bias current
P = power dissappation on the tube

For an EL34 in a class AB amp (aka my Marshall and Splawn), P = 25 watts. For 70% (or "hot"), then P = 25*0.7 = 17.5 watts. 50% - P = 12.5 watts. It takes 2 seconds with a calculator and a piece of paper to figure out the window you want to aim for. All the Weber calculator does is run these numbers for you.

 

[nbarts]

If your amp has external bias points these videos can be helpful

https://youtube.com/watch?v=xtKJuTUVZMA
https://youtube.com/watch?v=c0e5bTa3MZo

You need to be careful not to get shocked!