Amp Bias 101

[maddnotez]

Bias Rite or mulimeter?

Is one safer? Sticking probes into an amp socket is not something that interests me too much.

I have watched a few videos and have been reading up but it is still new to me.


Any links or videos you may have please post.

I am looking to start Biasing my own amps and I am not looking to kill myself in the process. Regardless of what people say, it is not $40 for a good tech to Bias your amp. At least not where I live. It is easily double and triple that price and having amps these days that allow multiple power tube types I can see myself wanting to do alot of tube testing and not having to pay alot of money for someone else to do it every time I want a swap.

The biggest tip I have read was to only use one hand. Keep the other in your back pocket to either A) Ground yourself or B) Do not accidentally touch something you should not touch with your free hand by mistake.

Any other tips or tricks?

 

[311boogieman]

There are obviously lots of smart talented people on this forum that can give you their tips and tricks but I did write a blog on biasing an amp that does not have external test points or VR pots. Therefore, it is loaded with safety tips. It also has pictures and a detailed description of the steps to follow. I went a bit overboard because I felt that some of the other biasing descriptions on the net left out certain steps or at least steps that I take.


http://splawnfans.proboards.com/thread/ ... g-tutorial <----


It has been reviewed by several amp techs/builders for accuracy but if anyone sees anything they disagree with or feel is missing please let me know

Short answer - get a probe AND a meter. Probe is always the way to go as you cannot always trust external test points. Also, please keep in mind these 2 things:
a.) My blog is for my amp which is a Splawn Quick Rod
b.) Bias your amp with your ears and not the numbers but DO NOT play the amp with the probe plugged in unless you've got a high end probe.

tl/dr

1.) Buy matched power tubes
2.) unscrew chassis from amp (not rubber feet)
3.) DO NOT touch the insides of the amp (potential shock)
3.) rest chassis upside down on something safe, or against the wall
4.) install bias probe, then multimeter, then turn on. Read milliAmps. Never detach probe or MM while amp is on.
5.) Turn tiny brass screw (not white plastic one) to the left counter-clock wise is colder right clock wise is hotter

use one hand, preferably with a rubber glove, stick your other hand in your pocket, where tennis shoes, use rubber/plastic handled screwdriver.

 

[maddnotez]

Cool thanks man. Great tutorial btw, most informative one I have seen so far.

I was also considering just buying a single probe from Euro tubes because I already have the ohm meter, which I beleive also reads MV. Will double check on that.

 

[Kapo_Polenton]

Not a huge fan of the bias probes.. found it to be more of a pain in the ass to install them, bias, uninstall them etc.. if i am in there anyway, one end of multimeter to the chassis ( I use one of the screw holes in top of chassis and you should probably use a small alligator clip to fix it to ground and keep your hand in your pocket), the other to pin 8 or 1 of output tube and bingo.. get your reading, adjust bias. (provided you have replaced ground wire on each power tube socket with a 1-ohm resistor to ground, i'd think more amps do this nowadays, my 800 had it already.) That is pretty much what the bias probe does anyway but allows you to have hands off.

Of course if helps to know what your plate voltage is and that is the key to determining how "hot" you run. I usually get the plate voltage and then use weber or other online bias calculator to let me know where the money zone will be. (pin 3 on output tubes, other end of multi meter to ground) http://www.webervst.com/tubes1/calcbias.htm

Don't kill yourself.

Always amazed at the fact that more amps didn't have bias points on the back of the amp. Love that feature in more modern amps.

 

[maddnotez]

Not a huge fan of the bias probes.. found it to be more of a pain in the ass to install them, bias, uninstall them etc.. if i am in there anyway, one end of multimeter to the chassis ( I use one of the screw holes in top of chassis and you should probably use a small alligator clip to fix it to ground and keep your hand in your pocket), the other to pin 8 or 1 of output tube and bingo.. get your reading, adjust bias. (provided you have replaced ground wire on each power tube socket with a 1-ohm resistor to ground, i'd think more amps do this nowadays, my 800 had it already.) That is pretty much what the bias probe does anyway but allows you to have hands off.

Of course if helps to know what your plate voltage is and that is the key to determining how "hot" you run. I usually get the plate voltage and then use weber or other online bias calculator to let me know where the money zone will be. (pin 3 on output tubes, other end of multi meter to ground) http://www.webervst.com/tubes1/calcbias.htm

Don't kill yourself.

Always amazed at the fact that more amps didn't have bias points on the back of the amp. Love that feature in more modern amps.

Agreed, every amp should have bias points and trims.

Ty for the info. To me that sounds a bit more dangerous and complicated. Something an experienced user would benefit from but I've never done a bias so I need a noob friendly and safe process. That's why I think I will go the probe route or bias rite.