To Bias an Amp with 8 pin dual bias probe...

[charvel750]

This may sound like a few dumb questions, but can anyone help me answer them? To bias a 50 watt Marshall tube head, I would connect the dual bias probe to the amp with the 2 tubes and check the millivolts reading, while connected to the multimeter, right? Marshall recommends 45 mv per tube, so while I monitor the value on my multimeter I would turn the bias adjuster inside the amp until the value reads 45 mv, right?

I have some experience working with electronics, but not with biasing. Is there any danger using the bias probes, in terms of high voltage that can kill? Like the leads that go to the multi meter, is that lethal voltage or ok to touch as you make the connections? Please let me know, thanks. This is what I'm talking about:



So what are the steps to take to bias the amp with that tool?

[charvel750]

Nobody?

[Cygnus X1]

DO NOT TOUCH THEM!

Connect your meter with that amp unplugged and discharged.
Adjust the bias pot until the recommended MV rating is achieved.

Then turn off the amp...discharge it, and disconnect everything.

If you have two meters, then put one on the B+ rail to make sure there is no residual DC voltage present.

[charvel750]

That goes against everything I've read and studied about the current in that set up. It is supposedely extremely low in voltage, like biasing a JCM 2000, the prongs are connected to the mm and that draw is very low, nowhere even close to being felt, let alone lethal. Doesn't the bias probe take the low draw from pin #8? Isn't that the mv range? I'm confused by your answer, Cygnus.

[Rupe]

That goes against everything I've read and studied about the current in that set up. It is supposedely extremely low in voltage, like biasing a JCM 2000, the prongs are connected to the mm and that draw is very low, nowhere even close to being felt, let alone lethal. Doesn't the bias probe take the low draw from pin #8? Isn't that the mv range? I'm confused by your answer, Cygnus.

You are correct. I know Cyg has amp building experience so I'm not sure where that reply is coming from.

[DonP]

Some of the other pins are at high voltage.

If you accidentally touch the wrong pin...

If you just touch the bias probe stuff you will be OK.

Basically, you pull the tubes, plug in the probe sockets, plug the tubes back in.

Power up and take you mV readings. Find the bias trim pot and adjust to get the correct mV readings for the tubes you are using.

[Cleveland Metal]

Yeah, I was thinking the probe leads are sending mV, which is well, pretty low.

[Cygnus X1]

Some of the other pins are at high voltage.

If you accidentally touch the wrong pin...

If you just touch the bias probe stuff you will be OK.

Basically, you pull the tubes, plug in the probe sockets, plug the tubes back in.

Power up and take you mV readings. Find the bias trim pot and adjust to get the correct mV readings for the tubes you are using.

Exactly.
I should have been more specific.

Yes, the current at the bias point is very small.
The adjacent DC currents are lethal.
Plugging the tube based bias checker isn't that big of a deal, but many people do it like I do, accessing the tube socket from underneath and clipping a 1 ohm resistor in and connecting a meter.
Thank you, Don.

While you're at it, remember that bias can drift as the amp warms up.
Let it get to normal operating temperature when checking it.

[emperor_black]

I just bought the single probe version. I'm planning on biasing my Carvin in a couple of weeks. I'll let you know which parts of the amps NOT to touch soon. i.e. if I'm still alive.

[phill_up]

charvel750, I re tubed my amp with pre-amp and power tubes. I work with commercial and industrial electricity and the only thing that I have found that can store electricity is the capacitors and capacitors are only used on single phase circuits and are discharged with a blahblah ohm bleed resistor or just shorting a screw driver between the terminals to disipate the charge.
What does this mean to you? nothing if you are just going to replace the tubes and bias it. Just dont touch any thing you arent supposed to. maybe bengal65 will be along in a bit he has done quite a bit of it. until then watch this video "clicky"

After you do it you will snicker at the money you saved and you can come back the next day and tinker with it again if you want.

[Bengal65]

Here's a long discussion with pics:

http://www.jcfonline.com/forums/showthread.php?t=86764&highlight=biasing

[DonP]

or just shorting a screw driver between the terminals to disipate the charge.

No! Don't short with a screw driver! it drains the caps too quickly and can cause damage. Let the bleeder resistors drain them slowly.

[Bengal65]

No! Don't short with a screw driver! it drains the caps too quickly and can cause damage. Let the bleeder resistors drain them slowly.

+1 Here is a bleeder already made up. Scroll to bottom of page.

http://www.studiosoundelectronics.com/ampkits.htm

[Cygnus X1]

I don't like to jump in on these discussions.
I'm no expert by any means.
Bengal65, DonP, and others do a fine job.

But at the same time there is so much bad information that it is simply scary to think about the consequences. That is why if I do jump in, I will recommend extreme caution. Chances are, nothing will happen. Most modern amps have bleeder resistors.
Then the well meaning guy that has been working on otherwise safe amps gets hold of an Ampeg V4 that will hold 550VDC for several hours on a row of four huge filter caps
thinks things are cool, is suddenly smacked with enough current to induce a cardiac arrest...and the bad thing about DC current...it's steady, it will literally weld to your circulatory system, and you won't have any idea what just hit you. A Champ amp can do the same....

I build, I don't do much troubleshooting/repair. I am familiar enough with what I have built to "see" what is going on without much trouble. Poking around inside amps that I am not intimately familiar with raises my "scare" factor so I don't do it much. But the rules stay the same.

I use a 5K 5W resistor on alligator clips to discharge caps.
I keep it on as I work, and clip a 10 dollar meter on the B+ rail to make sure I haven't induced a charge somewhere while working on the amp. Strange but true...soldering in the right places can recharge caps, even though the amp is unplugged.

[phill_up]

I don't like to jump in on these discussions.
I'm no expert by any means.

But at the same time there is so much bad information that it is simply scary to think about the consequences. That is why if I do jump in, I will recommend extreme caution. Chances are, nothing will happen. Most modern amps have bleeder resistors.
Then the well meaning guy that has been working on otherwise safe amps gets hold of an Ampeg V4 that will hold 550VDC for several hours on a row of four huge filter caps
thinks things are cool, is suddenly smacked with enough current to induce a cardiac arrest...and the bad thing about DC current...it's steady, it will literally weld to your circulatory system, and you won't have any idea what just hit you. A Champ amp can do the same....

I build, I don't do much troubleshooting/repair. I am familiar enough with what I have built to "see" what is going on without much trouble. Poking around inside amps that I am not intimately familiar with raises my "scare" factor so I don't do it much. But the rules stay the same.

I use a 5K 5W resistor on alligator clips to discharge caps.
I keep it on as I work, and clip a 10 dollar meter on the B+ rail to make sure I haven't induced a charge somewhere while working on the amp. Strange but true...soldering in the right places can recharge caps, even though the amp is unplugged.

I assume I am the one that is throwing the bad information around so let me apologize. I am very comfortable around electricity and seem to forget i am on a guitar forum and not talking to my coworkers. At the same time there is no need to go through all the things that will happen to you medically unless you have a medical license.
Austin police chief shocked with 50,000 volts in Taser demonstration

"Clicky"

All I'm saying is you are just changing the freaking tubes and making a bias adjustment. There is no need in trying to re-invent the wheel. How many people have changed a light bulb with the light switch on? woohoo! Glass is an excellent insulator, meaning non conductive. I also recommend that if someone doesn't feel good about their electrical skills take it to a professional. Do not attempt electrical work after a few beers or standing in water and if you get the shit knocked out of you I cannot be held responsible for your actions!