USING A BIAS PROBE

thunder970 · Post by **thunder970** » Sun Nov 21, 2004 9:55 pm

I hear alot about get close with the probe then tweak it by ear. The question is... Do I leave the bias probe in while I tweak to make sure I dont go to hot? or should I tweak it then pull a tube and check where it is? also I'm having another volume drop from time to time and I have the back off with a fan on it. The amp is cool as can be so I'm not suspecting heat as the problem anymore. I squirted some cleaner on a plug and put it in and out of the effects loop a few times and it stopped so far. I'm thinking when I pull the chassis to do a bias check that I will just douche the hell out of the jacks. Is this safe to douche them? should any precautions be taken? Thanks..

flemingmras · Post by **flemingmras** » Sun Nov 21, 2004 10:08 pm

Yes you should leave the bias probe in as you tweak to keep from going too hot.

Yes it's safe to douche the jacks(just like women, amps gotta have their "jacks" cleaned at sometime LOL). Summers Eve works better than Massengill IMHO.

Just kidding. Use contact cleaner. Flood them, then blow the entire amp out with either a compressor or a can of compressed air before turning on again.

Jon

thunder970 · Post by **thunder970** » Mon Nov 22, 2004 12:16 am

LOL, Thanks jon. I don't have either compressor or a can of air at the moment. Can I just let it sit for some time to dry out?

thunder970 · Post by **thunder970** » Mon Nov 22, 2004 3:42 pm

well?

myker · Post by **myker** » Tue Nov 30, 2004 3:04 pm

i got one of those things, but how do you hook it up to your meter?
mike

flemingmras · Post by **flemingmras** » Tue Nov 30, 2004 4:54 pm

The probe should have a red and a black lead.

You hook the black lead to the COM terminal on the meter and the red to the VOLTS-OHMS terminal.

Then you'll measure the bias in millivolts. So if the reading is 35mV, then your bias current is 35mA.

Hope this helps.

Jon

myker · Post by **myker** » Sun Dec 05, 2004 6:27 pm

thanks flemingmras
mine is from weber, it has a white black and red lead, that is what was kinda strange.
mike

thunder970 · Post by **thunder970** » Mon Dec 06, 2004 8:50 am

wow, I have a weber as well but mine only has red/blk. It also has the switch for reading current and voltage which I really like.

myker · Post by **myker** » Tue Dec 07, 2004 9:47 pm

i havent even used mine yet? i woder why i have three leads
mike

white room · Post by **white room** » Wed Dec 08, 2004 2:48 pm

I have been using a single bias probe for about 2 years and it is a good tool to have and the safest way to set the bias but it can be time consuming with a 100 watt amp with shuting down, removing the BP, moving to a another tube, blah,blah,blah.

I thought about putting in some 1 ohm resistors and was about to until I read a post from Jon ( I think) recently on the shunt method and how to do it and it cut the time by more than in half.

I can quickly read voltage and amperage this way now.

Don't know that this relates to the topic but thought I would throw it in there.

Dax · Post by **Dax** » Wed Dec 08, 2004 7:38 pm

I don't think you can get ant easier or faster than the 1 ohm resistor method. Measure voltage at pin 3, look at a bias chart and check between the 2 lugs of the resistor - Done!

white room · Post by **white room** » Thu Dec 09, 2004 12:10 pm

I just put one lead on the HT fuse and measure at pin 3 for amperage readings and for voltage readings I just take the lead off the HT fuse and ground it to the chassis and, again , read pin 3 (setting the meter accordingly for each of course)

Question: When using the resistor method; do you solder the resistor between pin 8 and ground or are you guys talking about a different resistor method? Just curious as I wonder if I'm thinking of something different than what your talking about

Dax · Post by **Dax** » Thu Dec 09, 2004 12:57 pm

Thats what i am talking about, there is bus wire from pin 8 to pin 1 and then pin 1 to ground on the output tubes. take the wire off from pin 8 to 1 and insert a 1 ohm 2 watt resistor. Leave pin 1 to ground. You then just put your meter on both sides of the resistor. Set the VM to milliamps. your reading is your bias setting. The number may be + or - doesn't matter.

white room · Post by **white room** » Thu Dec 09, 2004 1:16 pm

Thanks, I wasn't sure if that was it or not

myker · Post by **myker** » Sat Dec 11, 2004 9:09 pm

You guys are right! i havce no idea why i bought that thing, i guess i just wanted to see how it was built! i guess thats why we are all here, we like messing with stuff.
mike