Biasing with a decent mm and bias probe?

  • Thread starter Thread starter Beandust
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Beandust

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Is this a good method, or better yet are the probes safe/dependable? I have biased about 5 amps this way and recently had a tube just flake out when i pulled it. Put it in the probe socket then fired the amp up. Speaker load connected of course. Guitar pluged in minimal volume on amp with volume rolled off on guitar. I tried the outside tube bias was good. Pulled the inside tube to see if it was a consistent reading. Same process and fired the amp up and i get this horrible static sound at a loud volume. Immidiatley turned off. Removed the probe then turned amp.on same issue. So did infry the damn tube doing this or did it ju t coincidentally die at this exact moment? Not a tech by any means. I just THOUGHT i knew how to safely bias a set of tubes. Now i am all freaked out. Not sure if this probe/multimeter method is the way. Biasrite has mixed reviews, so any help would be greatly appreciated. I know tubes have a higher failure and consistency rate these days but just played the amp before i did this and it sounded really good as usual. Had been about 8 months since install i have read its good to check the bias as i usually just put in tubes anually 10-12 months. The bias pro and not the $20probe?
 
Bias probe with MM is fine. I use them all the time.

One thing though, handle the the tubes very carefully when they are hot/warm. It's easier to cause internal damage to the tube when they get jarred, bumped, and/or dropped while hot.

Other than that, could be a socket problem on the amp.
 
CrazyNutz":korr6n0r said:
Bias probe with MM is fine. I use them all the time.

One thing though, handle the the tubes very carefully when they are hot/warm. It's easier to cause internal damage to the tube when they get jarred, bumped, and/or dropped while hot.

Other than that, could be a socket problem on the amp.
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Thank you for the reply. I agree and i handle the tubes with care as mentioned. Insent the amp to the builder as i was thinking something like you mentioned( socket) or resister or something. I was told one of the tubes was arking.. so it was the tube is what that answer means? I wasnt given any detail other than that. Seemed odd as i had just checked the tube beside it. I am freaked out to bias my amp now. Never had a problem and felt confident in doing this simple task with care. I always grab the tubes with a soft cloth of type to avoid even dirt/grease from my fingers. The amp is a BE100
 
I think you just had a tube that was on it's way out, and you just helped it along the way by disturbing it. It's like touching a deteriorating hose, and then springing a leak.
You are fine, don't freak out, none of this is rocket science, and as for worrying about grease from your fingers, don't, that does not apply to tubes.

Another thing to keep in mined, do not try to play the amp with the bias probe installed. Other than that your are good.
 
CrazyNutz":19pb92jz said:
I think you just had a tube that was on it's way out, and you just helped it along the way by disturbing it. It's like touching a deteriorating hose, and then springing a leak.
You are fine, don't freak out, none of this is rocket science, and as for worrying about grease from your fingers, don't, that does not apply to tubes.

Another thing to keep in mined, do not try to play the amp with the bias probe installed. Other than that your are good.
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I really appreciate your weighing in on this as i felt maybe the probe could be a problem. I have some tubes i want to hear in this amp. El34 based of course and i cant imagine taking someplace to swap and bias. Will do. I hope indeed thats all it was. Thanks again! U Rock
 
The amp should have the volume and all knobs turned down and no guitar hooked up when biasing.but that just sounds like a bad tube.
 
One thing with bias probes if they are used a lot, the contacts may need to be re-tensioned and that may have been your issue in that the tube wasn't making good pin contact.


Also most bias probes are reading cathode current which is plate current + Screen current. So you are getting a slightly higher reading than the actual plate current which is not bad.

But keep in mind tube vendors only match based on plate current and transconductance. So seeing if your tubes are matched with bias probes is not very accurate since you have the screen current in the mix, but it will tell you if they are way off.
 
baron55":bl6qvna3 said:
One thing with bias probes if they are used a lot, the contacts may need to be re-tensioned and that may have been your issue in that the tube wasn't making good pin contact.


Also most bias probes are reading cathode current which is plate current + Screen current. So you are getting a slightly higher reading than the actual plate current which is not bad.

But keep in mind tube vendors only match based on plate current and transconductance. So seeing if your tubes are matched with bias probes is not very accurate since you have the screen current in the mix, but it will tell you if they are way off.
Click to expand...
Sounds like the $20 probe should be replaced. If for nothing more than piece of mind. Ive used at a dozen times i would say. Thanks for the useful info!!
 
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