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Post by Jerry P on Oct 21, 2004 12:13:02 GMT -5
You're welcome! Anytime Eddie. Jerry
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newo
New Member
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Post by newo on Dec 27, 2004 0:48:25 GMT -5
Hello great info for DIY biasing quick question Is it possible to replace the resistor with a different value to bias it, and if so wot value should i look at installing because i dont have any tools to check the bias i was looking at getting it just out of crossover which i think is like 32 - 33 thanks
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Post by Jerry P on Dec 27, 2004 18:33:30 GMT -5
Hello great info for DIY biasing quick question Is it possible to replace the resistor with a different value to bias it, and if so wot value should i look at installing because i dont have any tools to check the bias i was looking at getting it just out of crossover which i think is like 32 - 33 thanks You can change the resistor with one of a different value but if you don't measure the bias to see where it's set it's just a crap shoot. No two sets of tubes will draw the same current so there's no magic number that works for all 6L6's. Jerry
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newo
New Member
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Post by newo on Dec 27, 2004 22:50:25 GMT -5
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Post by Jerry P on Dec 28, 2004 14:17:50 GMT -5
well what if I say I'm going to install a new set of jjs (just to narrow it down ) what value or is still a crap shot * Could I replace the resistor with a with a value double it effectivly doulbling whatever the bias is now? ?? All JJ's draw differently that's why we buy matched sets of tubes. The resistor value isn't proportionate to the bias voltage so doubling it would not do what you want. Your best bet is to take it to a tech to have it done properly. Jerry
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Post by Sigel on Apr 12, 2005 2:37:57 GMT -5
Hey Jerry, just bought a 5150 and wanted to attempt to try to do the bias mod.I have never biased an amp before and this is my first tube amp, so I want to make sure on a few things...
1. When doing the mod, is it as 'simple' as just taking out one resistor then soldering a pot/resistor assembly to the cap?
2.When adjusting the bias (I'm guessing you do this by adjusting how open/closed the pot is?) are all 4 tubes adjusted at once? Or do you have to adjust each one? If you have to adjust them seperately how do you do that?
3. Whats the simplest way to find out where to set the bias, how many mA?
4. Lastly, slightly O/T from the mod, but when installing new tubes, how to you properly bias the amp to adjust to the new tubes?
Thanks for your time.
P.S. I tried asking over at harmony central but I wasent getting as detailed responses as I needed.
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Post by Jerry P on Apr 13, 2005 7:06:09 GMT -5
Hey Jerry, just bought a 5150 and wanted to attempt to try to do the bias mod.I have never biased an amp before and this is my first tube amp, so I want to make sure on a few things... 1. When doing the mod, is it as 'simple' as just taking out one resistor then soldering a pot/resistor assembly to the cap? 2.When adjusting the bias (I'm guessing you do this by adjusting how open/closed the pot is?) are all 4 tubes adjusted at once? Or do you have to adjust each one? If you have to adjust them seperately how do you do that? 3. Whats the simplest way to find out where to set the bias, how many mA? 4. Lastly, slightly O/T from the mod, but when installing new tubes, how to you properly bias the amp to adjust to the new tubes? Thanks for your time. P.S. I tried asking over at harmony central but I wasent getting as detailed responses as I needed. Be very careful if you're gonna try the mod. The bias circuit is very important and if you do it wrong you can cause problems. It's not a real difficult mod, but it must be done right. You're just changing that resistor from a fixed value to something that's adjustable. When you adjust the bias, you're adjusting the negative bias voltage to all 4 power tubes. The amount of bias voltage controls the amount of current the tubes will draw. There's one adjustment for all 4 tubes, that's why you wanna buy matched tubes. I recommend www.dougstubes.com. To measure bias you'll need some way to monitor the current draw. The safest way to do it is to buy a bias probe of some type. There's many different ones out there. www.webervst.com sells a nice one that comes with directions. I think there's info on biasing on his site too. To know where to set the bias you'll need to know the plate voltage of the power tubes. Weber sells a bias probe that also allows you to measure that too. There's some math involved to figure out where to set the bias after that. Bias is a range, you can set the tube anywhere within that range to get the tone you want. Set it on the cold side and the amp will be buzzier, tighter, brighter. Set it hotter and the amp will be fat, powerful, and looser. You wanna find a happy medium that will give you the tone you want and not damage the amp or tubes. There's no one magic number that works for everyone or every amp. Jerry
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Post by chris on Jun 16, 2005 18:51:31 GMT -5
Hey just found this forum today, very cool. Ive been thinking of getting the 5150 and trying the bias mod. what is the link to how to do it again? Also if someone wants to try other mods you perform will you sell them the info on how to do it. Maybe you could make up a cdr with picks and info, etc for those of those who like to do are own stuff and sell them. Thanks Chris gringo699c@yahoo.com
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Post by thrash on Dec 23, 2005 2:18:13 GMT -5
Jerry, so on the 5150 II there will already be a pot there with the resister attached and that resister just needs replaced. Am I correct? Do you have a pic of that by chance? Thanks
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Post by Jerry P on Dec 29, 2005 9:22:24 GMT -5
Jerry, so on the 5150 II there will already be a pot there with the resister attached and that resister just needs replaced. Am I correct? Do you have a pic of that by chance? Thanks On the 5150II the pot is already there so all you have to do is change the resistor value. Sorry, I don't have any pics of that. Jerry
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srv69
New Member
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Post by srv69 on Apr 1, 2006 1:56:25 GMT -5
I was wondering if there is a link I'm missing or what because I cant find the actual bias mod in the forum.
Thanks in advance
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Post by Jerry P on Apr 1, 2006 13:40:24 GMT -5
Click where it says 5150 Bias Mod in the very first post of this thread. I have to change my colors, it's hard to see the link. Jerry
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Post by fullpoint on Apr 16, 2006 13:25:50 GMT -5
Can anyone tell me the preferred wattage rating for the trimpot?
I have a .75 watter laying around here somewhere but I'm willing to bet that's too lightweight.
Any recommendations?
Thanks.
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Post by erocku on Jun 7, 2006 12:04:55 GMT -5
Hey Jerry, I see there are two main bias meter's out there... Bias Rite and Bias King. Any one that you recommend. And also I see with the Bias rite, they have an adapter for the preamp tubes. When bias'ing the amp, you bias the power section, right?
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Post by Jerry P on Jun 10, 2006 17:25:52 GMT -5
I've never used either so I can't really say which is better. The Weber Bias Rite looks real nice and easy to use. It seems like a good unit for someone that doesn't have electronics knowledge. I use a scope and meter to bias, I don't use a bias probe. I just built myself one I'm going to use on some jobs. Jerry
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