Just got interested in biasing my 2555x Silver Jubilee, but I've got no experience doing this. I was hoping I could get some advice on the whole process before I even start. All I know is that around 70mv is a good number for the tubes. Any saftey precautions since the amp needs to be all the way on while I'm doing this?
New To Biasing: Advice?
- Thread starter Ren335
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SpHj
Well-Known Member
Nope, you’ve got external test points on the back. Plug your multimeter in, and set to 70mA per side. That’s it. You don’t have to take the chassis out and expose the circuit board so no massive safety precautions
MonstersOfTheMidway
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Agree. With external bias ports, bias should be a breeze.
I just change the power tubes on my stock JVM410H last weekend. One of the things I did before installing the new tubes was that I turned the bias pots way down with the old tubes installed (I could have done this with no tubes in the amp as well, but I was curious about the bias measurement at the lowest setting. The reason I turned down the bias was: 1) I was installing power tubes that were a little different in the specs what was previously installed, and 2) so that none of the tubes start red plating on me in case one was bad. Additionally, I highly suspected that the previous tubes were going bad, so I didn't want to install new tubes using a bias pot setting from the previous tubes. If the previous tubes were not worn to death and/or if I was using the same exact power tube as previously installed, then maybe I would just leave the bias pots alone, but I decided to error on the side of caution and slowly turn things up with the new power tubes.
What I described is no hard and fast rule but rather a particular way of doing things for my particular situation. If all is well with your current power tubes and you are confident in the quality of the new power tubes, you can just install the new ones and proceed with checking the bias for the new power tubes (write down whatever setting you choose). Then, after an 30 minutes to an hour of playing, recheck the bias measurement; if the bias measurement has drifted from what you previously installed, then you can adjust it if you like (write down your measurements again). Then, play you amp again for another 30 minutes to an hour and recheck the bias; if there is less change in the bias measurement, you can either make one last adjustment or just leave it. Maybe in two weeks (or at the end of February) you can recheck the bias again to see if it's holding steady. Most properly functioning amps and properly functioning power tubes will require checking/adjusting the bias 2-3 for new power tubes before the power tubes "settle."
But it's your amp: feel free to check the bias as often as you like. The bias pots and test points are external on the 2555X, so it's really no hassle anyways. Good luck with your gear and hope you are enjoying your amp.
I just change the power tubes on my stock JVM410H last weekend. One of the things I did before installing the new tubes was that I turned the bias pots way down with the old tubes installed (I could have done this with no tubes in the amp as well, but I was curious about the bias measurement at the lowest setting. The reason I turned down the bias was: 1) I was installing power tubes that were a little different in the specs what was previously installed, and 2) so that none of the tubes start red plating on me in case one was bad. Additionally, I highly suspected that the previous tubes were going bad, so I didn't want to install new tubes using a bias pot setting from the previous tubes. If the previous tubes were not worn to death and/or if I was using the same exact power tube as previously installed, then maybe I would just leave the bias pots alone, but I decided to error on the side of caution and slowly turn things up with the new power tubes.
What I described is no hard and fast rule but rather a particular way of doing things for my particular situation. If all is well with your current power tubes and you are confident in the quality of the new power tubes, you can just install the new ones and proceed with checking the bias for the new power tubes (write down whatever setting you choose). Then, after an 30 minutes to an hour of playing, recheck the bias measurement; if the bias measurement has drifted from what you previously installed, then you can adjust it if you like (write down your measurements again). Then, play you amp again for another 30 minutes to an hour and recheck the bias; if there is less change in the bias measurement, you can either make one last adjustment or just leave it. Maybe in two weeks (or at the end of February) you can recheck the bias again to see if it's holding steady. Most properly functioning amps and properly functioning power tubes will require checking/adjusting the bias 2-3 for new power tubes before the power tubes "settle."
But it's your amp: feel free to check the bias as often as you like. The bias pots and test points are external on the 2555X, so it's really no hassle anyways. Good luck with your gear and hope you are enjoying your amp.
anitoli
Well-Known Member
Just make sure you have a dummy load or a speaker connected when you fire that up.
MarshallDog
Well-Known Member
I always turn the volume and gain down to zero just to be safe.
I know this is a real rookie question, but what multimeter do I buy and how do I use one? People say use whichever, but I don't exactly know what settings to use. What setting should the multimeter be pointing to? (All I see is 20mA, 200mA, 200V, etc. Which do I choose!?)
Any help is very much appreciated.
Any help is very much appreciated.
Jethro Rocker
Well-Known Member
In your case as you want to read something in the 70mA scale, set it to the closest number above that, in your example, 200mA.
Get anything with a digital display they're easier to read. Place neg probe on the neg pin on 2555 and positive on the correct pin for first side. Take the reading. Then do same for second side. They're easy to run for that type of application.
Get anything with a digital display they're easier to read. Place neg probe on the neg pin on 2555 and positive on the correct pin for first side. Take the reading. Then do same for second side. They're easy to run for that type of application.
anitoli
Well-Known Member
You don't need anything fancy or expensive so don't be forking out for a Fluke. The first three on this link will be fine:
https://www.newark.com/w/c/test-mea...D1Vn07RnDqF9bmdxZ3naihmyHD29cSdp7aRu25dtFqsFw
https://www.newark.com/w/c/test-mea...D1Vn07RnDqF9bmdxZ3naihmyHD29cSdp7aRu25dtFqsFw
Harbor Freight Multimeters are like $6. I wouldn't risk an expensive Fluke or something doing biassing on amps. This is the one I use. I keep my bias probe hooked to it at all times and use this meter just for working on my amps.
https://www.harborfreight.com/7-function-digital-multimeter-63759.html
Robus
Well-Known Member
Don't electrocute yourself.
Delete!!
Well-Known Member
I just bought a cheap Craftsman multimeter ($20) and a $12 cable kit on eBay and it all works great.
Jeff Hudson
Active Member
As a add on, when you are setting bias, there is a balancing act going on between the pairs. So you will need to check the opposite pair when you adjust one side until they balance out. I always do this with my amps, because it seems to help sound and tube life, or it's just because I get my OCD fix in.
paul-e-mann
Well-Known Member
Step 1: clean off each capacitor with your tongue...
mickeydg5
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Buy the most expensive multimeter you can afford.
Bias the 2555X at approximately 84mA per side.
Tell me how you like it.
Chris-in-LA
Well-Known Member
I had some cheaper multimeters, readings all over the place. I couldn’t even get a solid reading in my wall voltage when I was trouble shooting a household breaker problem. I did buy a Fluke, it was more meter than I’ll ever need but it gives rock-solid readings for building and working on amps.
