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 Post subject: re biasing question
PostPosted: Fri Mar 31, 2017 8:53 am 
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Mark II

Joined: Tue Feb 20, 2007 3:12 pm
Posts: 65
A month ago my ROV blew two fuses after I went from 6L6 to EL34. I did switch the bias switch to EL34. Took it to my tech and he said that the switch is just an approximation of the right bias for the selected tube. It wasnt close enough and thats why i blew fuses, and a tube. After discussion of what I was looking for we went with 6V6's. Heres my question, he said he rebiased the amp for these 6V6. Is that possible? I always believed that Mesas were fixed bias and one could not go under the hood and adjust the bias. One of the reasons im asking is that I want to go back to 6L6's but im thinking if he did rebias the amp for the 6V6, then i would have to have him rebias it again for the 6L6...make sense???

Thanks


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 Post subject: Re: re biasing question
PostPosted: Fri Mar 31, 2017 12:34 pm 
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estring wrote:
A month ago my ROV blew two fuses after I went from 6L6 to EL34. I did switch the bias switch to EL34. Took it to my tech and he said that the switch is just an approximation of the right bias for the selected tube. It wasnt close enough and thats why i blew fuses, and a tube. After discussion of what I was looking for we went with 6V6's. Heres my question, he said he rebiased the amp for these 6V6. Is that possible? I always believed that Mesas were fixed bias and one could not go under the hood and adjust the bias. One of the reasons im asking is that I want to go back to 6L6's but im thinking if he did rebias the amp for the 6V6, then i would have to have him rebias it again for the 6L6...make sense???

Thanks

The amp can be re-biased, but this requires either changing a resistor in the bias network on the mainboard, or adding a trim pot in it's place.

For future reference, if you ask your tube vendor to supply tubes that are in Mesa's spec's for bias, there is no need to change anything. The correct range of EL34 would have worked without incident as that is what the circuit is designed for. The switch is not an approximation, it is the correct bias setting for tubes that are selected to run properly in the amp. If you had used Mesa branded EL34's, or tubes that met Mesa's spec's there would not have been a problem.

Now that the amp has been modified, there is no telling what that bias spec is now.

Dom

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 Post subject: Re: re biasing question
PostPosted: Fri Mar 31, 2017 2:18 pm 
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Mark II

Joined: Tue Feb 20, 2007 3:12 pm
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Thanks, I got the tubes from Dougs tubes...he advertises mesa specs and I have bought plenty from him before for my other mesas. But Ive always used 6L6. Now, since he rebiased it for these 6V6, im suspecting I cant simply throw in some 6L6 i have laying around since this current draw is not the stock draw the amp was designed with.


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 Post subject: Re: re biasing question
PostPosted: Fri Mar 31, 2017 2:51 pm 
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I have bought all my power tubes from Doug, three different Mesa's and he has always gotten it right on (I verify with a bias probe). I'm surprised you had an issue.

You are correct that the bias may not work with 6L6's depending on how the amp was biased.

Dom

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 Post subject: Re: re biasing question
PostPosted: Fri Mar 31, 2017 4:22 pm 
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Mark III
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Location: Chicago, IL
I think you need a new tech.

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 Post subject: Re: re biasing question
PostPosted: Sun Apr 02, 2017 8:16 am 
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Mark II

Joined: Tue Feb 20, 2007 3:12 pm
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deeman wrote:
I think you need a new tech.

What makes you say that?


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 Post subject: Re: re biasing question
PostPosted: Mon Apr 03, 2017 9:10 am 
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Mark III
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Either he doesn't know what he is doing, or he is trying to charge you for work he didn't do.

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 Post subject: Re: re biasing question
PostPosted: Tue Apr 04, 2017 8:40 am 
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Mark II

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deeman wrote:
Either he doesn't know what he is doing, or he is trying to charge you for work he didn't do.
Well, I doubt either of those is true. Hes been a trusted tech in Cleveland since the 90's. Plus, I brought my amp in "not working" and he returned it "working." I fail to see how this makes him incompetent or a rip-off. I thought maybe you knew something else about the amp that I wasnt aware of. He's a good guy and has done good work for me in the past.
BTW, Mesa responded to the same question I asked here. They said it is possible to re-bias the amp. The concern would be making sure I match the new tube to those new values. What mesa can do is get the amp back to its original stock value if I wanted.

Thanks


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 Post subject: Re: re biasing question
PostPosted: Tue Apr 04, 2017 9:15 am 
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Mark III
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Fair enough, I'm just skeptical by nature.

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 Post subject: Re: re biasing question
PostPosted: Tue Apr 04, 2017 9:48 am 
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estring wrote:
..... BTW, Mesa responded to the same question I asked here. They said it is possible to re-bias the amp. The concern would be making sure I match the new tube to those new values. What mesa can do is get the amp back to its original stock value if I wanted.......

I thought I cleared that up already.

domct203 wrote:
The amp can be re-biased, but this requires either changing a resistor in the bias network on the mainboard, or adding a trim pot in it's place.


Your tech can also return the amp to Mesa's specs.

Dom

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 Post subject: Re: re biasing question
PostPosted: Tue Apr 04, 2017 9:58 am 
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Mark II

Joined: Tue Feb 20, 2007 3:12 pm
Posts: 65
domct203 wrote:
estring wrote:
..... BTW, Mesa responded to the same question I asked here. They said it is possible to re-bias the amp. The concern would be making sure I match the new tube to those new values. What mesa can do is get the amp back to its original stock value if I wanted.......

I thought I cleared that up already.

domct203 wrote:
The amp can be re-biased, but this requires either changing a resistor in the bias network on the mainboard, or adding a trim pot in it's place.


Your tech can also return the amp to Mesa's specs.

Dom


Yes you did and yes I know he can return to stock... I was simply clarifying for another post.. Thanks for your help.


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 Post subject: Re: re biasing question
PostPosted: Tue Apr 04, 2017 9:59 am 
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Mark II

Joined: Tue Feb 20, 2007 3:12 pm
Posts: 65
deeman wrote:
Fair enough, I'm just skeptical by nature.

:lol:


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