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 Post subject: Triple XXX Bias Q
PostPosted: Sun Jan 23, 2011 11:17 am 
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I want to verify the bias on my 120w Triple XXX. The previous owner replaced the 6L6 power tubes with EL34s, I understand there is a switch and adjustment for either type of tube. I just want to verify that it was done correctly. I also understand that many prefer to measure plate current but since I don’t have a bias probe at the moment, I’d like to check the voltage at the test points. Can anyone tell me what the measured volt range is for EL34s and 6L6s? Does it matter what the Standby, Volume and Gain settings should be?

Thanks.


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 Post subject: Re: Triple XXX Bias Q
PostPosted: Sun Jan 23, 2011 11:41 am 
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42-43 VDC at bias test points for EL34. 55 VDC for 6L6's

here is something I read but since i have not been educated on the matter, not sure if it is true. I just go by the MANUFACTURERS manual on my XXX.

An update 10/09/07 We just measured the bias on a XXX in the EL34 mode and with the bias set at 42 volts using the factory test points (Peavey recommends 42.5) the bias was a whopping 57mA which is over 100% of dissipation! You will burn up your tubes with all but the coldest grades of EL34's or KT77's. With the bias set to 45 volts at the factory test points the bias went way down to a very acceptable 37mA. So in this particular amp a variation of only 3 volts at the factory test points made an incredible 20mA difference in bias!!

you can go to http://www.eurotubes.com/euro-k.htm

I have always been told that as long as your switch located next to the bias pot is correct for the tubes installed (6l6 for 6l6 or el34 for el34) then you really cannot hurt your amp by adjusting the bias knob either extreme. Just go by what sounds right to you. You might just go thru tubes faster.?? I have done both schools of thought on this - when i went by what sounds good to me, I then hooked up to the bias test points & my 6L6's were actually reading about 51 VDC, so from what i understand, from the statement above is that now that the bias is set at 51 VDC, my tubes are running actually hotter than at the
recommended 55 VDC.... but then i thought well i should just do what peavey recommends and go to 55 VDC, so i did.

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 Post subject: Re: Triple XXX Bias Q
PostPosted: Sun Jan 23, 2011 12:37 pm 
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Thanks. So can I assume that the front panel settings are irrelevant?

twirlyboggs wrote:
42-43 VDC at bias test points for EL34. 55 VDC for 6L6's

here is something I read but since i have not been educated on the matter, not sure if it is true. I just go by the MANUFACTURERS manual on my XXX.

An update 10/09/07 We just measured the bias on a XXX in the EL34 mode and with the bias set at 42 volts using the factory test points (Peavey recommends 42.5) the bias was a whopping 57mA which is over 100% of dissipation! You will burn up your tubes with all but the coldest grades of EL34's or KT77's. With the bias set to 45 volts at the factory test points the bias went way down to a very acceptable 37mA. So in this particular amp a variation of only 3 volts at the factory test points made an incredible 20mA difference in bias!!

you can go to http://www.eurotubes.com/euro-k.htm

I have always been told that as long as your switch located next to the bias pot is correct for the tubes installed (6l6 for 6l6 or el34 for el34) then you really cannot hurt your amp by adjusting the bias knob either extreme. Just go by what sounds right to you. You might just go thru tubes faster.?? I have done both schools of thought on this - when i went by what sounds good to me, I then hooked up to the bias test points & my 6L6's were actually reading about 51 VDC, so from what i understand, from the statement above is that now that the bias is set at 51 VDC, my tubes are running actually hotter than at the
recommended 55 VDC.... but then i thought well i should just do what peavey recommends and go to 55 VDC, so i did.


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 Post subject: Re: Triple XXX Bias Q
PostPosted: Sun Jan 23, 2011 1:10 pm 
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the front panel settings as in gain, master vol, etc ?
maybe i am misunderstanding you, but i like to keep the master vol dimed.
cahnnel vol to at least 4, gain down to say 9-10 oclock, and then roll guitar volume down from 5-10.
adjust the mid,treb,bass to taste
but what ever......
i play mostly funk & boogie & some rock
here is my xxx 2-12 with my single coil guitar (custom shop strat)
http://www.youtube.com/user/twirlyboggs?feature=mhum

yeah i hit the wrong chord at 1:36 whoops, should a beeen the 6/9.

then i have more with my humbucker guitar (custom 22) on the above site

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 Post subject: Re: Triple XXX Bias Q
PostPosted: Sun Jan 23, 2011 1:44 pm 
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As long as you go by the Peavey factory settings you are ok and won't come close to frying the tubes. Would never believe any calculations of bias unless you measure it yourself with either a bias meter or specific knowledge of how to measure tube current correctly. 57ma's could have been for 2 tubes for all we know and probably was because 3 volts of negative grid bias voltage does not equate into 20 ma's of tube current difference. Bias meters are a very useful tool and worth their weight in gold.


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 Post subject: Re: Triple XXX Bias Q
PostPosted: Sun Jan 23, 2011 3:17 pm 
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No. I'm askingif the front panel has to be dialed in a certain way when checking tube bias.

twirlyboggs wrote:
the front panel settings as in gain, master vol, etc ?
maybe i am misunderstanding you, but i like to keep the master vol dimed.
cahnnel vol to at least 4, gain down to say 9-10 oclock, and then roll guitar volume down from 5-10.
adjust the mid,treb,bass to taste
but what ever......
i play mostly funk & boogie & some rock
here is my xxx 2-12 with my single coil guitar (custom shop strat)
http://www.youtube.com/user/twirlyboggs?feature=mhum

yeah i hit the wrong chord at 1:36 whoops, should a beeen the 6/9.

then i have more with my humbucker guitar (custom 22) on the above site


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 Post subject: Re: Triple XXX Bias Q
PostPosted: Sun Jan 23, 2011 3:29 pm 
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USE a Bias probe cost 20 bucks or less With a DMM and do it right.
Be patient and do it right Check (your) Plate voltage do the math for the tubes you are using.
THIS IS DANGEROUS DO NOT ATTEMPT UNLESS YOU KNOW WHAT TO DO.
AND ((DISCHARGE CAPS FIRST))


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 Post subject: Re: Triple XXX Bias Q
PostPosted: Sun Jan 23, 2011 3:38 pm 
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oh yeah, sorry bout that

No. If the amp had tremolo then i would turn the intensity to 1 and speed to 1, and then turn the tremolo off with a footswitch. Other than that I don't believe you have to place the other controls a certain way.

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 Post subject: Re: Triple XXX Bias Q
PostPosted: Sat Aug 06, 2011 8:29 pm 
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KTB wrote:
As long as you go by the Peavey factory settings you are ok and won't come close to frying the tubes. Would never believe any calculations of bias unless you measure it yourself with either a bias meter or specific knowledge of how to measure tube current correctly. 57ma's could have been for 2 tubes for all we know and probably was because 3 volts of negative grid bias voltage does not equate into 20 ma's of tube current difference. Bias meters are a very useful tool and worth their weight in gold.



I hate to revive a post this old but this is incorrect and people searching for the information might do some damage without correction. The readings are not for two tubes but one. This is actually pretty accurate for what can happen in a peavey amp. My 6534+ with e34L's cant go down to 42.5 volts or its in meltdown territory. Put it at 45-46 and its cool as a cucumber. I dont know why or how but it just is.


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 Post subject: Re: Triple XXX Bias Q
PostPosted: Sun Aug 07, 2011 10:44 am 
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purpledc wrote:
KTB wrote:
As long as you go by the Peavey factory settings you are ok and won't come close to frying the tubes. Would never believe any calculations of bias unless you measure it yourself with either a bias meter or specific knowledge of how to measure tube current correctly. 57ma's could have been for 2 tubes for all we know and probably was because 3 volts of negative grid bias voltage does not equate into 20 ma's of tube current difference. Bias meters are a very useful tool and worth their weight in gold.



I hate to revive a post this old but this is incorrect and people searching for the information might do some damage without correction. The readings are not for two tubes but one. This is actually pretty accurate for what can happen in a peavey amp. My 6534+ with e34L's cant go down to 42.5 volts or its in meltdown territory. Put it at 45-46 and its cool as a cucumber. I dont know why or how but it just is.


That's because you don't quite understand what's actually going on here and the post is not incorrect. What is happening is you are confusing negative grid bias voltage with tube current. So if the negative grid bias voltage goes up the tube current goes down and vice versa so by incresing the voltage on the back of the amp you are running the tubes colder not hotter.

This is the most misunderstood subject of all tube amp parameters.
Peavey was smart however in not letting end-users have enough bias to fry their amps and made the negative grid voltage range fixed to a point to where end-users couldn't fry their amps. Marshall wasn't as fortunate as the JCM 2000 is the highest rated problem amp on the market which has real bias pots that can fry an amp in a second if not biased correctly and it happens quite frequently.


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