Biasing an amp

[RiffRaff]

got a a question - I'm picked up some new power tubes for my amp (EL34's) - and I have a weber bias probe to use between the tubes and the socket so I won't need to install 1 ohm resistors on the tube sockets to measure the voltage drop - the tubes are matched but I'm well aware there are still differences - when I go to bias the amp should I base my bias point (between 40 and 45 mv's) on the tube with the highest mv measurement or set it based on an average between all four tubes? My amp does has a single bias trim pot so I guess I'm shooting for the best match between the outside and inside set of tubes?? Any insights would be welcome.

tx!

 

[Curly]

Re: Biasing an amp

well, if the tubes are matched, you should be able to bias to any tube and it will be ok - a swing of a few ma's isn't a big deal

if you're concerned, you can always double check each tube, although it's time consuming

 

[STRATDELUXER97]

Re: Biasing an amp

There's actually a formula that's based on your incoming voltage from the wall,your tube's dissipation factor,and the plate voltages...40 to 45 millivolts is kind of high...I always keep mine at around the 35 millivolt area...Never near or at 40 though...The higher these readings,the faster you run your tubes out of life...I run my BF Fenders at the 30 to 34 millivolt range and I do use the formula...I need to find the formula again because you can't just make random adjustments unless you have some facts on how the amp is operating...I bias every amp I own,but I use 1 ohm cathode resistors across each output tube..Your bias meter is the same thing,but all boxed up into a nice safe package..

John

 

[RiffRaff]

Re: Biasing an amp

Strat,

Check this out - it's on the weber site:Bias Calculator

Here's another interesting site:
Bias Information

 

[Kent S.]

Re: Biasing an amp

well, if the tubes are matched, you should be able to bias to any tube and it will be ok - a swing of a few ma's isn't a big deal

if you're concerned, you can always double check each tube, although it's time consuming

If the tubes are off then you you have to do an average ... but like Curly said, they won't be off by much if they are matched. That bias voltage is dependent on the characteristics of that tube, in relation to it's plate voltage. Other than using an oscilloscope to see what effect the bias is having one the signal your kinda at the mercy of what they tell you in those bias probe manuals (which is fairly accurate, I think they are within 5% or so).
If you write down the reading you get on each tube and then take an average you are safe as the difference won't be much difference between the tubes (from what they read before). And I'm pretty sure when you are using the scope method that is the same thing you actually wind up doing.
I'd be interested in seeing the formula that John is referrring to, as I'm not sure what he's referring to there, unless it's some kind of offset for local voltage differences (e.g. ... 117VAC vs 120VAC) once the bias is set.
I'll have to wait and see, I haven't got a clue as to what it might be ... sounds handy though.

 

[mrid]

Re: Biasing an amp

Strat,

Check this out - it's on the weber site:Bias Calculator

Here's another interesting site:
Bias Information

GREAT STUFF!

 

[STRATDELUXER97]

Re: Biasing an amp

If the tubes are off then you you have to do an average ... but like Curly said, they won't be off by much if they are matched. That bias voltage is dependent on the characteristics of that tube, in relation to it's plate voltage. Other than using an oscilloscope to see what effect the bias is having one the signal your kinda at the mercy of what they tell you in those bias probe manuals (which is fairly accurate, I think they are within 5% or so).
If you write down the reading you get on each tube and then take an average you are safe as the difference won't be much difference between the tubes (from what they read before). And I'm pretty sure when you are using the scope method that is the same thing you actually wind up doing.
I'd be interested in seeing the formula that John is referrring to, as I'm not sure what he's referring to there, unless it's some kind of offset for local voltage differences (e.g. ... 117VAC vs 120VAC) once the bias is set.
I'll have to wait and see, I haven't got a clue as to what it might be ... sounds handy though.

There's a formula that gets you the correct range of cathode current to your output tubes that's based on the dissipation of the tubes in watts(Usually 25 to 30)...And you plug in the wall voltage you see coming into "your" amp and the plate voltage...This gives you exact bias current readings...It's kind of like an ohms law formula where you multiply and divide the numbers...Years ago some of the amp building gurus over at The Ampage turned me on to it...it's overkill I Know,but it's the most accurate readings to strive for...I Know where to set my amps up across the 1 ohm cathode resitors I have in all of my tube amps...One of these days I'll locate that formula! :smack:

The crossover notch method by using an oscilliscope is the least accurate method...Each time you remove that notch,you get different readings if you were to hook your meter up and look at the readings across cathode resistors...The cathode resistor readings are solid and accurate and very easy to obtain everytime you bias your amp..The oscilliscope is very plus or minus in accuracy...

John

 

[Kent S.]

Re: Biasing an amp

There's a formula that gets you the correct range of cathode current to your output tubes that's based on the dissipation of the tubes in watts(Usually 25 to 30)...And you plug in the wall voltage you see coming into "your" amp and the plate voltage...This gives you exact bias current readings...It's kind of like an ohms law formula where you multiply and divide the numbers...Years ago some of the amp building gurus over at The Ampage turned me on to it...it's overkill I Know,but it's the most accurate readings to strive for...I Know where to set my amps up across the 1 ohm cathode resitors I have in all of my tube amps...One of these days I'll locate that formula! :smack:

Cool, I'd love to see it ...

The crossover notch method by using an oscilliscope is the least accurate method...Each time you remove that notch,you get different readings if you were to hook your meter up and look at the readings across cathode resistors...The cathode resistor readings are solid and accurate and very easy to obtain everytime you bias your amp..The oscilliscope is very plus or minus in accuracy...

John

Well that's the way I was taught (as were many others), and it still works; However it does have some detractions (as mentioned by Aiken and his crew) ... the oscilloscope method though is a bit more accurate than the bias probe as my understanding is that you can get within 5%, but not more, still that's very good to say the least without a scope.With a scope you are looking for the effect of the bias, not necessarilly what's exact for the
tubes.
I wouldn't go so far as to say that the scope is very plus or minus ... BUT, I have noticed that most guys that use 'em still go back (once within range), and do an audible test as well as watching the scope, so they are fine tuning it by ear. So I guess it is open to interpretation though, and if you got a scope might as well use it ... I am very interested in the alternative approach though. Groove Tubes has a book with their version of the bias probe that sounds a bit like what you are referring to, but I think theirs was just a chart of approximate current draw values per wattage and tube types though. :cool3:

 

[Gearjoneser]

Re: Biasing an amp

I use a mA bias tool. With the chassis on it's side, plugged into the cab, I monitor the numbers while I'm strumming the guitar with my right hand, and my left hand on the mini-screwdriver/bias pot. I try to keep the mA near their parameters, while also listening to the response of the amp on clean and gain tones. When the amp sounds it's boldest and tightest, the numbers are usually right where they should be.
Then, I move the bias tool around to other tubes, just to make sure the numbers are all pretty much equal. I'm not sure about voltage, although it's interesting that the numbers are similar to mA values. I've found that most amps sound best when 6L6's are around 32-34 mA and EL-34's are 36-40 mA. Funny, that the voltage values are often those same numbers. I still go by my ears though, mostly.

 

[STRATDELUXER97]

Re: Biasing an amp

Cool, I'd love to see it ...


Well that's the way I was taught (as were many others), and it still works; However it does have some detractions (as mentioned by Aiken and his crew) ... the oscilloscope method though is a bit more accurate than the bias probe as my understanding is that you can get within 5%, but not more, still that's very good to say the least without a scope.With a scope you are looking for the effect of the bias, not necessarilly what's exact for the
tubes.
I wouldn't go so far as to say that the scope is very plus or minus ... BUT, I have noticed that most guys that use 'em still go back (once within range), and do an audible test as well as watching the scope, so they are fine tuning it by ear. So I guess it is open to interpretation though, and if you got a scope might as well use it ... I am very interested in the alternative approach though. Groove Tubes has a book with their version of the bias probe that sounds a bit like what you are referring to, but I think theirs was just a chart of approximate current draw values per wattage and tube types though. :cool3:

Kent....Basically I'm just rattling off what Ken Fischer told me many years ago about the different methods of biasing...Shunt,Cathode resistor,and Oscilliscope...The shunt method is pretty dangerous and the cathode method is quick and easy..Easier on the meter also! :wink: Ken knows his stuff like no other...I Know Aiken and Trace Allen(Voodoo Amps)is a friend of mine and knows his stuff also...These guys help keep me informed... :dance:

 

[Kent S.]

Re: Biasing an amp

Kent....Basically I'm just rattling off what Ken Fischer told me many years ago about the different methods of biasing...Shunt,Cathode resistor,and Oscilliscope...The shunt method is pretty dangerous and the cathode method is quick and easy..Easier on the meter also! :wink: Ken knows his stuff like no other...I Know Aiken and Trace Allen(Voodoo Amps)is a friend of mine and knows his stuff also...These guys help keep me informed... :dance:

Cool deal John. I think a lot of people got told to use the scope method, just because that was the accepted method of doing it at the time, and it is good to see what effect the bias is having upon the signal. Ken also says that ceramic caps don't resonate correctly in a tone circuit (in a guitar that is), I have one in one of mine and it's very resonant ... but that doesn't mean that what Ken is saying is BS ... in fact I'm sure there is a reason that he says such. Kinda like when Kinman was speaking about his explanation of cable capacitance and his fix for it (in his case, I see what he is getting at, but I think he's off his rocker a bit).
The scope method may very well turn out to be the method that depends mostly on personal interpretation ( read also error) ... I dunno on that. I'm curious as to whether they are adjusting ... (they,... we are are talking about three totally different people, with probably three different ways of doing things ...they ... what a boneheaded expression on my part ... :smack: ) for the tube performance (letting to characteristics of that tube be whatever it may), or the actual performance that they would like the tube to display (within it's operating limits). Interesting. Well it's time for me to sleep ... thanks again John, and let me know if you find that formula, as I'd be interested in looking it over, Thanks again, later.

 

[STRATDELUXER97]

Re: Biasing an amp

Cool deal John. I think a lot of people got told to use the scope method, just because that was the accepted method of doing it at the time, and it is good to see what effect the bias is having upon the signal. Ken also says that ceramic caps don't resonate correctly in a tone circuit (in a guitar that is), I have one in one of mine and it's very resonant ... but that doesn't mean that what Ken is saying is BS ... in fact I'm sure there is a reason that he says such. Kinda like when Kinman was speaking about his explanation of cable capacitance and his fix for it (in his case, I see what he is getting at, but I think he's off his rocker a bit).
The scope method may very well turn out to be the method that depends mostly on personal interpretation ( read also error) ... I dunno on that. I'm curious as to whether they are adjusting ... (they,... we are are talking about three totally different people, with probably three different ways of doing things ...they ... what a boneheaded expression on my part ... :smack: ) for the tube performance (letting to characteristics of that tube be whatever it may), or the actual performance that they would like the tube to display (within it's operating limits). Interesting. Well it's time for me to sleep ... thanks again John, and let me know if you find that formula, as I'd be interested in looking it over, Thanks again, later.

Kent...I have it written down somewhere? Most likely with my Tremoverb amp info...I'll look for it and post it..The guys on the Ampage were the ones that gave me the formula..May have been Randall or Steve Ahola,Trace etc??

John