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Biasing amp calculations, please help

cisspcism

Member
Messages
2,078
There was a thread like this but it didnt address my questions.

I look at this http://www.webervst.com/tubes1/calcbias.htm
and the rate EL34 as 25 watts and 6L6 as 30 watts. People at Eurotubes say to rate El34, 6l6, 5881 all as 25 watts. Who is right?

Plus to caculat bias you take the
wattge from a single tube so in my case lets say a an EL34 which gives you 25 watts. You then take the 25 watts and devided it by the plate volatge which lets say is 450V

so you would have 25/450 = 0.055 you would than should for what I am gathering is mostly recommended is 60% or 55% of the 0.055( which is 100% dissipation) o this gives you .033 for 60% which is right about average according to the website.

so I just want to make sure my formula is correct and most important how to rate the tubes in wattage. I dont wasnt to go by the eurotubes recommendation of saying the 6L6s are 25 watts and end up biasing an amp wrong if they are wrong by saying 6l6 are 25 watts when the Weber site was correct
 

cisspcism

Member
Messages
2,078
You see here is a quote from the eurotubes wbesite, they even say thay you should bias between 65% to 75% dissipation which most of you is saying never do at all. So it sounds like these guys at eurotubes might be eating to many paint chips


Most Marshall Mark II's and 900's will be between 460 to 480 plate volts and the older Super Lead and Super Bass Marshall's sometimes run as high as 530 plate volts.
So if you're biasing an 800 or a Super Lead, Super Bass or Marshall Major then I highly recommend measuring the plate voltage. If you're biasing a Mark II or 900 then you can use 470 plate volts or you can measure it.
The formula for calculating the bias is the dissipation of the tube divided by the plate voltage which gives you 100% and then you can bias anywhere between 65% up to 85% of this. Most amps will just start to come out of crossover distortion at 65% and so we recommend using 70%. You can of course go up to 85% but you will wear out your tubes quicker and brown out the tone a little.
For example 25 watts dissipation can be used for just about any new production EL34, 6L6, KT77 or KT66 tube and most amps on average will have about 470 plate volts (see the list below). So 25 watts divided by 470 plate volts equals .053 or 53 milliamps. So .052 X .7 = .037 or 37mA.
Just to illustrate how little difference your plate voltage makes lets use 450 volts. So 25 watts divided by 450 plate volts equals .055 or 55 milliamps. So .055 X .7 = .0385 or 38.5mA. You can see that 20 volts only makes a difference of 1.5mA in where you would set the bias.
 

rockon1

Platinum Supporting Member
Messages
13,189
By design specification a 6L6GC is a 30 watt tube. It does seem however most treat them as 25 watt tubes for biasing figures. While plate voltage variance doesnt change the ma idle setting too much the difference between calcutaing them at 25 or 30 watts does. 450v with a 25 watt tube set at 60% is 33 ma but for 30 watts @ 450v its 40 ma Hmmmm... Bob
 

cisspcism

Member
Messages
2,078
the 60 watt fender hot rod deville uses two 6L6 tubes, is that what your looking for.

no, thats not what I am asking.

if I have a single 6L6 tube and I had to say what wattage the tube was, what would I say?

Just because an amp is 60 watts and has two 6l6 tubes in it isnt accurate enough, heck the bogner shiva uses 2 el34 tubes and is 80 watts not 50
 




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