One Last Power Tube Biasing Question

[theapprentice]

okay sorry guys for asking too many questions about tube amplifiers (I'm really getting anxious with the tubes replacement ideas). Correct mew if I'm wrong, but can a matched set of power tubes be replaced with another matched set of the same kind (6L6, EL34, 6V6) with the same ratings but made by a different company, without the need for biasing?

I just learned the proper way of determining the bias current from the Laney forums (www.laney-posse.co.uk) and I was very happy that I finally figured out how biasing works (still scared about the electrocution risks though ). The way I understood it, the only reason why you'd want to bias an amp is 1.) You're going to use a set of power tubes different from the ones that came stock with the amp, and 2.) to see if the tubes still perform within the correct bias range.

So what do you think people?

[Vim Fuego]

You can switch tubes by changing the bias, usually if this has been designed into an amp, there's a bias switch on the back that can be set for EL34 or 6L6. Otherwise you'll have to change a few resistors.
Jim

[theapprentice]

I would like to ask what would happen if I put a new power tube set of the same type but from a different manufacturer into my amp, and if it would need any biasing afterward?

I checked my LC50 and the power tubes had the brand "TAD" (Tube Amp Doctor) printed on them. I would like to replace them with TungSol EL34s when I get the chance, but I am worried about biasing. I was wondering if it were still required if it were the same thing but produced by different factories?

[Mustaine-who?]

Originally Posted by theapprentice

I would like to ask what would happen if I put a new power tube set of the same type but from a different manufacturer into my amp, and if it would need any biasing afterward?

I checked my LC50 and the power tubes had the brand "TAD" (Tube Amp Doctor) printed on them. I would like to replace them with TungSol EL34s when I get the chance, but I am worried about biasing. I was wondering if it were still required if it were the same thing but produced by different factories?

You should always bias new tubes...they are matched to each other, but not matched to the bias setting of your amp...unless you have say a Boogie and use Boogie tubes.

It won't hurt to not bias them...but as you already know...they saund the best when biased properly. If you remember what your bias was for the tubes you just installed, you could contact say Doug at dougstubes or eurotubes and ask them to send you a set thatmatches your current bias...but I would even bias them.

Just my $.02

[theapprentice]

Originally Posted by Mustaine-who?

You should always bias new tubes...they are matched to each other, but not matched to the bias setting of your amp...unless you have say a Boogie and use Boogie tubes.

It won't hurt to not bias them...but as you already know...they saund the best when biased properly. If you remember what your bias was for the tubes you just installed, you could contact say Doug at dougstubes or eurotubes and ask them to send you a set thatmatches your current bias...but I would even bias them.

Just my $.02

Thanks for the reply. I'm still confused however, especially since the instructions I got from the site did not seem to measure any parameter of the tubes except the max dissipation wattage, and the only thing needed to be determined in the amp itself was its plate voltage.

The only reasons I could infer for rebiasing from the information available to me are:
1.) the bias current may "drift" in time, resulting in poor sound and performance,
2.) some tube brands sound better when biased "hotter" and so biasing is used here for tonal preferences, or
3.) different matched sets draw different currents, and so rebiasing will make up for "insufficient" or "excessive" current flow to the new power tubes which drew a different amount of bias current compared to the previous set.

Is this correct?

[guitar-turk]

Originally Posted by theapprentice

Thanks for the reply. I'm still confused however, especially since the instructions I got from the site did not seem to measure any parameter of the tubes except the max dissipation wattage, and the only thing needed to be determined in the amp itself was its plate voltage.

The only reasons I could infer for rebiasing from the information available to me are:
1.) the bias current may "drift" in time, resulting in poor sound and performance,
2.) some tube brands sound better when biased "hotter" and so biasing is used here for tonal preferences, or
3.) different matched sets draw different currents, and so rebiasing will make up for "insufficient" or "excessive" current flow to the new power tubes which drew a different amount of bias current compared to the previous set.

Is this correct?

it is not a big deal..there is a trimpot inside of the amp and it adjust the current which flows from tubes
""""first you have to know the chaises voltace-your amps voltage is 400-420 volts
""""max power for your tube forl el34 25 watts for each(and chouse the power litle bit lower than its range lets say that ""20"" watts for each el34 tubethis will give you a nice power tube compresion and long lasting tube life)
p=Vx I
20=400xI I will be 50 ma(0,05A) for each tube.you may use diffent biasing technics for example
1-total amp current measurment(you should know the current flow in your preamp sectionyou have to calculate the current floe when your power tuber are out)
forexample.....preamp section current is 30 ma and you want to bias your amp use fuse port or stand by part-- and try to bias it 2*50(el 34 neads)+30 ma(that preams needs)=and adjust total current to 130 ma.thats the more easy way
2-install a 1 ohm 3 watt resistors from cothode and ground of tubes..and measure the voltage of this resistors 2 pins..
for example if you want to bias for 50 ma you should measure the voltage of the pins of resistor must bu V=IxR V=0.05*1 V=0.05 volts 50 mv try with the trimpot to get 50 mv that the cathode and ground connected pins of resistor..

best regarts
cihan

[jim777]

Eurotube's tube biasing FAQ, with a video
https://ssl.eurotubes.com/euro-i.htm?page=faq#4.