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GMC Forum > Discussion Boards > VINTAGE GMC > Community Activities and Tutorials > Ask an Instructor > Andrew Cockburn
Ivan Milenkovic
I have recently bought a new amp, and changed the power amp tubes. The amp is marshall tsl601 combo and it has two JJ E34L power tubes in there. Now the guy in the service hasn't biased them, and since I don't know much about that stuff, and you're an expert, I was wondering could you help me out and give me some tips how to do it myself.

So is biasing really necessary, and if it is, can I do it (i don't have any electronical knowledge except some soldering skills smile.gif ).
on the amp there's a small plate just above the tubes that serves for biasing, I can give you the picture if needed.

Thanks in advance mate, I appreciate the support
MickeM
While waiting for Andrew here's some reading I saw at the site I bought my tubes from. He also has a video.

http://www.eurotubes.com/euro-m.htm

Could be well worth the effort to get one of these bias-sockets.


Andrew, hope you don't mind smile.gif


EDIT: I just read that for your amp it's just plug n' play with a multimeter.
Good luck biasing! smile.gif
JVM
Oooh, congrats on the amp Ivan smile.gif
Ivan Milenkovic
Thanks guys. MickeM, I have seen that video, but there he shows only the head bias job, and I'm not quite sure how I should do it step by step, because he uses a slightly different method. unsure.gif
Andrew Cockburn
Well gosh, I'm jolly cross that you stepped in here Micke given all your posts on the secret mods board about people who answer in instructor forums wink.gif

Ivan, when you swap tubes you should really re-bias as I am sure you are aware. There are many ways to Bias tubes depending on the design of the amp - usually you need to know the bias voltage/current, then change a resistor until you get the required reading on a meter. Some amps, the A.C.1 included have a variable resistor with a testpoint so it is easy to measure and adjust, some amps need a soldering iron and opening up of the top, it varies. Looks like the tsl601has the testpoint too ..

Damn you Micke you answered jolly well and leave me little to say ... Ivan you need a multimeter, and all you need to do is measure the voltage, and adjust the variable resistor until the voltage is in the range that is on that web page, should be a 5 minute job once you have the multimeter ready to go.

If you want more detailed instructions I'm happy to help you figure it out!
MickeM
QUOTE (Andrew Cockburn @ Oct 21 2008, 03:07 AM) *
Well gosh, I'm jolly cross that you stepped in here Micke given all your posts on the secret mods board about people who answer in instructor forums wink.gif

Yes for real instructors, I didn't think that applied here since you're an instructor in theory.

laugh.gif
Ivan Milenkovic
Any help is appreciated guys thanks. smile.gif I'll get multimeter and try to bias the tubes myself. I hope everything will turned out nicely. unsure.gif
tsl60 model is one of the easiest to bias as I found out on that eurotubes site. It has 3 finger prongs and a trim pot that serves for biasing. Anyway, enough of smart talk, I don't understand the bias job at all - if you see the next post here where I say I finished the bias job, that will mean I survived the thing smile.gif
OrganisedConfusion
QUOTE (MickeM @ Oct 21 2008, 10:17 AM) *
Yes for real instructors, I didn't think that applied here since you're an instructor in theory.

laugh.gif

laugh.gif That's an awesome line smile.gif

Biasing is easy as many have said smile.gif
Ivan Milenkovic
Job done guys, just to inform you, I have bought a multimeter, and dialed in 75mV on the tubes. And I didn't get electrocuted! smile.gif

Thanks for your help. biggrin.gif
MickeM
QUOTE (Ivan Milenkovic @ Oct 22 2008, 11:44 PM) *
Job done guys, just to inform you, I have bought a multimeter, and dialed in 75mV on the tubes. And I didn't get electrocuted! smile.gif

Thanks for your help. biggrin.gif

Glad you're still alive biggrin.gif Electricity is spooky, in this case the mV you won't feel.
A friend of mine showed me with a car battery, he grabbed both the minus and the plus pole with one hand each and what happened was... nothing. He said the resistans through the one arm, through the upperbody and then through the other arm was too much wink.gif I'd never think of trying that myself before but appearantly not dangerous at all. Be sure to use both hands, don't reach the poles with the fingers of the same hand mellow.gif
Ivan Milenkovic
Thanks for the tip man, I'll be sure to memorize that smile.gif

btw I have bought the worst kind of brand new multimeter there is price was 540RSD < 1$ laugh.gif

Alcron ac-210 biggrin.gif


So it didn't work at start, so I had to open it and fix one connection, and then I could get it working. This is one of the reasons I was afraid I don't get electrecuted - I thought somebody sold me a toy multimeter.. laugh.gif

Andrew Cockburn
Well done Ivan!

And ... I'll get you for that Micke!!!!
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