First time bias help

[snoggin]

Ok so I bought a /13 sjt 10/20 in the emporium.. It cam ewith one of the 6v6 groove tubes broken.. I figured no biggie I can use the opportunity to get a bias rite and some new tubes and do my first self bias.


I also want to be alive to go to work on monday:roll
I got some nos 6v6 gt tubes from kca and figured good to go.. I look at my amp and there is no external bias trimmer sooooooo. I guess i have to open it up... I did and can see the trimmer so I just want to run my procedure by you. I also called freed yesterday and he said I should check with him re the nos tubes to make sure they are compatible. I will do that

I already have the chassis out by the way and am still alive. if i am careful do I have to worry about discharging the caps?I am thinking to hang the chassis upside down so the trimmer faces up and the tubes down and just go about my business. I know to have the amp plugged into a speaker, , volume down.

I will wear my tennies on the concrete.

What else do I need to consider. I know about the weber bias chart which I think seems to make sense

[phsyconoodler]

You might want to check with /13 and see if the amp is cathode biased.If so you don't need to bias it.It may be with 6v6 tubes.

[Dave C]

From info on Fred's website the 10/20 appears to be cathode biased in 10w mode and probably fixed bias(adjustable) in 20w mode. I'd check the voltage on pin 5 in 20w mode to see if the bias pot you located has any effect...I'm reasonably certain it will. I'd set it in 20w mode to 50-70% of max plate dissipation leaning more towards the lower end of the scale. The 10w mode appears to be cathode biased and not adjustable without a resister change...so I'd leave that alone unless it was really out of whack for some reason.

I wouldn't bother draining the filter caps since you'll be working on a live amp anyway to bias...just keep one hand in your pocket to avoid taking a hit across the chest. Be careful and if you have any doubts...take it to a tech...it's not worth getting hurt. Good luck and let us know how you make out.