[ishtob]

I finished building my Minimax, and is just trying to bias all the stuff at the moment. The tube bias is giving me some problem. diyforums says that the prefered range for the tube voltage from the 2 test points are each supposed to be half the powersupply (~12-14V)but mine is at 21V with my RA1(L&R) trim pots all the way closed. I tried moving the trim pot and stuff wiht no success. the voltage is not moving anywhere.
help?

 

[tomb]

Quote:

Originally Posted by ishtob /img/forum/go_quote.gif I finished building my Minimax, and is just trying to bias all the stuff at the moment. The tube bias is giving me some problem. diyforums says that the prefered range for the tube voltage from the 2 test points are each supposed to be half the powersupply (~12-14V)but mine is at 21V with my RA1(L&R) trim pots all the way closed. I tried moving the trim pot and stuff wiht no success. the voltage is not moving anywhere. help?

My guess is that you need to turn the trimmers the other way.

Because there are three tubes with widely varying gains that can be used with the MiniMAX, the trimmers couldn't be selected to adjust a particular tube over their whole travel. As a result, you may find that the tubes bias within 5 turns or so of a 20+ turn trimmer.

Another thing to check:
Are you sure the tubes are lit? Make certain that you get a glow from both, otherwise, any other effort is fruitless.

Also, you may find that biasing them at 15V may sound better to you. It depends on your headphones and other factors. The tubes wll have the most voltage swing biased at 1/2 the power supply voltage, but they will sound better at higher plate voltages. Just something to try ...

 

[ishtob]

thanks tomb!

i did exactly what you said, turning the other direction did the trick. I didn't realize how many times i need to turn before the voltage starts dropping. I was afraid to send too much voltage being sent through. Its the opposite direction as the DB

 

[ishtob]

I am currently running 14V with my Grado HF-1, it sounds pretty good so far. Can't wait till the tubes are broken in.

 

[tomb]

Quote:

Originally Posted by ishtob /img/forum/go_quote.gif I am currently running 14V with my Grado HF-1, it sounds pretty good so far. Can't wait till the tubes are broken in.

Glad to hear it! The HF-1 is a perfect headphone to use with the MiniMAX - I love mine!!