FMIC Model T Biasing

[Jr]

can anybody explain to me what this feature does and why you need to re-bias the amp...i'm totally lost...thanks

Chris

[youDEVILyou]

i posted the same thing a while back..


but yea you unscrew the panel in the back get some type of electircal meter and get readings. i dont know

i wish some one would tell me specificaly what type of meter i need adn if the amp is supposed to be in stand by or on. the manual says that the standby switch shold be switched "ON" so...in stand by or out of stand by.

[Anonymous]

Devil - On this site, listed under Service Manuals, you will find one for the Fender Model T.  There is a section concerning tube replacement and the biasing procedure.  For us boobs that can read, it clearly states a digital voltmeter and the voltage settings.  If this is confusing, then you would be better off taking the amp to a qualified service person.

Jr - Bias determines the operating point of the tube, and can be set either by measuring the current through the tube (the Fender Model T uses "sensing resistors" in the cathode circuit that transform the current level to a small voltage reading), or by measuring the voltage on the grid (as was done with the Sunn Model T).

Adjustment of the bias maybe necessary when replacement tubes are installed - all tubes are not created equal, and even tubes from the same manufacturer and assembly line can vary in their conductance over a wide tolerance range.  Adjustment of the bias allows you to compensate for tube tolerances.

[Rocketboy]

Even if you're not changing the finals, it's good to check the bias voltage and balance from time to time, particularly if the amp is moved around (gigged) a lot.

A decent digital volt meter (DVM) can be bought from Radio Shack for about $25.00.  I have found the Svetlana 6550C to conform (with good tone) to the setting specified in the manual.  Since this excellent tube is generally available, it's a good one to use for replacement.  Power tubes wear like tires and brakes - it depends how you use them.  If you play the amp hard 30 or more hours per week, expect to replace the finals every year or so.  Lighter use requires less frequent replacement.  The bias adjustment test point on the 300T rear panel is a good way to check final tube wear.  When it becomes difficult to obtain the values specified in the manual, it's probably time for new finals.  Don't forget the T-100mA fuses (1 per tube pair).  These are sometimes challenging to find, and it's a good idea to buy a dozen or so to have around.  Pay attention to the final tube status LED's on the amplifier deck.  Also, it wouldn't hurt to read the manual.

Taking a course in basic electronics at any community college (or self-study for, say, an amateur radio license) will do a lot more than simply allow one to use the excellent bias feature of the 300T, and is highly recommended.

 

[youDEVILyou]

yea i read the manual dude.


i set the meter to the settings. it didnt work..

[Rocketboy]

So sorry I wasted your time.

[youDEVILyou]

no not you

thanks for the help.

the dude above who said i should read the manual.