Tools / Process for Rebiasing DRRI

[Matt-o-caster]

Hi,

I've done my own guitar setups for years but have never attempted to work on amps. After spending a lot of time browsing various online forums and reading Ritchie Flieger's book on guitar and amp maintenance, I have decided I'd like to get into amp maintenance.

I would like to start by replacing the power tubes and rebiasing my DRRI. I would also like to do the "bright cap mod" (removing C10 capacitor) to this amp. From my preliminary research, I know I probably need to buy a multimeter, some sort of bias probe (possibly Ted Weber's?), and a decent soldering iron.

Can anyone provide advice as to what devices I will need -- including product recommendations -- and the processes by which I should perform the undertakings described above? I am on a budget and don't want to spend a ton of $$. But I am thinking about this as an investment of sorts (I'd like to avoid bench fees from now on, especially if what I am doing can easily be learned and performed at home).

Thanks very much!

[Blue Strat]

Best bet for rebiasing is a "Bias Probe". You won't even have to "pull the chassis" to use one of these on your particular amp because the bias pot is accessible from under the chassis.

Bias Rite, Comp Bias, Bias King, etc. are are viable products.

[leofenderbender]

A Bias Rite & a screw driver will do it.

[Matt-o-caster]

Thanks, Mike. BTW, I'll be putting Tung Sol 6V6s I purchased from you a few weeks ago. :)

Do I need to do any discharging in order to safely adjust the bias? Also, do you have a recommended mA setting for a DRRI using those Tung Sols?

[Matt-o-caster]

Can anyone recommend a multimeter? How about this one?

http://www.labsafety.com/search/multimeter/+631/52423/95331/?isredirect=true

I'm thinking about just picking up Weber's Bias Rite Adapter Head and using in conjunction with a multimeter. More bang for the buck, it seems.

[RussB]

All you need is a DMM


BIAS INSTRUCTIONS.THIS METHOD IS BETTER AND SAFER THEN THE KEN FISCHER NEGATIVE SHUNT METHOD,THAT HAS PROVED TO BE A FUSE AND METER KILLER ,EVEN FOR THE MOST CAREFUL. STEP ONE MEASURE TRANSFORMER .WITH METER ON OHMS TAKE RED LEAD TO CENTER TAP OF OUTPUT TRANS,THIS IS EASY ENOUGH TO FIND IT WILL COME OUT OF THE SAME PORT THAT THE PLATE WIRES COME OUT OF IN MOST CASES. PUT THE BLACK WIRE TO EACH SET OF PLATE WIRES,PIN 3 OUTPUT TUBES YOU WILL SEE A NUMBER ,GET A SHARPIE AND WRITE IT DOWN ON THE RESPECTIVE SIDE OF THE SOCKET YOU MEASURE IT FROM . EXAMPLE 58.3 51.5 .YOUR NUMBERS COULD VARY GREATLY . WHEN YOU WRITE THIS DOWN ,YOU WILL NEVER HAVE TO MEASURE IT AGAIN FOR THAT AMP.NOW WITHOUT REMOVING THE WIRES WITH THE RED ON THE CT AND THE OTHER ON THE PLATE TURN THE AMP ON ALL DIALS AT 0 SET METER TO MILIVOLTS ...TAKE DOWN THAT NUMBER ,IT WILL BE SMALL .EXAMPLE 2.3 3 .1 ON THE OTHER SIDE ONCE AGAIN THIS IS NOT ANYTHING BUT AN EXAMPLE TURN AMP ON STANDBY SWITCH BLACK TEST LEAD TO OTHER PLATE ,,,,WRITE BOTH NUMBERS DOWN ON A PIECE OF PAPER .IN COLUMNS .FINAL TASK SET METER TO HIGH DC VOLTAGE ,PUT BLACK LEAD TO GROUND RED LEAD TO PLATE ,WRITE THAT NUMBER DOWN EXAMPLE 450 .CHECK OTHER SIDE 450 OR WHATEVER . IF YOU ARE WORRIED YOU ARE GOING IN TO HOT CLIP POSITIVE WIRE TO PIN 5 BEFORE YOU START ANYTHING,WHEN AMP IS ON YOU WILL SEE A NEGATIVE NUMBER DIAL TRIM POT TO ITS HIGHEST NEGATIVE ...THAT WILL START YOU AT BONE COLD BIAS HOPEFULLY . SO YOU DONT INCINERATE TUBE BEFORE IT GETS TO THE DIALED IN ZONE. NOW TAKE THE SECOND NUMBER IN THE MILIVOLT MEASURE SAY ITS 4 .7 ...DIVIDE IT WITH THE FIRST NUMBER USE A CALCULATOR UNLESS YOU HAVE A DOCTRINE IN MATH WITH THE SAME SIDE YOU WROTE THE TRANSFORMER IMPEDENCE NUMBER IN THE CHASSIS TAKE THAT NUMBER AND MULTIPLY IT WITH THE PLATE VOLTAGE YOUR AMP IS GETTING AT IDLE. THE FIRST TWO NUMBERS WILL DEFINETLY INDICATE HOW MANY WATTS YOUR AMP IS MAKING THE TUBES DISAPATE AT. FOR EL34S 17 OR 18 IS THE TICKET . THIS DOES NOT REFLECT HOW MUCH POWER THEY WILL MAKE .THIS IS THE SAFE ZONE FOR ANY EL34 FOR 6L6 I WOULD NOT EXCEED 22 WATTS SO THE TRICK IS SECOND READING FIRST ON THE CALC DIVIDED BY FIRST NUMBER ...TRANS WINDING MULTIPLIED BY LAST NUMBER PLATE VOLTAGE . IT WILL TAKE A BIT OF PRACTICE BUT YOU WILL NEVER BE IN THE DARK AGAIN ABOUT WHAT YOUR RIG IS DOING . YOU CAN LEARN THIS OR KEEP PAYING SOME BOZO THAT KNOWS THE SAME THING 40 PLUS DOLLARS TO DO THE SAME THING. TRACY NORTON SHOWED ME THIS AND POUNDED IT INTO MY HEAD. GOD BLESS HIM IT IS BETTER SAFER AND MORE ACCURATE THEN ANY FAG BIAS METER OR NEG SHUNT METHOD TO DATE . DO PULL YOUR HEAD OUT OF YOUR HINDQUARTERS AND LEARN HOW TO DO IT. IF YOU HAVE ANY QUESTIONS I WILL BE HAPPY TO YELL AT YOU ANYTIME YOU HAVE A MIND SINCERLY .... TERRY M KILGORE ALIAS TUBE TRAMP AKA SGT OD . NOW GO PISS UP A ROPE LOL

[Blue Strat]

Thanks, Mike. BTW, I'll be putting Tung Sol 6V6s I purchased from you a few weeks ago. :)

Do I need to do any discharging in order to safely adjust the bias? Also, do you have a recommended mA setting for a DRRI using those Tung Sols?


Around 20mA is just about right.

[Matt-o-caster]

Can I discharge the entire amp by simply removing the rectifier tube, turning the amp on and standby off (off = play), and waiting for several minutes for the other tubes to draw out all the current? I'd like to do that bright clip mod but don't want to leave my son without a father. :worried

[RussB]

removing the recto tube won't do anything for you

Fender's use bleeder resistors across the filter caps. They discharge the caps. Pull the chassis and measure the voltage, you'll see what I mean.

If you're not comfortable, have a tech do it.

[Matt-o-caster]

I'm comfortable doing it so long as I have adequate instructions. It seems like many online resources suggest what many people consider to be bad practice. Can you suggest a proven, safe process?