EVH 5150III 100watt Head Bias Trouble

Discussion in 'Gear & Equipment' started by jon rabbitt, Sep 6, 2017.

  1. jon rabbitt

    jon rabbitt SS.org Regular

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    Hey all you 5150III 100watt Head users out there I have some info that may help you in getting the bias set correctly without having to pay a shop a ton of money and then not knowing if they did it correctly or better yet did it at all!! So first thing is first, get yourself a decent volt meter that can read up to at least 600 volts DC. Now we are ready to go... start by taking the amp out of the box. This is done by removing the six screws on the bottom of the head. The four rubber feet can stay in place. Remove the black mesh screen cover on the rear of the amp the front one can stay in place. Gently remove the amp by pulling it out from the rear. Now what I did is I layed the amp up against my speaker cab so I had access to the underside of it where the circuit board is ya know all the shit that looks like it should be in the control room of a boring 747? Lol Ok now we are ready to go. Start by making sure the amp is connected to your speaker cab just like you were gonna play. Now plug it in and turn on the power. On the bottom of where the four power tubes plug in there is a small circuit board. On that board you will find the bias test points positive and negative. Then the volt meter to miliamps DC and check the reading at the bias test points. Write down your findings that is where the bias is right now. Now you wanna measure the plate voltage. This is done by measuring at pin 3 of any power tube and ground. Pin 3 is the 3rd pin to the right of the power tube line up pin. This reading can be taken at the same board as the bias test points. You will see at the bottom of each power tube small drops of solder that correspond to each pin. This is a VERY high voltage area so don't get high before doing this! Pin 3 and a good ground like the chassis of the amp will be fine will give you the plate voltage reading. I got 489 when I did mine. This number is now used to calculate where the bias needs to be. Also make sure the amp is in play mode off of standby when you take the last two readings. The current bias which is taken at the bias test points on the power tube board and the plate voltage which is pin 3 of any power tube and ground. Now comes the formula to calculate the bias. Most 6l6 power tubes are a 25 watt tube and you want the bias set to at least 70% of the plate voltage so it's 25 divided by the plate voltage times .7 this will give you the setting for each tube so multiply that times four cause you have four tubes and that is the number you want at the bias test point. My bias from the factory was 89.4 MV and my calculated bias came out to 143 so you can see they are set cold from the factory. The bias trim pot is located on the far side of the main circuit board. It says bias adj on the board with an arrow so it's hard to miss. Turn clockwise to increase and counter to decrease. I was able to get mine to 135 before it was maxed out. Much better than 89. Id need a hotter grade tube to get more but until I retube it that will be great. My amp sounded better right away. Time was much fuller and less buzzy and notes were very clear. If the bias trim pot seems to not wanna move fender puts a drop of glue on it so it cant, just turn it and you will break the bead of glue and be good to go. I hope this helped anyone who is daring enough to do this. I'm just like EVH where I have to play with everything and modify and adjust and break and fix and all that. If you have any other questions feel free to email me at jonxlh99@yahoo.com hope my hour long post helped someone out there!! Rock on. By the way my band HaddonField is playing the rock n shock on Oct 14 at the palladium in Worcester so if you are near there be sure to check it out. We are all original and we promise to bring rock music back to where it left off in 1990!!!
     

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