mansfieldguitars
Well-known member
Hello all. It has been some time since I posted on here. Some will remember me from the bias mods I was producing while back. I have since closed down the business due to lack of time. I work a full time job and quite honestly I was getting a ton of overseas orders through my website which just got to be to time consuming. I also took my downtime to focus on my playing and joined another band and really got back to what I enjoyed about guitars and amps in the first place. That being said i have been contacted by some through Facebook and still produce the mods upon request. Those interested can contact me @ [email protected].
The real reason for the post is I have just recently noticed some misinformation regarding my bias mod so I just wanted to explain just what it does.
There has been some question on the values I chose for the pot and fixed resistor. First I want to clarify that the fixed resistor is in SERIES (not parallel as some have said) with the trim pot. The reason for this is if the trimpot should open there would be enough resistance in the circuit to protect the amp. I usually used a 3.9k or a 10k. In the case of the stiletto, I used the 10k. For the Rectifiers I used the 3.9k.
As for the pot value I chose double the value of the resistor I was swapping out. The Stiletto is a 39K so so the combination of the 50k pot and 10k series resistor was approx double the 39k we were replacing. That puts us around the middle range on the pot with plenty of room to go up or down. On the Rectifiers we replaced a 22k resistor so I chose a 50k pot with a series resistance of 3.9k.
I also wired the trimpot as a variable resistor so if the wiper connection were to lift you would get the full resistance of the pot and series resistance combined. That would run the tubes very cold and again keep the amp from blowing.
All this was just neater and easier done on the circuit board. I also used 20 turn trimpots for accuracy and any movement due to vibrations would only yield small changes in bias voltage. I really never thought that was an issue but I guess it could happen although not likely as the bourns pots I use are high quality.
One more thing I read and just wanted to clarify. Most already know this but I recently read different on here. When resistance increases the tubes run colder due to more negative voltage at the grids of the tubes. When resistance goes down the tubes run hotter due to negative voltage going to ground instead of the grids of the output tubes. THIS IS WHY SERIES RESISTORS ARE IMPORTANT. You never want to have no resistance in the circuit.
Again anyone interested in my kits if you just have questions I can be reached at [email protected] or on Facebook under Mansfield Guitars. I am also going to post this in the rectifier and lonestar sections just to share this info with everyone.
Thanks
John
The real reason for the post is I have just recently noticed some misinformation regarding my bias mod so I just wanted to explain just what it does.
There has been some question on the values I chose for the pot and fixed resistor. First I want to clarify that the fixed resistor is in SERIES (not parallel as some have said) with the trim pot. The reason for this is if the trimpot should open there would be enough resistance in the circuit to protect the amp. I usually used a 3.9k or a 10k. In the case of the stiletto, I used the 10k. For the Rectifiers I used the 3.9k.
As for the pot value I chose double the value of the resistor I was swapping out. The Stiletto is a 39K so so the combination of the 50k pot and 10k series resistor was approx double the 39k we were replacing. That puts us around the middle range on the pot with plenty of room to go up or down. On the Rectifiers we replaced a 22k resistor so I chose a 50k pot with a series resistance of 3.9k.
I also wired the trimpot as a variable resistor so if the wiper connection were to lift you would get the full resistance of the pot and series resistance combined. That would run the tubes very cold and again keep the amp from blowing.
All this was just neater and easier done on the circuit board. I also used 20 turn trimpots for accuracy and any movement due to vibrations would only yield small changes in bias voltage. I really never thought that was an issue but I guess it could happen although not likely as the bourns pots I use are high quality.
One more thing I read and just wanted to clarify. Most already know this but I recently read different on here. When resistance increases the tubes run colder due to more negative voltage at the grids of the tubes. When resistance goes down the tubes run hotter due to negative voltage going to ground instead of the grids of the output tubes. THIS IS WHY SERIES RESISTORS ARE IMPORTANT. You never want to have no resistance in the circuit.
Again anyone interested in my kits if you just have questions I can be reached at [email protected] or on Facebook under Mansfield Guitars. I am also going to post this in the rectifier and lonestar sections just to share this info with everyone.
Thanks
John
