VK112 Bias Pot Mod

[pburgbahamut]

Well its not that I'm afraid to do it or anything. Its just that I take out the chassis for my Carvin X100b anyways to do a bias. So I'm kind of used to it.

[CharlieP]

Well its not that I'm afraid to do it or anything. Its just that I take out the chassis for my Carvin X100b anyways to do a bias. So I'm kind of used to it.

It is just as easy to have the convenience of biasing without taking the chassis out. Also how to mount the pot is figured. Other style you will have to figure it out. The 50k Weber pot should work even better for your amp with the 39k resistor, allowing you more adjustment range. It really makes a tube change and bias quick and easy.

[CharlieP]

Just an update here! I acquired my second VK112 and did a mod to it. This time I used (2) 6.8K resistors. One in and one out in series path. Used the same Weber 50K Bias Pot as the first time. Using two resistors was just a convenience of attaching. Last time I used (1) 15K resistor and for the second path connection I had soldered (2) clipped resistor leads together to bridge the connection from board to pot. I felt that may create a future fail point with vibration.

[docz]

Did you have to take out the PCB to access the solder points? Or did you just top solder them?

DocZ

[CharlieP]

Did you have to take out the PCB to access the solder points? Or did you just top solder them?

DocZ

You have to flip the board to solder correctly. Cut the cable tie binding the standby switch to get slack enough and take the standby loose also. Gives you enough slack to flip the board and put a wood spacer under board to keep it from falling down in the chassis while you are working. There is also a small cable that can be unplugged from the power tube board. First time I did this mod I didn't unplug it the second one I did and it was a little easier.


Warning: Do not attempt this unless you fully understand how to discharge caps. High voltage and dangerous.

[CharlieP]

Updating a bias pot that is 25 K from Weber WP253-slot-lock. Use of the 25 K will give you a better range of adjustment.

[jazbo8]

Just curious what're the plate and screen voltages after adjusting for a "hot" bias, people seems to prefer running the 6L6s that way in the amp.

Found a part of the answer on this old post - the voltages before any "hot" bias adjustment is made.