Add Bias Pot to Mesa Boogie Mark I reissue

[stratomaster]

Hello,

I've searched this forum and the web looking for instructions on how to add a bias pot to this particular amplifier to no avail. I was wondering if any of you had experience with this or would know what to do after looking at a schematic.

I recently purchased one of these used, and while it had the Mesa marked tubes in it, they were all graded differently and one of them was even red-plating. I'd like to be done with all this Mesa-tube buying business and just put a bias pot in the amp to allow me to purchase any matched quad I want and have it run safely in the amp. Thanks in advance.

[stratomaster]

http://www.tubefreak.com/mkirga_2.jpg

Here is a link to the power amp section of said schematic. The bias supply has a few resistors in there that I think may make good candidates. I just don't know which to replace with a pot. Thanks.

[defaced]

I'd remove the 100k and 83k and install a 15k pot wired as a variable resistor in series with a 33k resistor. You can tweak these values a bit, but that will get you in the ballpark. You'll be able to bias a little bit colder, and a bit warmer.

[stratomaster]

Thanks so much! I figured it was one of those, but didn't want to chance it being the 15k between the caps. I went ahead and replaced the 82k with a 22k in series with a 100k cerment precision trimmer--and left this combo in parallel with the 100k resistor that was already on the board. That way I could have a wide range of bias options available with a bigtime buffer built in (25 turns worth of adjustment!). Got a set of tubes in there now that I feel safe operating (no redplating yet!).

While I was at it, I noticed a fried 470 ohm resistor on one of the plates--so I just replaced them all with flameproof 1W 470 ohm resistors, so we'll see how it behaves. While much better, this one ain't out of the woods just yet.

[FUCHSAUDIO]

Add a 1-ohm 5-W resistor on the primary B+ lead to the output transformer, and you can measure the total output current. With a matched set of tubes, you want about 140 ma total...Thats 140 MV across the resistor (measured in DC volts).

It's not easy to install a resistor for each tube in Mesa's unfortunately.

[Bruce / Mission Amps]

Add a 1-ohm 5-W resistor on the primary B+ lead to the output transformer, and you can measure the total output current. With a matched set of tubes, you want about 140 ma total...Thats 140 MV across the resistor (measured in DC volts).

It's not easy to install a resistor for each tube in Mesa's unfortunately.

Instead of a 5 watter, I think it would be more then safe to use 1 watt 1 ohm resistor there too...maybe a metal oxide flame proof to be safer.

[stokes]

I would just replace the 83k,if that is actually what is in the amp with a 100k to 150k cermet pot.If you use the 1ohm resistor on the cathode you must be sure it is exactly 1 ohm.Better to use the transformer shunt method or get a bias-rite type thing.The transformer shunt method is done by connecting the red lead of a miliameter to the B+ or red lead of your OT and the black lead of the meter to the plate of the tube,read directly in ma's.More exact than the 1 ohm method.