5150 III Bias?

[bsp01]

Quick question....

I'm sure this is not a difficult thing to do. I understand the 5150 III has bias adjustment inside, but I have no idea how you use it, and the test points.

I'm equipped with a DMM and that's about it. I do not trust any of the local techs as they tend to break everything they touch, so I have to figure out how to do it myself.

Can anybody walk me through it?

 

[RockNRun]

I'm no expert but I do have the amp and it's a piece of cake really. I'm sure we all know the danger involved in futzing around with a plugged in amplifier....but otherwise, yeah, once you've removed the chasis, flip her over, blah blah, you'll see in the top center of the board two small contact points actually labeled "bias (+)" and "bias (-)"...these are for your multimeter obviously. Someone else can tell you which setting the multimeter should be one and what not, but when you check yours, it'll most likely be really cold/low - mine was reading at "15ma" from the factory! The trimpot to adjust the bias is on the far left (when looking at the bias contact points dead on, not upside down) and it's blue, you can't miss it. Start slowly turning it clockwise while holding the testers on the contacts, you'll see the level on the meter rise. Try to get it up there to 28ish (ma). Let it sit for a bit, repeat, etc. Good luck. Be careful.

Also, there are much better breakdowns for proper biasing on youtube. Check em out!

 

[just42dave]

There you have it

 

[bsp01]

Thanks!

 

[the4thlast1]

Yeah keep in mind that the EVH seems to like a somewhat cold bias..... Something like 25ma was the sweet spot on mine.. If you go over 30 it starts to get undefined and mushy.

 

[bsp01]

Just for my own confidence, what do you guys set your multimeter to read when you do this? mA, mV ??

 

[Drkorey]

good questions - I'll have to check my 5153 micro when it ships in Oct., hopefully it will be a fairly similiar layout

 

[bsp01]

So did the bias, with a little help from the internet. I could not get it to 120mV or 30 mA / tube, it would only hit about 110, adjustments beyond that would just seem to make the reading climb without stopping. Is that a normal thing when you go past the range a set of tubes can handle?

It does sound better than it did for sure, a little less buzz/fizz and it's warmer sounding.

For the record it was DC mV that I set my DMM to.