Milkman amps bias

David Gram · Post by **David Gram** » 19 Jun 2019 2:27 pm

Hi everyone!

Can somebody explain to me how the amp bias works on a Milkman Pedal Steel mini 40w? I've got an older version with 2x10 alnico speakers in it and on the back it says "ADJ BIAS" with some sort of jack input. I emailed Tim about it a while ago but didn't get a response from him yet.

I love the way the amp sounds but would like a little less breakup (mine starts breaking up at around 3,5) and would love to get a bit more headroom around that volume. Maybe the Bias can do a little bit of that before I have to buy an external speaker cabinet with a bigger speaker. Does anyone have any thoughts on this?

Thanks!
- David

Bob Hoffnar · Post by **Bob Hoffnar** » 19 Jun 2019 3:57 pm

A bigger speaker won't change the amount of clean headroom. A more efficient speaker will though.

Wait for Tim to get back to you.

Paul Sutherland · Post by **Paul Sutherland** » 19 Jun 2019 6:17 pm

I believe Tim designed the Mini with the bias adjustment clearly labeled and visible on the back panel so that knowledgeable owners could adjust the bias as needed. It's not hard to do. You do need to have the tools to do the job.

You should not just start turning the knob without having a meter to tell you what's happening. And you should know in advance what value you are hoping to achieve. The internet has many sites that talk about biasing an amp with 6L6 power tubes.

Tim Marcus · Post by **Tim Marcus** » 20 Jun 2019 7:19 am

Hi David, I never received an email from you.

Sometimes they end up in my junk folder and I never get to see them.

The bias adjustment is to set the idle current draw of the output tubes. If its set too high, you will lose headroom. I like to bias the 40W Mini between 30-35mA per side. That will get you the optimal headroom and wattage.

You might be losing headroom because of those alnico speakers. I do not think I have ever built a 40W Mini with 10" Alnico speakers so they were probably replaced by another owner. Some speakers are more efficient than others, as Bob suggests.

Bill A. Moore · Post by **Bill A. Moore** » 20 Jun 2019 8:41 am

Tim I have never seen one of your amps, is the 1/4" jack to connect a meter?
(Just curious!)

Paul Sutherland · Post by **Paul Sutherland** » 20 Jun 2019 9:21 am

My previous comment is misleading when I talked about turning a knob. There is no knob. The thing that looks like a jack input, and is labelled "adj bias" is a port where you insert a small flat head screw driver. That's what you turn to adjust the bias. Turn it clockwise and the power tubes run hotter, and counter clockwise to run cooler.

On my bias test kit, the meter is attached to the porcelain tube sockets that are inserted between the power tubes and the chassis.

Tim Marcus · Post by **Tim Marcus** » 20 Jun 2019 9:50 am

Bill A. Moore wrote:Tim I have never seen one of your amps, is the 1/4" jack to connect a meter?
(Just curious!)

no - that would be a bad idea. People would plug a speaker into it or a guitar. I have had customers ask me what that extra plug was for... its not a plug. Its a recessed potentiometer that should not be touched unless you know how to bias an amp

Bill A. Moore · Post by **Bill A. Moore** » 20 Jun 2019 10:13 am

Thanks, surprised the OP didn't see the adjustment slot.

David Gram · Post by **David Gram** » 20 Jun 2019 3:40 pm

Thank you guys,

Thanks Tim, I'll take it to my amp tech to read the current bias and maybe adjust it if needed. I got this one in Europe second hand it has 1 Alnico jupiter and 1 ceramic jupiter 2x10 in total. Sounds amazing, also for guitar, but on bigger stages and higher volumes the headroom is a little bit squashed.

thanks for your reply!
David