Minimax amp newbie question

in the image below you can see that the left connectors are for rca inputs. the center 3 are for adjusting the bias for the right left and ground channels.

 

So, you would "plug" them into a multimeter (or rather, a multimeter into the miniMAX).

 

You will set the multimeter to measure DC voltage and then using adjustment potentiometers (accessed via screw junctions from the top panel), you will set the proper bias (or voltage) setting for the tubes you are using. The miniMAX does not self--bias, so, you need to perform that step yourself whenever you change out the tubes. It is a relatively simple task, but, an essential one to insure the tubes you are using are properly adjusted to deliver the optimum sound quality.

 

You can get a functional multimeter for VERY little money:

 

    http://www.harborfreight.com/7-function-digital-multimeter-90899.html

 

In any case, when you buy a kit from Beezar, I believe TomB includes one to insure you are covered.

there actually trimmer pots. you use a hex wrench/allen key to adjust the bias on the tubes. you use your multimeter to test the voltage over each tube, and if it is not half of the input voltage ( minimax input voltage is 27 volts) you must adjust the trimmer until you reach the correct voltage, which in this case is 13.5 volts over each tube. the link you included in your first post explains it more.

 

before you build this amp i suggest reading up on how tubes work in audio applications. also do some research on biasing and the different classes tubes can be biased to. it can get pretty complex, but for the minimax basically you want 13.5 volts over each tube, and the trimmer pots on the back are what accomplish this. bias changes as tubes break in so for the first few hours of operation you need to monitor the voltage and adjust the bias accordingly.

Actually, on the miniMAX, using the recommended trim pot, the bias adjustment is best done with one of these:

 

   http://beezar.com/catalog/product_info.php?cPath=29&products_id=65

 

Although, with care and patience, a flat-head 3/32" screw driver can be used as well.

 

Your multimeter plugs into the back panel jacks and the adjustment is made using the trim pots accessable from the top panel. The jacks on the back bring out the adjustment points to make it more convenient (no need to disassemble the chassis to change and bias tubes).

ooops my mistake

 

havent actually built this yet (planning to)

just going off of what ive read

Now that's a disturbing visual.

Thanks, now I'll never be able to look at it the same.

BTW, some tubes benefit from bumping up the bias point a bit. The tradeoff is less overall usable volume, which is usually not problem. It all depends upon your phones and how loud you like to listen to your music. In other words, you might want to experiment a bit. The tip jacks and trim pots make this a relatively straightforward process.

Just an FYI, but the trimmer adjustment tool is also included in the kit.

 

If you refer to this page: http://www.diyforums.org/MiniMAX/MiniMAXsetup.php , you'll see that there are 5 trimmers on the MiniMAX that need adjustment.  This is not much different than the M3 or PPAV2, where you have to adjust three trimmers - plus the one on whatever power supply you pick to use for those amps.  The difference with the MiniMAX is that tubes are added into the mix and then the power supply is also on the same PCB as the rest of the amp.

 

On the MiniMAX PCB, there's one trimmer that's need to set power supply voltage and there are two needed to set the Class A bias on the output diamond buffer.  These three trimmers don't need adjustment - ever again - once you settle on the final adjustment and enclose the amp in the case.  Tubes are all different, though, and bias should be set on each one when plugging a different one into the amp.  Because of that, the custom Beezar MiniMAX case included holes in the case top for adjusting the tube trimmer screws and the tip jack plugs on the back.  That way, you can adjust the bias on the tubes while the amp is enclosed in the case.

 

The old revMH Millett Hybrid (formulated from Pete Millett's design by n_maher and drewd) used one of the Hammond cases with the slide-out lid.  A lot of builders just left the lid off so that they could adjust the tubes whenever they changed them.  The MiniMAX case is a single extrusion, though, and so the adjustment capability for the tubes was accounted for in the case design.  

 

Sorry to resurrect an old thread but I felt my question pretty much falls under the titel of this thread.

 

I just bought a Whiplash built miniMAX from the forums and as I live in Sweden, I need to get a new wall wart. From what I've read I need one with 24VAC @ ~400mA output, which turned out to be quite difficult to find. The only 24VAC that I found was this: http://tinyurl.com/cxlu4tx. But I guess it should work fine? If you know where to find one with 230V input cheaper, let me know as that one is kinda "pricey" at ~$20.

Sorry, but I get a blank page when I try to access that link.  Ideally, you'd need a 500ma 24VAC walwart.  However, higher amperage ratings than 500ma are not going to cause any issues at all, except perhaps to your wallet.  I would rather that you didn't go below 500ma, though.

 

EDIT: OK, I directly posted the text of the link and that worked.  The walwart you've selected will work just fine - assuming it's the right one to plug into your wall power.
 

 

Edited by tomb - 11/15/11 at 6:34pm