[Oberst Oswald]

Have a problem... DB Bias. I turned the pots clockwise until I heard the click and took a reading: 1.7mV. Seemed safe so I set the power supply voltage to 27v. Set Tube Bias to Left 13.57, Right 13.63. All is well...so I thought When back to adjust the DB Bias to 110mV but I could not get the pots to adjust properly to get the mV climbing. Smelled smoke and shut of fast. Too late...think I burned out the QB1R transistor because now the board lights up for a few seconds and than slowly dims off. Now I can buy new transistors etc but I don't know whats wrong with why I can't get the pots to adjust. I have a good camera but I don't know how to post pictures to show you my build. Pretty sure I did a good job building it and triple+ checked everything but I'm hopelessly lost now on where the problem is and what I need to do or buy to repair it. Thanks in advance for all help.

 

[tomb]

Quote:

Originally Posted by Oberst Oswald /img/forum/go_quote.gif Have a problem... DB Bias. I turned the pots clockwise until I heard the click and took a reading: 1.7mV. Seemed safe so I set the power supply voltage to 27v. Set Tube Bias to Left 13.57, Right 13.63. All is well...so I thought When back to adjust the DB Bias to 110mV but I could not get the pots to adjust properly to get the mV climbing. Smelled smoke and shut of fast. Too late...think I burned out the QB1R transistor because now the board lights up for a few seconds and than slowly dims off. Now I can buy new transistors etc but I don't know whats wrong with why I can't get the pots to adjust. I have a good camera but I don't know how to post pictures to show you my build. Pretty sure I did a good job building it and triple+ checked everything but I'm hopelessly lost now on where the problem is and what I need to do or buy to repair it. Thanks in advance for all help.

We need to see the pics. From what you describe, it's not anything with how you were performing the setup. Moreover - QB1 is not really in the "critical path" of the buffer - it only sets the current bias. Other parts in the buffer will fry long before it does if something is wrong. Instead, I suspect some parts are out of place in the buffer.

Head-Fi has its own photo posting system - try that. Or, you can e-mail them to me at Beezar and I'll post them.

 

[tomb]

Just an FYI if any others were following this, but Oberst Oswald and I discovered his problem offline and he is working on it.

 

[WyldRage]

I build one, but I am having troubles with biasing.

The LEDs lit up, the tubes were heating up and the power supply is at 27 VDC. When I first plugged it in, the left side tube was at 500 mV and the right side 200 mV. The DB were less than 5 mV.

Now, turning it on/off has no effect. The mesures (except power) are slowly decreasing, measuring LT 250 mV and RT 11 mV about 2 hours since I started it. I repeat: turning it on or off has NO effect on the measures.

Since this is my first project, I have no idea what's wrong.

Here are pics:

 

[tomb]

Quote:

Originally Posted by WyldRage /img/forum/go_quote.gif I build one, but I am having troubles with biasing. The LEDs lit up, the tubes were heating up and the power supply is at 27 VDC. When I first plugged it in, the left side tube was at 500 mV and the right side 200 mV. The DB were less than 5 mV. Now, turning it on/off has no effect. The mesures (except power) are slowly decreasing, measuring LT 250 mV and RT 11 mV about 2 hours since I started it. I repeat: turning it on or off has NO effect on the measures. Since this is my first project, I have no idea what's wrong. Here are pics:

You must install jumpers at RB8L/R and RB9L/R. Refer to this page:
Millett Hybrid MiniMAX BJT Transistors

"Jumpered out" means you install and solder a spent resistor lead into the pads at each position. There must be a zero-ohm connection across those pads for the amp to work.

 

[WyldRage]

Cheers, must have missed that.

It's done! Haven't listened toit yet though.

 

[tomb]

Quote:

Originally Posted by WyldRage /img/forum/go_quote.gif Cheers, must have missed that. It's done! Haven't listened toit yet though.

So, have you listened to it now?

Keep in mind that those Black Gates will take awhile to break-in.

 

[WyldRage]

I like them. They've got good bass and good definition. I had decided to build one for a future pair of headphone (Audeze LCD-2) and all I've got right now are Senn IE8. I like them, but the amp is a bit wasted on these.

Absolutely love the glow.

 

[tomb]

Quote:

Originally Posted by WyldRage /img/forum/go_quote.gif I like them. They've got good bass and good definition. I had decided to build one for a future pair of headphone (Audeze LCD-2) and all I've got right now are Senn IE8. I like them, but the amp is a bit wasted on these. Absolutely love the glow.

That's great to hear! BTW, the MiniMAX and MAX do very well with Orthos.

 

[iamthecheese]

Sheeee lives!

I started my kit from Beezar yesterday and spend my day off today sucking down solder fumes.
Surprisingly enough to me it turned on first try and stayed powered for about a minute before I turned it off. No smoking anything on it

Haven't checked the HP out yet...I'm off to go check voltages and bias everything

 

[tomb]

Quote:

Originally Posted by iamthecheese /img/forum/go_quote.gif Sheeee lives! I started my kit from Beezar yesterday and spend my day off today sucking down solder fumes. Surprisingly enough to me it turned on first try and stayed powered for about a minute before I turned it off. No smoking anything on it Haven't checked the HP out yet...I'm off to go check voltages and bias everything

Great to hear! Be sure to let us know how the rest of it goes!

EDIT: I assume that means it went OK without the fuse.

 

[iamthecheese]

Ok... So I wired the RCA jacks to see if I could get sound. I didn't

. So i go to check voltages...PSU is I get set to 27.0 V and as I'm going to check the DB I notice only 1 heatsink is hot and the rest are cool. If your look at the front of the amp the far left heatsink is the only one that is hot, the other 3 are cool.

Help please?

 

[tomb]

Quote:

Originally Posted by iamthecheese /img/forum/go_quote.gif Ok... So I wired the RCA jacks to see if I could get sound. I didn't . So i go to check voltages...PSU is I get set to 27.0 V and as I'm going to check the DB I notice only 1 heatsink is hot and the rest are cool. If your look at the front of the amp the far left heatsink is the only one that is hot, the other 3 are cool. Help please?

You need to give us some logical, organized measurements. Study this page carefully and work calmly through the PS voltage setting to the tube bias, and then to the DB bias for adjustment and measurement:
http://www.diyforums.org/MiniMAX/MiniMAXsetup.php

Then come back and tell us what you've adjusted and what the specific measurements are. Remember - if you turned down the DB trimmers as recommended, the transistors are OFF. They won't develop heat. However, there are enough mismatches in some of the currents to perhaps let heat develop in one or two even if you have them turned down.

So, please go back - be methodical, logical, and in sequence adjust, measure, then tell us what you've got.

EDIT: Sorry - forgot the link.

 

[iamthecheese]

Here are my measurements after turning RB12l/r 4 times counter clockwise and RA1l/r 10 times counter clockwise.

v+ to ground = 27.0 V

TA2l to ground = 2.4v
TA2r to ground = 0.5v

TB1l with Ta2l = 0.49v
tb2l with ta2l = 0.49v
tb1r with ta2r = 0.0 v
tb2r with ta2r = 0.0 v

Same heating problem.

 

[tomb]

Quote:

Originally Posted by iamthecheese /img/forum/go_quote.gif Here are my measurements after turning RB12l/r 4 times counter clockwise and RA1l/r 10 times counter clockwise. v+ to ground = 27.0 V TA2l to ground = 2.4v TA2r to ground = 0.5v TB1l with Ta2l = 0.49v tb2l with ta2l = 0.49v tb1r with ta2r = 0.0 v tb2r with ta2r = 0.0 v Same heating problem.

OK - if you've truly measured 0.49V on TB1L-TA1L and TB2L-TA2L, then that's dangerous. That's 490mV or about 220ma. Turn that trimmer down NOW! Clockwise is DOWN for the buffers, not counter-clockwise!!!

Once you do that, leave the buffer alone. Try to see if you can bias the tubes first. Note that the trimmers will go 25 turns, so you've got quite a bit more to adjust. Sometimes the tubes may bias only in the last 5 turns out of 25. Keep a watch on the meter - turn until you can see the voltage change. You can't burn up anything with the tube bias.

Also - when you go back to the buffer, note that you should have your meter set to read in mili-Volts, not Volts.


EDIT: There's an additional link on that same page that may help you further, here:
http://www.diyforums.org/MiniMAX/setup/DB-Bias.htm (it's a pop-up window on that page).