[chadbang]

After my recent rousing success with biasing my wonderful new amp, I was reading more about setting it up at the DIY forum and saw that the BJT DBs must also be biased. I saw this was tricker and approached it with caution.
 
The left side seems to fall within spec (around 32ma), but the right side is problematic. It read (ever so briefly) about 23ma, but after several tries getting a reading (with minimal adjustments to the trim pot -- not even enough to change the numbers) it now is blowing fuses in my DMM!
 
It also seem the heatsink the TB1R buffer is mounted on is running hotter than the rest.  
 
Was I right to use the ground right next to the TB1R checkpoint? What happened! I'm so sad as this was the last bit to check! (of course....)
 
Now I'm afraid I'll have to call in outside help, which I can ill afford.

 

[chadbang]

Ah well, I just did it a completely unscientific way. I just keep dialing down that DB trim pot until they were the same temperature. I give up. Time to close the case! All I know is it sounds damn fine, that's good enough for me!

 

[tomb]

Quote:

After my recent rousing success with biasing my wonderful new amp, I was reading more about setting it up at the DIY forum and saw that the BJT DBs must also be biased. I saw this was tricker and approached it with caution.
 
The left side seems to fall within spec (around 32ma), but the right side is problematic. It read (ever so briefly) about 23ma, but after several tries getting a reading (with minimal adjustments to the trim pot -- not even enough to change the numbers) it now is blowing fuses in my DMM!
 
It also seem the heatsink the TB1R buffer is mounted on is running hotter than the rest.  
 
Was I right to use the ground right next to the TB1R checkpoint? What happened! I'm so sad as this was the last bit to check! (of course....)
 
Now I'm afraid I'll have to call in outside help, which I can ill afford.

No, you were NOT RIGHT to measure TB1R with Ground.  This webpage clearly indicates which points to use for measuring DB bias:
http://www.diyforums.org/MAX/MAXsetup.php
 
TB1R gets measured with TA2R and TB1L gets measured with TA2L.  Both must be read in mV.  It's true that you're trying to adjust for ma (50ma is ideal), but measuring current would require breaking the circuit and wiring the meter in parallel.  That's impractical, so we measure the voltage across the power resistors (RB10 and/or RB11) in each channel and divide by the resistance (2.2R) to get ma (I = V/R).  For 50ma, you should read 110mV from TB1 to TA1 in each channel.

 
 

 

[chadbang]

Quote:

No, you were NOT RIGHT to measure TB1R with Ground.  This webpage clearly indicates which points to use for measuring DB bias:
http://www.diyforums.org/MAX/MAXsetup.php
 
TB1R gets measured with TA2R and TB1L gets measured with TA2L.  Both must be read in mV.  It's true that you're trying to adjust for ma (50ma is ideal), but measuring current would require breaking the circuit and wiring the meter in parallel.  That's impractical, so we measure the voltage across the power resistors (RB10 and/or RB11) in each channel and divide by the resistance (2.2R) to get ma (I = V/R).  For 50ma, you should read 110mV from TB1 to TA1 in each channel. 
 

Wow! Thank you. Now I've got it. I looked and looked and the directions and there they were staring me right in the face. Phew, thank you so much! So now I measured between TB1L and TA2L and set it at 90mv. And I measured between TB1R and TA2R and set that at 90mv (I picked 90mv just to play it safe and it falls in the middle of the "acceptable range". Is this correct. Thanks, man. Sorry to bug you again. I'm totally new to this!
 
 

 

[chadbang]

Hmm. Interesting. At a higher voltage setting (someone had it set really low! 35 and 28mv!) the sound seems a little "drier" at the higher voltage - the midrange a little less rich. Is this possible?
 
Edit: Ah, I think I know why, I was getting more volume out of it so I dialed down the volume. Very please to be on the road to perfecting this masterpiece. Hands down the best headphone amp I've owned over the years -- and i"ve had quite a few. It also shines for tube rolling. I find it more responsive than other tube headphone amps I've tried that with!

 

[chadbang]

Okay, next issue... Now, after setting what should be optimized setting, now I blowing the main 1 amp 250v fuse every few times it's turned on or off. Anyone have any suggestions?

Does anyone want to take a look at this thing for me. I'll pay shipping both wats. I just ordered some better nichicon caps and $26 sprague bypass caps from beezar. Maybe I should let someone just do this thing for me. Seal it up, and I'll never touch it again.:mad:

 

[chadbang]

Well, I did some research and it seems that there may be a rush of current induction causing this (don't know why it just started) So I went to a 1amp slow blow and this seems to have solved the problem. Anyone see a problem with that change? I also have 1.6amp fast blows. Anyone have a preference? 

 

[tomb]

Quote:

Well, I did some research and it seems that there may be a rush of current induction causing this (don't know why it just started) So I went to a 1amp slow blow and this seems to have solved the problem. Anyone see a problem with that change? I also have 1.6amp fast blows. Anyone have a preference? 

The fuses in a MAX should always be Slo-Blo. A 1A slo-blo is perfect.