[BobSaysHi]

Hey guys, quick question about the tube bias.
 
When I turn the amp on the bias is ~14.5 V. It takes about an hour to get down within good levels. This is normal right? Nothing for me to be concerned about?
 
Also, this amp sounds absolutely awesome with my dt880s, and my new hd650s are coming today and I cant wait to hear them. Thanks tomb, for helping me out.

 

[Beefy]

That sounds all good, Bob.

 

[BobSaysHi]

I figured. I just like to be cautious.
 
HD650s are here. Sounds fan-freaking-tastic.

 

[tomb]

Great to hear, Bob!  As Beefy confirmed, it takes about an hour or so before the bias quits dropping.  What you describe is perfectly normal.  Those 12AE6's make a good pairing with Sennheisers.

 

[jdkJake]

Quote:

Those 12AE6's make a good pairing with Sennheisers.

Ain't that the truth!

 

[nate911]

Sorry if I'm reviving an older thread, but I just finished my Minimax (connected to a grubDAC)--sounds amazing!
I had some problems drilling out the tube sockets but tomb helped me out (I ordered his kit).
I had a fun time building this amp; thanks for a great kit and amp. Here are some pics:
 

 

[tomb]

Quote:

Sorry if I'm reviving an older thread, but I just finished my Minimax (connected to a grubDAC)--sounds amazing!
I had some problems drilling out the tube sockets but tomb helped me out (I ordered his kit).
I had a fun time building this amp; thanks for a great kit and amp. Here are some pics:
 

Nice work, Nate!  Those are great pics with the tube lighting, too!
 

 

[jdkJake]

Sorry if I'm reviving an older thread, but I just finished my Minimax (connected to a grubDAC)--sounds amazing!

Never feel you need to apologize for resurrecting a build thread with a new build.

It is always a great day when a new amp comes to life! Congrats!

 

[nate911]

 
Quote:

Originally Posted by tomb /img/forum/go_quote.gif

 
Nice work, Nate!  Those are great pics with the tube lighting, too!
 

Thanks! Yea I snapped some good pics, beautiful amp and case.
 

Quote:

Never feel you need to apologize for resurrecting a build thread with a new build.It is always a great day when a new amp comes to life! Congrats!

Indeed, thanks!

 

[FallenAngel]

Getting a weird DB bias with my latest build - can't set DB bias on one channel lower than 135mV, while the other one I can set normally to 80mV.  I think the soldering job is fine, but I didn't match the transistors very well.
 
Perhaps the PN4392 is burned, otherwise, the 2K trimpot inline with 100R is still too high...  Maybe the CSS transistors are poorly matched, but I wasn't expecting it to have such crazy results...

 

[tomb]

Quote:

Getting a weird DB bias with my latest build - can't set DB bias on one channel lower than 135mV, while the other one I can set normally to 80mV.  I think the soldering job is fine, but I didn't match the transistors very well.
 
Perhaps the PN4392 is burned, otherwise, the 2K trimpot inline with 100R is still too high...  Maybe the CSS transistors are poorly matched, but I wasn't expecting it to have such crazy results...

You're correct.  Miss-matched transistors are not going to prevent you from turning down the bias below 135mV and the CCS transistors are not going to have an effect on this (unless they're 5088's or something really weird).  I suspect something else is going on - bad trimmer, bad PN4392 as you suggest (although that implies zero bias), or maybe you misplaced a resistor.  It should be easy to compare with the good channel, just be very methodical.  It's almost like proof-reading our own essay paper: it's really hard to see our own mistakes (assuming that's it, that is).  Let us know what you find.

 

 

[tomb]

BTW, I was planning on making a bigger announcement (with some SASE freebies from Beezar), but I've started supplying 1.15K and 11.5K resistors for the CCS (RA8 and RA9 - L/R).  It brings the CCS current down to 0.565ma and cuts the distortion in half.  IMHO, the difference is easily noticed in the listening.  I never thought that little bit of change (the original MiniMAX was 0.650ma) would make that much of a difference.  However, after working with Dsavitsk's Torpedo prototype over the last year, he's taught me that small changes in the CCS can have a big effect.  I've tested with RMAA extensively, before and after - with old CCS and new CCS currents, and the 1.15K/11.5K resistors make a real difference.\
 
 
EDIT: Dang, I thought I clicked to edit the post above.  Oh, well ...

 

[tomb]

Oops!  I knew I shouldn't have posted that from memory!  The distortion on the MiniMAX is improved by about 1/3 (still audible, IMHO).  The MOSFET-MAX's distortion is the one that's cut in half.  Regardless, major improvements either way with either amp.  Of course, the optimum loads still apply - MiniMAX is best for all-around and high impedance and the MOSFET-MAX does best with low impedance.

 

[MrDavis]

 
Quote:

Oops!  I knew I shouldn't have posted that from memory!  The distortion on the MiniMAX is improved by about 1/3 (still audible, IMHO).  The MOSFET-MAX's distortion is the one that's cut in half.  Regardless, major improvements either way with either amp.  Of course, the optimum loads still apply - MiniMAX is best for all-around and high impedance and the MOSFET-MAX does best with low impedance.

 
I'm still a noob at this, so I just want to make sure I'm ordering the correct resistors.
 
1.15 KOhms
http://www.mouser.com/Search/ProductDetail.aspx?R=RN55D1151FB14virtualkey61300000virtualkey71-RN55D1151F
 
11.5 KOhms
http://www.mouser.com/Search/ProductDetail.aspx?R=RN55D1152FB14virtualkey61300000virtualkey71-RN55D1152F

 

[tomb]

Quote:

 
Quote:

Oops!  I knew I shouldn't have posted that from memory!  The distortion on the MiniMAX is improved by about 1/3 (still audible, IMHO).  The MOSFET-MAX's distortion is the one that's cut in half.  Regardless, major improvements either way with either amp.  Of course, the optimum loads still apply - MiniMAX is best for all-around and high impedance and the MOSFET-MAX does best with low impedance.

 
I'm still a noob at this, so I just want to make sure I'm ordering the correct resistors.
 
1.15 KOhms
http://www.mouser.com/Search/ProductDetail.aspx?R=RN55D1151FB14virtualkey61300000virtualkey71-RN55D1151F
 
11.5 KOhms
http://www.mouser.com/Search/ProductDetail.aspx?R=RN55D1152FB14virtualkey61300000virtualkey71-RN55D1152F

Yes - those are absolutely correct.  BTW, if you send Beezar an SASE - I'll send them to you for no extra charge.