[zwack]

All clear.
I did order some new tubes indeed. 

 
The bias of my tubes stops dropping after half an hour or so. They drop half a volt.
Another thing I noticed is that the "louder" tube also sounds a bit more sparkling or better said: the other tube sounds like there is a curtain hanging between me and the sound. I hope that the new tubes, or a combinations of those old and new make both channels sound equally sparkling without the imbalance. I have a good feeling that this will be fixed. Still the amps sounds already very good. I'm very pleased with it!

 

[zwack]

Well I finished my build. It was a fun build and I'm very pleased with it. I published an article (complete with pictures) about it on my website:
 
Here's the link: http://ce-designs.net/index.php/my-projects/headphone-amplifiers/articles-the-millett-hybrid-minimax
 
please note that I still need to replace the current volume knob with a nice black knob  
 
Thanks to all for the needed support!

 

[tomb]

Quote:

Well I finished my build. It was a fun build and I'm very pleased with it. I published an article (complete with pictures) about it on my website:
 
Here's the link: http://ce-designs.net/index.php/my-projects/headphone-amplifiers/articles-the-millett-hybrid-minimax
 
please note that I still need to replace the current volume knob with a nice black knob  
 
Thanks to all for the needed support!

Thank you for such a nice review!  It's humbling to read the efforts and results of builders like you!
 
BTW - I was able to find four of the tubes we spoke about in 12FM6 (RCA grey plates) and they'll be shipped out tomorrow.  Unfortunately, the 12FM6 has reached a point of scarcity that there's very little to pick from.  AFAIK, Beezar has the largest stock anywhere, and it's not much - perhaps about 150 at this point in time.
 
Just a couple of comments -

• 0.22uf Vitamin Q's are still in stock at Beezar, but they may not last long.  I have a few dozen, but no more.  The MiniMAX was originally based on the 0.18uf Vitamin Q's because they will fit on the PCB with the custom Beezar case, whereas the 0.22uf won't.  I still sell what I have remaining mainly for the MOSFET-MAX which doesn't have a custom case and leaves all options open.
• There is a revised headphone relay-delay that we (cetoole and I) applied to the MOSFET-MAX.  It uses a 24VDC regulator and relay, and deletes the zener diode altogether.  Testing was done on the MiniMAX, so it's very easy to retrofit.  The advantage is that the higher voltage regulator and relay actuate much quicker.  The turn on phase is almost devoid of transient spikes and the turn-off phase is so much quicker that transients are minimized in that regime as well.  I recently updated the MOSFET-MAX website with details of the new relay scheme (MOSFET-MAX Relay-Delay) and it shows a photo of a MiniMAX PCB with the parts and their mounting particulars:

 

 
As can be seen in the photo above, the mods are fairly simple - the biggest change is the use of a 78L24 TO-92 regulator in place of the BD-139.  The middle lead of the 78L24 goes to the left pad at DM1.  DM1 and RM1 are deleted completely.  It would be best to use a transient suppressor in place of DM2, but it's not strictly necessary if you want to try it out.  Also, CM3 should be rated at 35V, but I've never experienced a failure with the standard MiniMAX caps as specified.  Replacing DM2 with an SA48C may promote a longer life for the relay and perhaps reduces the transients even more than the diode.  It's something fairly easy to try anyway.
 
 
P.S. I socketed the relay as shown in the pic. You have to snip off some of the pins to make it work and even two DIP-8 sockets work as well (as shown in the pic).

 

[zwack]

BTW - I was able to find four of the tubes we spoke about in 12FM6 (RCA grey plates) and they'll be shipped out tomorrow.  Unfortunately, the 12FM6 has reached a point of scarcity that there's very little to pick from.  AFAIK, Beezar has the largest stock anywhere, and it's not much - perhaps about 150 at this point in time.

 
I just read that my tubes where shipped. Thanks for all the efforts to serve my request! Very much appreciated!
 
Regarding the spikes. They're actually not that bad. Just a soft click when the relay is actuated. The click is a bit louder when the relay opens again, but certainly not that loud that it would damage the headphone.
 
btw I corrected the text about the 22uf Vitamin Q's on my website  

 

[tomb]

Quote:

 
I just read that my tubes where shipped. Thanks for all the efforts to serve my request! Very much appreciated!
 
Regarding the spikes. They're actually not that bad. Just a soft click when the relay is actuated. The click is a bit louder when the relay opens again, but certainly not that loud that it would damage the headphone.
 
btw I corrected the text about the 22uf Vitamin Q's on my website  

Many thanks - I hope the tubes work out!  If they don't, though, please let me know and I'll do what I can with the stock that I have! 

 

[zwack]

I have a good feeling that the tubes will work out. 
Next Friday a friend of mine is coming over to listen the amplifier. I will probably be building another MiniMAX if he really likes it! 

 

[lordearl]

Is anyone using the Minimax with Beyerdynamic DT880s? I have the 600 ohm impedance version and plan to use jumpers at R8 & R9. Any feedback would be greatly appreciated as I am ok at soldering, but TERRIBLE at de-soldering.

 

[tomb]

Quote:

Is anyone using the Minimax with Beyerdynamic DT880s? I have the 600 ohm impedance version and plan to use jumpers at R8 & R9. Any feedback would be greatly appreciated as I am ok at soldering, but TERRIBLE at de-soldering.

Jumpers are good.  Also be sure you use the 12AE6 and adjust the tube bias for the greatest voltage swing, which is the 13.5V midpoint of a 27VDC setting at V+ and GND.

 

[lordearl]

Finally finished my build this afternoon, however there's a slight hitch - I keep getting 0mV when measuring TB1L (or TB1R) with reference to TA2L. This also happens with the right channel.
Everything else appears fine (volts in 27v, tube bias 13.5) and I can't have smoked the bd137, etc, as I've checked the orientation numerous times.

Any suggestions would be most helpful

 

[tomb]

Finally finished my build this afternoon, however there's a slight hitch - I keep getting 0mV when measuring TB1L (or TB1R) with reference to TA2L. This also happens with the right channel.
Everything else appears fine (volts in 27v, tube bias 13.5) and I can't have smoked the bd137, etc, as I've checked the orientation numerous times.

Any suggestions would be most helpful

 
Back up a minute and be very careful what you're typing or what you're measuring.

  To bias the buffer, you need to measure mV between TB1L (or TB2L) with reference to TA2L.  That's for the left channel.  For the right channel, it's TB1R (or TB2R) with TA2R.
 
If you want to be really simple about it, all those test points are doing is allowing you to measure the voltage across RB10L (or RB11L) for the left channel or RB10R (or RB11R) for the right channel.  You could just as easily pick one of those resistors and measure the voltage across the leads - that's the bias for that channel.  There are two sets of resistors - RB10 and RB11 for each channel, that's why there are two test point choices for each channel.  However, unless the transistors in the buffer are grossly unmatched, the bias is going to be pretty close between RB10 and RB11 for each channel.
 
Anyway - see if that clears some things up for you and let us know.  On the other hand, if what you typed above is a typo, then try it again and let us know specifically what you're measuring.  You should be able to measure about 20-30mV even with the transistors un-biased.

 

[lordearl]

Thanks Tom, yes was a typo, however I was measuring the correct points, results are as follows;

TB2R-TA2R: 0.0mV
TB1R-TA2R: 0.0mV
Voltage across RB10R: 0.0mV
Voltage across RB11R: 0.0mV

The left side reads exactly the same.

The voltage is 27.1v DC and I set the bias on each tube to 13.5v.
After a minute or two the bias always goes to roughly the 16v mark on both tubes.
Also the LED (LEDC) keeps wildly changing color.

Does this help in narrowing down the issue?

 

[tomb]

Thanks Tom, yes was a typo, however I was measuring the correct points, results are as follows;

TB2R-TA2R: 0.0mV
TB1R-TA2R: 0.0mV
Voltage across RB10R: 0.0mV
Voltage across RB11R: 0.0mV

The left side reads exactly the same.

The voltage is 27.1v DC and I set the bias on each tube to 13.5v.
After a minute or two the bias always goes to roughly the 16v mark on both tubes.
Also the LED (LEDC) keeps wildly changing color.

Does this help in narrowing down the issue?

 
Well, the center "Color Change" LED is supposed to keep wildly changing color.

 
As for the rest, you've got something amiss in the buffer.  As I stated up there, you should be able to read 20-30mV even with the RB12 trimmers turned all the way down.
 
Time for some pics.

 

[lordearl]

No worries - pics attached.  Hopefully the resolution is OK.
By the way, sound is actually making it through the amp - it is highly attenuated (volume is less than the output of the source, even with the pot all the way up, and the sound is rather distorted in bursts).
 
 

 

 

 

 

 

 

 

 

 

 

[tomb]

  No worries - pics attached.  Hopefully the resolution is OK.
By the way, sound is actually making it through the amp - it is highly attenuated (volume is less than the output of the source, even with the pot all the way up, and the sound is rather distorted in bursts).
 

 
I'm sorry to say that you have the output transistors soldered to the PCB on the wrong side of the heat sinks.  Look at the first page in the "Notes on the Millett Hybrid MiniMAX" document that was included in your kit.

 
Or here:

Swap them out to the other side and let me know if things work after that.  If not, I'll send you some more transistors.

 

[lordearl]

Argh!
And I was attempting to be so meticulous - two fundamental errors, putting the BD137 in the wrong spot, TWICE!
 
How important is the matching?  I can try to source two BD137 from a local electronics store.
 
Oddly enough, I have two pairs of BD139/BD140 already mounted to heatsinks here with me, will they work at all?