[schubert]

So higher is better on the mosfets?  I biased mine to the middle of the acceptable range (220mV), figuring that would be optimum.  What happens if you set bias to the maximum and it drifts up?
 
While I'm at it, I have a question about tube bias.  At one place on the site it says that tube bias should be 1/2 the supply voltage, which in my case is half of 27.5 or 13.75V.  The heater resistor section says that 12.6V is optimum.  Should I try to get as close to 12.6V as possible, or is there some good reason to nail it (it would be like nailing jello to the wall) at 13.75V?

 

[tomb]

 
Quote:

So higher is better on the mosfets?  I biased mine to the middle of the acceptable range (220mV), figuring that would be optimum.  What happens if you set bias to the maximum and it drifts up?
 
While I'm at it, I have a question about tube bias.  At one place on the site it says that tube bias should be 1/2 the supply voltage, which in my case is half of 27.5 or 13.75V.  The heater resistor section says that 12.6V is optimum.  Should I try to get as close to 12.6V as possible, or is there some good reason to nail it (it would be like nailing jello to the wall) at 13.75V?

The heater circuit and the plate voltage are separate and not connected - same as for any tube, not just the low-voltage ones in the Millett.  It just happens that since the tubes themselves are low plate voltage, they end up being in the same range - a coincidence.  The heater voltage is related to the supply voltage and the size of the R1 resistor.  It will use the supply voltage and cut it in half (the tube heaters are connected in series).  Then the R1 is used to tweak the final value to get it as close to that 12.6V as practical.
 
Anyway, unless you have a huge transformer feeding your power supply, I would back off to just 27V even.  That extra 0.5V may mean your MAX's PCB power supply is not removing as much ripple as it should.
 
As for the tube bias, which is the plate voltage - yes, 1/2 of the supply voltage will give you the most voltage swing.  However, the tubes will actually sound better (less distortion) with higher voltages.  The downside is that this cuts into the voltage swing.  You may find that the tubes sound better biased to 15V, but then you only have a swing from 15V to 27V on the high side.  For low impedance phones, which is where the MOSFET-MAX really shines, setting the bias to 15V will have little disadvantage.  You might notice it with Senns.

 

 

[dbfreak]

 
Quote:

 
Yes - looks like I need to correct the "tu", but polarity doesn't matter.  What you're reading is the voltage across the RB10 resistor in one case and the RB11 resistor in the other - for each channel.   The readings will be slightly different, because there are complementary differences and slight errors in matching/resistors, etc. - but they should be fairly close within the same channel.  It really ends up being an "either-or" type thing.  Although, I personally find which one reads higher and then use that for safety's sake.
 
Divide the voltage reading by the value of the resistor (they're 2.2R) and that gives you the milli-ampere bias.  I would try to get them up to 120ma (~264mV).

 

Is that 264mV AC Voltage?

 

[dbfreak]

Finished MOSFET MAX - Sounds wonderful and I am amazed at the clarity and detail of the music.  A few shots of the valves glowing...
 

 

 

 

 
Thanks much tomb for anwering my questions and providing some of the parts and board.

 

[ovrclkd]

Quote:

Finished MOSFET MAX - Sounds wonderful and I am amazed at the clarity and detail of the music.  A few shots of the valves glowing...
 

 

 

 

 
Thanks much tomb for anwering my questions and providing some of the parts and board.

Awesome job DB .... I may have to look for that Diamond Plate ....

 

[dbfreak]

re:  "Awesome job DB .... I may have to look for that Diamond Plate ....

"
 
Thanx!  It's available at DragonPlate (online shop).  It's actually carbon fiber veneer and it's woven into a 3D pattern.  Forgot to mention, the bass on the MAX is right there (deep and controlled).

 

[schubert]

 
Quote:

 
The heater circuit and the plate voltage are separate and not connected - same as for any tube, not just the low-voltage ones in the Millett.  It just happens that since the tubes themselves are low plate voltage, they end up being in the same range - a coincidence.  The heater voltage is related to the supply voltage and the size of the R1 resistor.  It will use the supply voltage and cut it in half (the tube heaters are connected in series).  Then the R1 is used to tweak the final value to get it as close to that 12.6V as practical.
 
Anyway, unless you have a huge transformer feeding your power supply, I would back off to just 27V even.  That extra 0.5V may mean your MAX's PCB power supply is not removing as much ripple as it should.
 
As for the tube bias, which is the plate voltage - yes, 1/2 of the supply voltage will give you the most voltage swing.  However, the tubes will actually sound better (less distortion) with higher voltages.  The downside is that this cuts into the voltage swing.  You may find that the tubes sound better biased to 15V, but then you only have a swing from 15V to 27V on the high side.  For low impedance phones, which is where the MOSFET-MAX really shines, setting the bias to 15V will have little disadvantage.  You might notice it with Senns.

 

Thanks, TomB.  Well, I learn something new every day.  I had equated the tube bias with the heater voltage!  Now I'll have to see if I finally have enough holes in this thing to adjust everything from the outside. 

 
When you mention voltage swing, my experience with cmoys and virtual grounds pops into mind - any similarities here?
 
I do have a biggish transformer - it's a wall wart, Triad WAU24-1800 (24VAC, 1800mA, 52.5W) - should I worry about that extra 0.5V?  I suppose I could try to measure the ripple, but for me that would be a major undertaking.  I used 27.5V because the site gave that as nominal.
 
Speaking of transformers, dbfreak - yours looks interesting - what are the details?

 

 

[dbfreak]

The MOSFET MAX is running circles around my Beta22 when it comes to playing albums like Metallica, Judas Priest, etc.,  (bass slam is  unbelievable)!

 

[schubert]

No Judas Priest in my collection, but there is a palpable sense of space with this amp while playing Bach cantatas - so much so that at first I thought the speakers were playing!
 
My only reservation so far remains the piano sound - I've only found one piano album it likes so far.  It's hard and clattery in the upper midrange-low treble.  Maybe the Elnas will take care of that - if they only would get here!

 

[dbfreak]

Transformer is a wall wart (purchased from Beezar) that was removed from it's original housing and mounted in a new enclosure.  While it was out, I resoldered all of the connections (some were really nasty), installed a better MOV and relocated the LED.  The main reason I did this was to enable the use of an IEC power connector from the transformer box to the wall outlet.  This makes it easier to use since I don't have to find a wall outlet that accepts the original wall wart's screw that seems to line up only with certain wall outlets.
 


 
Quote:

 
Thanks, TomB.  Well, I learn something new every day.  I had equated the tube bias with the heater voltage!  Now I'll have to see if I finally have enough holes in this thing to adjust everything from the outside. 

 
When you mention voltage swing, my experience with cmoys and virtual grounds pops into mind - any similarities here?
 
I do have a biggish transformer - it's a wall wart, Triad WAU24-1800 (24VAC, 1800mA, 52.5W) - should I worry about that extra 0.5V?  I suppose I could try to measure the ripple, but for me that would be a major undertaking.  I used 27.5V because the site gave that as nominal.
 
Speaking of transformers, dbfreak - yours looks interesting - what are the details?

 

 

[dbfreak]

I have some classical piano only CD's and will try them out after a complete burn in.

 
Quote:

No Judas Priest in my collection, but there is a palpable sense of space with this amp while playing Bach cantatas - so much so that at first I thought the speakers were playing!
 
My only reservation so far remains the piano sound - I've only found one piano album it likes so far.  It's hard and clattery in the upper midrange-low treble.  Maybe the Elnas will take care of that - if they only would get here!

 

[schubert]

 
Quote:

Transformer is a wall wart (purchased from Beezar) that was removed from it's original housing and mounted in a new enclosure.  While it was out, I resoldered all of the connections (some were really nasty), installed a better MOV and relocated the LED.  The main reason I did this was to enable the use of an IEC power connector from the transformer box to the wall outlet.  This makes it easier to use since I don't have to find a wall outlet that accepts the original wall wart's screw that seems to line up only with certain wall outlets.

 

 

Good idea - I didn't even bother to try plugging mine into the wall (it's huge)  - I just use an extension cord.  I suppose that's the lazy man's way out! 

 
I'm now testing out the "bass slam" - bass drum whacks in Copland, Mussorgsky and Grainger - so far so good!

 

 

[dbfreak]

Actually, the extension cord idea is the smart way out.

 
Quote:

 
Good idea - I didn't even bother to try plugging mine into the wall (it's huge)  - I just use an extension cord.  I suppose that's the lazy man's way out! 

 
I'm now testing out the "bass slam" - bass drum whacks in Copland, Mussorgsky and Grainger - so far so good!

 

 

[dbfreak]

After a +24 hours burn-in I've got the MOSFET MAX set with the PS @ 27V, each tube (Philco 12FM6) at 13.5V and the MOSFETS at 120mA.  I put in a Stevie Ray Vaughan CD (Couldn't Stand the Weather) and was blown away by the MAX's ability to handle the bass and guitar notes in Tin Pan Alley through a pair of Grado 225's.  For a second, I thought of my Pioneer SX1250 through a pair of HPM-100's sounding as nice.

 

[tomb]

Wow!  Glad to hear you're enjoying the MOSFET-MAX.  I'm humbled at your comments, but actually - I think many people who seem to easily dimiss the venerable Millett Hybrid design would be quite shocked to hear a current-edition Millett Hybrid MOSFET-MAX.