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scootsit said:

Perfect. I've never even gotten to 12 o'clock on the volume. Thanks, Tomb.

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Keep in mind that this seems to work best with 12FM6 tubes.  I'm not sure 12AE6's and 12FK6's have as much advantage at the higher voltage.  It just seems to suit the 12FM6 the most.

Maybe this has been addressed before, but here goes anyway:
 
To avoid adjusting bias on the tubes, which seems to vary more than any other bias on my MAX, would it be possible to use a voltage regulator IC (possibly adjustable depending on tube in use) to provide a stable tube bias VDC?
 
I am curious since I am not aware of commercial tube hybrid amps (Schiit, etc) requiring bias adjustments...

Quote:

modmax said:

Maybe this has been addressed before, but here goes anyway:
 
To avoid adjusting bias on the tubes, which seems to vary more than any other bias on my MAX, would it be possible to use a voltage regulator IC (possibly adjustable depending on tube in use) to provide a stable tube bias VDC?
 
I am curious since I am not aware of commercial tube hybrid amps (Schiit, etc) requiring bias adjustments...

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There's a catch: commercial tube hybrid amps (Schiit, etc.) do not use three completely different types of tubes.  And by different, I mean entirely different plate loadings.  They are designed for one tube, period.  Then everyone looks for all the other tubes that are functional equivalents and tries them for different sound characteristics (tube rolling).  Or, they go further and choose not-quite equivalent - but perhaps similar - tubes for even more differences in sound.  Bottom line, all the mfr has to do is simply choose one bias-setting resistor (assuming a similar circuit) and that's that.
 
Secondly, think for a minute about the voltage difference in commercial amps and the Millett Hybrid circuit.  The Millett uses automobile radio tubes - tubes that were designed to be used where the maximum voltage on the plates was around 12V.  Say your bias on a Millett is 1 or 2 volts off of optimum.  That's 10 to 15% error in the bias setting - somewhat significant.  However, consider a commercial amp that doesn't use these DIY-friendly tubes (low-voltage, no killings or electrocutions involved): it uses 100, 200, maybe even 300V on the tube plates.  Granted, the tubes may vary somewhat proportionally in bias variance - but not completely.  The bias voltage errors from tube to tube then become scatter within those huge voltages.
 
The Millett Hybrid and its variants - the revMH Millett Hybrid, the Millett Hybrid MAX, the Millett Hybrid MiniMAX, and the Millett Hybrid MOSFET-MAX, use three different tubes with different gain structures.  It's liking saying a commercial amp can take a 12AX7 and run the same with a 12AU7 (it can't).  That's why the bias is adjustable with trim pots.
 
Now, it just so happens that the Millett-style space-charge tubes happen to vary quite a bit in their bias settings, even among the same tube types.  That's another reason for having the little trimmers.  I suppose you might say there's a price to be paid for not worrying about electrocution.
 
That said, there is a dirty little secret that I will share, but not recommend: assuming you stick with the same tube type, you can probably set the bias once and get away with simply changing tubes out and never re-adjusting.  Yeah, it won't have the optimum voltage swing if the bias is off, but it will still play and sound pretty good.  It won't if you take out a 12AE6 and replace it with a 12FK6, however.  The bias may be so far off that there isn't enough voltage swing to produce an output.  Hence, the adjustable-resistance trimmers.
 
 
P.S. The Starving Student Millett Hybrid does not fall under this category.  It's a completely different design than the original Millett Hybrid and its variants.  It uses high-voltage tubes at a medium voltage - and the circuit is designed for that one tube, only.

Quote:

tomb said:

There's a catch: commercial tube hybrid amps (Schiit, etc.) do not use three completely different types of tubes.  And by different, I mean entirely different plate loadings.  They are designed for one tube, period.  Then everyone looks for all the other tubes that are functional equivalents and tries them for different sound characteristics (tube rolling).  Or, they go further and choose not-quite equivalent - but perhaps similar - tubes for even more differences in sound.  Bottom line, all the mfr has to do is simply choose one bias-setting resistor (assuming a similar circuit) and that's that.
 
Secondly, think for a minute about the voltage difference in commercial amps and the Millett Hybrid circuit.  The Millett uses automobile radio tubes - tubes that were designed to be used where the maximum voltage on the plates was around 12V.  Say your bias on a Millett is 1 or 2 volts off of optimum.  That's 10 to 15% error in the bias setting - somewhat significant.  However, consider a commercial amp that doesn't use these DIY-friendly tubes (low-voltage, no killings or electrocutions involved): it uses 100, 200, maybe even 300V on the tube plates.  Granted, the tubes may vary somewhat proportionally in bias variance - but not completely.  The bias voltage errors from tube to tube then become scatter within those huge voltages.
 
The Millett Hybrid and its variants - the revMH Millett Hybrid, the Millett Hybrid MAX, the Millett Hybrid MiniMAX, and the Millett Hybrid MOSFET-MAX, use three different tubes with different gain structures.  It's liking saying a commercial amp can take a 12AX7 and run the same with a 12AU7 (it can't).  That's why the bias is adjustable with trim pots.
 
Now, it just so happens that the Millett-style space-charge tubes happen to vary quite a bit in their bias settings, even among the same tube types.  That's another reason for having the little trimmers.  I suppose you might say there's a price to be paid for not worrying about electrocution.

 
That said, there is a dirty little secret that I will share, but not recommend: assuming you stick with the same tube type, you can probably set the bias once and get away with simply changing tubes out and never re-adjusting.  Yeah, it won't have the optimum voltage swing if the bias is off, but it will still play and sound pretty good.

  It won't if you take out a 12AE6 and replace it with a 12FK6, however.  The bias may be so far off that there isn't enough voltage swing to produce an output.  Hence, the adjustable-resistance trimmers.

 
 
P.S. The Starving Student Millett Hybrid does not fall under this category.  It's a completely different design than the original Millett Hybrid and its variants.  It uses high-voltage tubes at a medium voltage - and the circuit is designed for that one tube, only.
 

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Thanks Tom. 
 
Say I wanted my tube bias to be 14VDC for all tube types, 12AE6, 12FM6, and 12FK6.  Would it be possible to use a voltage regulator, or some other type of component, to dynamically adjust the bias to 14VDC independent of the tube type?
 
Maybe this is not possible given the circuit topology of the MOSFET-MAX?

Quote:

modmax said:

 
 
Thanks Tom. 
 
Say I wanted my tube bias to be 14VDC for all tube types, 12AE6, 12FM6, and 12FK6.  Would it be possible to use a voltage regulator, or some other type of component, to dynamically adjust the bias to 14VDC independent of the tube type?
 
Maybe this is not possible given the circuit topology of the MOSFET-MAX?

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Anything's possible, I suppose, but I suspect you're talking some pretty sophisticated electronics.  I doubt seriously the voltage regulator itself would be that widely tolerant - it would "drop out" of regulation pretty easily.  Some of the variances with the tubes can be as much as 10V from one type to the other.  Again, that's we have trimmers and measure.  
 
Maybe someone else may have a better idea.

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tomb said:

Quote:

modmax said:

 
 
Thanks Tom. 
 
Say I wanted my tube bias to be 14VDC for all tube types, 12AE6, 12FM6, and 12FK6.  Would it be possible to use a voltage regulator, or some other type of component, to dynamically adjust the bias to 14VDC independent of the tube type?
 
Maybe this is not possible given the circuit topology of the MOSFET-MAX?

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Anything's possible, I suppose, but I suspect you're talking some pretty sophisticated electronics.  I doubt seriously the voltage regulator itself would be that widely tolerant - it would "drop out" of regulation pretty easily.  Some of the variances with the tubes can be as much as 10V from one type to the other.  Again, that's we have trimmers and measure.

 
 
Maybe someone else may have a better idea.

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I was actually thinking the same thing. My thought was that the current required would fry an aweful lot of regulators.

Not to mention that the 14VDC is just the bias point on the plates. There's also AC audio signal at this point. If you simply place a voltage regulator in there, the regulator will filter out the audio signal.
 
The only way to do this is to use a servo like in the CTH. But it gets complicated fast. You have to redesign the whole circuit at this point.

Showing my MOSFET-MAX amp with HE-400 + AKG K271 at the Austin head-fi meetup tomorrow!  Can't wait...

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modmax said:

Showing my MOSFET-MAX amp with HE-400 + AKG K271 at the Austin head-fi meetup tomorrow!  Can't wait...

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Cool!  Post some pics when you can!

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tomb said:

Cool!  Post some pics when you can!

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Austin head-fi meetup pics posted by member pabbi1 - my amps in one of those pics... 
 
http://www.head-fi.org/g/a/747422/austin-march-2013

Hi guys, I'm gonna finish up my build tonight. I've read over all of the construction notes on the mini max site. I have a question. All the biasing for this thing, is this done with the case on? Seems to me it would have to be without the case to measure your db biasing and measure voltages off the pcb reference points? So in other words, do I adjust the trimmers for db biasing and power supply voltage with the case off and then do the tube biasing off of the rear bias posts after the case has been assembled? Edit, this is on a mini max if that matters.
 
Thanks Trav

Quote:

tdubl07 said:

Hi guys, I'm gonna finish up my build tonight. I've read over all of the construction notes on the mini max site. I have a question. All the biasing for this thing, is this done with the case on? Seems to me it would have to be without the case to measure your db biasing and measure voltages off the pcb reference points? So in other words, do I adjust the trimmers for db biasing and power supply voltage with the case off and then do the tube biasing off of the rear bias posts after the case has been assembled? Edit, this is on a mini max if that matters.
 
Thanks Trav

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You are correct.  You can't reach the DB biasing points once it's cased up.  The idea is that the bias never changes - unless you're prepared to take it apart to re-adjust it to some new value.  On the other hand, every tube change needs to be checked for bias, thus the tip jacks in the rear and the little holes on top to poke a trimmer tool through.
 
Keep in mind that with DB bias, you're not referencing to Ground.  All that's going to show is 27VDC (assuming you set the PS at the recommended voltage).  Instead, you are using the points that bridge the push-pull resistors - RB10-L/R and RB11-L/R.  There are two forward points for each channel, one for each resistor.  Really though, the resistors are not going to be different, but the two points are convenient because of the crowded area between the heat sinks and the output capacitors (CA7, CA8).  Simply pick which one's easiest to get to, then the other measuring point is that channel's tube bias point.  So, if you've wired up the backplate, but haven't totally assembled it, you'd put one DMM probe in the proper channel tip jack.  Then the other probe goes into one of the two forward test points between heat sinks.
 
I've always recommended 110mV, which results in 50ma, but you can safely push it to 70ma with the MiniMAX custom case - it ventilates that well.  Be sure to use those tall square bumpers I supply, though, otherwise the intake slots on the bottom are covered up.

Awesome response. Thanks Tom. I'll post up results when i get get buttoned up. Thanks again

Hey guys, have you heard that Intel recently developed a new MOSFET? The transistor is evidently the same as the one in the ivy bridge chips. It allows for 37% faster switching and 50% less power use. They're called tri-gate transistors. Any chance one could be popped into a Millett? More here: http://en.wikipedia.org/wiki/Multigate_device#Tri-gate_transistors
 
Also, if they use that little power, wouldn't this be a great transistor for portable applications?
(If the transistors themselves were commercially available, of course)
 
 
Sorry for the way-off-topic-post

Scootsit. I'm not sure that's going to be any use in power applications, it looks to me that its not a discrete device so much as another way of packing more transistors into a memory chip.