New Millett Hybrid Maxed Amp - Page 431

Quote:

Originally Posted by ModMax 

Asked tomb this same question by email, but thought I would post it here to in case it helps others or others can offer some advice.

 

Does the following wiring diagram look correct for the MOSFET-MAX power socket to SPST switch and PCB with fuse?

 

 

 

 

 

Power_Switch_Wiring_Diagram.pdf 164k .pdf file

Yes. Tomb may want to chime in further.

Or is this alternate wiring preferred so that no power is supplied to any circuits until the SPST switch is thrown?

Diagram PDF attached

Quote:

Originally Posted by ModMax 

Or is this alternate wiring preferred so that no power is supplied to any circuits until the SPST switch is thrown?

 

Diagram PDF attached

 

Power_Switch_Wiring_Diagram_Alternate.pdf 164k .pdf file

This circuit will not work. You need to connect AC-2 to the other side of the power supply. You need to "close" the circuit for current to flow. This circuit has no return path, so no current will flow even with the switch "on".

There's plenty of information about wiring on the MAX website. I think this particular picture from the website describes well what you need to to:

This is exactly what your first schematic showed. The polarity is irrelevant.

Thanks Kim.  The pic is very helpful - thanks for including.

I think in my first schematic though, I made a mistake.  I had the power socket GND pin left as unconnected, but the pic above shows this pin connected for return path.  I guess I am a bit confused on the socket - why are there 3 contacts on this socket?  Just an extra + lead?

Quote:

Originally Posted by ModMax 

Or is this alternate wiring preferred so that no power is supplied to any circuits until the SPST switch is thrown?

 

Diagram PDF attached

 

 

 

 

Power_Switch_Wiring_Diagram_Alternate.pdf 164k .pdf file

I don't understand the question, as it currently is, no power is supplied until the switch is closed. That's how the switch works.

Quote:

Originally Posted by ModMax 

Thanks Kim.  The pic is very helpful - thanks for including.

 

I think in my first schematic though, I made a mistake.  I had the power socket GND pin left as unconnected, but the pic above shows this pin connected for return path.  I guess I am a bit confused on the socket - why are there 3 contacts on this socket?  Just an extra + lead?

That socket is totally typical as it comes with three pins.  It's meant to cut out a battery circuit if you plug in an AC Adapter.  The third pin is connected to the other non-center pin when no plug is connected.  Thus, you can run one lead of a DC battery circuit through the pins.  When the plug is inserted into the socket, the circuit to the battery would then be broken.  For our purposes, you just need the one pin that is "hot" when a plug is inserted, combined with the center pin.  I never remember for sure myself which is which.  I simply touch a DMM probe to the contact spring inside the socket and find out which pin has zero resistance by touching the other DMM probe.  Or, simply look at the photo above that Kim posted: center pin and far outside pin.

I didn't comment on your schematic before because it made no sense without context.  Are you building a point-to-point MAX or MiniMAX?  If not, then the PCB's are being used.   In all the MAXes/MiniMAXes, the DC power supply is onboard the PCB.  So all outside connections - walwart, adapter socket, down to the terminal block - are all AC current with no polarity.  Note that you still need two AC connections from the wall and walwart, however.  You can't simply create one by "teeing off" of an existing AC lead.

How do the Kiwame output resistors effect the sound? I got some laying around, and was thinking about swapping them into my Millet, but never got around to it. Does it really make an audible difference?

Would a 12AU6 work as a drop in tube? I'm just curious, found about a gazillion on ebay really cheap, curious how they might sound.

Quote:

Originally Posted by scootsit 

Would a 12AU6 work as a drop in tube? I'm just curious, found about a gazillion on ebay really cheap, curious how they might sound.

It might be a possibility for another amp design, but the 12AU6 is not a possibility as a drop-in replacement for the Millett Hybrid design.  Even discounting the fact that it's not a space-charge tube that runs with 12.6V as design-center plate voltage (the 12AU6 wants 250V!), the pinouts are critically different.  In the 12AE6, 12FM6, and 12FK6, the cathode is pin 2 and the plate is pin 7.  In the 12AU6, the cathode is pin 7 and the plate is pin 5.

Not a good swap if you want to keep things from blowing up.

Quote:

Originally Posted by tomb 

Quote:

Originally Posted by scootsit 

Would a 12AU6 work as a drop in tube? I'm just curious, found about a gazillion on ebay really cheap, curious how they might sound.

It might be a possibility for another amp design, but the 12AU6 is not a possibility as a drop-in replacement for the Millett Hybrid design.  Even discounting the fact that it's not a space-charge tube that runs with 12.6V as design-center plate voltage (the 12AU6 wants 250V!), the pinouts are critically different.  In the 12AE6, 12FM6, and 12FK6, the cathode is pin 2 and the plate is pin 7.  In the 12AU6, the cathode is pin 7 and the plate is pin 5.

 

Not a good swap if you want to keep things from blowing up.

I guess I should have actually taken the 30 seconds to read the pinout and more before asking. I just saw the same heater voltage/current and assumed...oops.

So, I have finally powered-on my MOSFET-MAX for the 1st time and in the process of biasing.  Currently, my values are as follows:

- Power Supply Voltage = 27.0 VDC

- Tube Bias = ~13.5 VDC for both tubes

- DB Bias = 0.0 mVDC for both L & R

I am using matched 12FM6 tubes I purchased from Beezar.com.  They are new so I expect fluctuations in Tube bias for a bit until they are "broken in".

My questions are:

- Is it enough to break-in the tubes by turning on the amp and (1) leaving it on for hours, and (2) occasionally checking and adjusting the tube bias, and (3) leaving the DB bias at 0.0 mV, and (4) running no audio through the amp?  

- How long is the typical break-in period for new tubes?

- Is there a suggested or best method to break-in new tubes?

- Should I wait for the tubes to break-in before adjusting DB bias since DB bias can have negative impact on transistor health?

Quote:

Originally Posted by ModMax 

So, I have finally powered-on my MOSFET-MAX for the 1st time and in the process of biasing.  Currently, my values are as follows:

 

- Power Supply Voltage = 27.0 VDC

- Tube Bias = ~13.5 VDC for both tubes

- DB Bias = 0.0 mVDC for both L & R

 

I am using matched 12FM6 tubes I purchased from Beezar.com.  They are new so I expect fluctuations in Tube bias for a bit until they are "broken in".

 

My questions are:

- Is it enough to break-in the tubes by turning on the amp and (1) leaving it on for hours, and (2) occasionally checking and adjusting the tube bias, and (3) leaving the DB bias at 0.0 mV, and (4) running no audio through the amp?  

- How long is the typical break-in period for new tubes?

- Is there a suggested or best method to break-in new tubes?

- Should I wait for the tubes to break-in before adjusting DB bias since DB bias can have negative impact on transistor health?

Nothing is going to take more than a few hours to break-in.  However, until you put bias current to the DB, nothing is going to get "heat-soaked."  There is no advantage to not biasing the MOSFETs and getting that going.  "Negative Impact" is in years if your heat sinks are done properly.  MOSFETs like to run hot.  I'm no expert on MOSFETs, but seems like I've heard and been told that they're not even linear in a non-biased state.

Just keep in mind also that tubes are not perfect little solid-state devices.  I liken them to mules when it comes to biasing.  If you have them within a quarter volt, you're doing fine.  Also, if you're using 12FM6's, I would up the bias to 15V on the MOSFET-MAX.  You will lose very little voltage swing and the 12FM6's operate much better with a little more voltage.  You shouldn't miss the voltage swing on the MOSFET-MAX because the MOSFETs each up a couple of volts for biasing, anyway.

Thanks, TomB.

Can you recommend a sweet-spot value for the DB bias?  MOSFET bias setup states 220 - 275 mV.  Anywhere in that range you could recommend?

Quote:

Originally Posted by ModMax 

Thanks, TomB.

 

Can you recommend a sweet-spot value for the DB bias?  MOSFET bias setup states 220 - 275 mV.  Anywhere in that range you could recommend?

Depends on your case.  If you have plenty of cooling, go for the 275mV (125ma).  If not, then stay on the low end.  I think 150ma is too much for the PCB/heat sink design, even for a well-ventilated case.  It will sound noticeably worse if you go below ~90 ma (<200mV), though.

I followed the case templates from the MOSFET-MAX site and the heatsinks are 1.5" and I bought the HiQ Heat Sink Mounting kits from Beezar.com.  Given this info, what would you recommend for a max DB bias?