[Beefy]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif So, if V is 27VDC, then we can establish "I" by choosing the correct value for R. If we were sizing it for the full current rating of the LED you mention above (it's typical), then we'd use 0.02A (20ma) for "I". Then R = 27VDC/0.02, or 1350 ohms.

Just for my own clarity...... do we have to take the forward voltage of the LED into account? Lets say it is 3V, then we use 24V in the equation, for 1200ohms?

 

[tomb]

Quote:

Originally Posted by Beefy /img/forum/go_quote.gif Just for my own clarity...... do we have to take the forward voltage of the LED into account? Lets say it is 3V, then we use 24V in the equation, for 1200ohms?

Yes - technically, you are correct. However, the exact voltage across the LED is not known for every case (typically it varies depending on the LED color). The direction of the effect of the LED is to lower voltage across the resistor, thereby reducing current. So it adds an additional safety factor that can be ignored in sizing the resistor. That's my own judgment call, though, by trying to pick a resistor size with reasonable certainty that it will work for everyone.

Someone else may want to calculate it more closely and look up the exact voltage drop of their particular LED and correct the exact voltage across the resistor accordingly.

 

[Beefy]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif That's my own judgment call, though, by trying to pick a resistor size with reasonable certainty that it will work for everyone.

Yeah, that's fine. Just double-checking for my own builds

 

[srserl]

I'm done populating the board except for the heat sink assemblies (yay)! I need to order the mounting kits, and I noticed that I almost have enough parts to build another max. Maybe try the mosfet max this time, but not sure...what are the pros and cons of the mosfet vs the bjt version? Is there an estimated timeframe for the new Mosfet Max web site?
Meanwhile, I have the case work to do for the current Max, and I also have the Millet starving student to listen to while I work.
Life is good!

Scott

 

[tomb]

Quote:

Originally Posted by srserl /img/forum/go_quote.gif I'm done populating the board except for the heat sink assemblies (yay)! I need to order the mounting kits, and I noticed that I almost have enough parts to build another max. Maybe try the mosfet max this time, but not sure...what are the pros and cons of the mosfet vs the bjt version? Is there an estimated timeframe for the new Mosfet Max web site? Meanwhile, I have the case work to do for the current Max, and I also have the Millet starving student to listen to while I work. Life is good! Scott

Glad to hear it!

About the MOSFET-MAX: I am still working on it and hope to have it done in the next week or two, at the most.

The pros and cons are probably equivalent to the differences between an M3 and a PPAV2.

The cons with MOSFETs are relatively easy: heat. They have to be biased much higher to obtain equivalent performance to the BJT. In the MAX, that means special considerations to the case. The easiest solution is to mount the board in the bottom slot of the standard Hammond (to make room for taller sinks), but that puts the tubes almost below the lid and lowers the pot and headphone jack below the centerline. It also pretty much precludes running the tip jack wiring underneath because there's no room under the board for the tip jacks themselves.

I'm going to try some special things to use the 1" sinks for the MOSFETs in at least one version, but haven't had the chance. Unfortunately, this is the busiest time of the year for me: both at home and at work. So, I'm still behind.

 

[rhester]

If you use the next size Hammond enclosure 1455H2201 you can still have the tip jacks included in the solution. It also makes it easy to add an Alien in the case. The MOSFETs are nice (my working version has the ES caps and VitaQs bypasing) with great low end extension, detail and thump (especially with the ES). The highs are a little different. I think they are more present sounding or up front than the BJTs, but have a little edge to them (especially conmpared to the BGs in the BJT builds). But I did dedcide to keep the MOSFET build and sell the BG BJT build, if that tells you anything.

 

[Beefy]

Quote:

Originally Posted by rhester /img/forum/go_quote.gif If you use the next size Hammond enclosure 1455H2201

1455T2201 perhaps?

Quote:

The MOSFETs are nice (my working version has the ES caps and VitaQs bypasing) with great low end extension, detail and thump (especially with the ES). The highs are a little different. I think they are more present sounding or up front than the BJTs, but have a little edge to them (especially conmpared to the BGs in the BJT builds)

That is almost exactly how I would describe the difference between my M^3 and the Max. The M^3 has a bit more slam at the low end, and a little more sizzle at the top end. Fairly similar through the middle.

 

[tomb]

Quote:

Originally Posted by Beefy /img/forum/go_quote.gif <snip>That is almost exactly how I would describe the difference between my M^3 and the Max. The M^3 has a bit more slam at the low end, and a little more sizzle at the top end. Fairly similar through the middle.

Well, thanks for that comparison, Beefy. I tend to shy away from direct comparisons these days - the MAX and the size of this thread have made us a bit of a target in certain circles.

Nevertheless, that's high praise, regardless - I know how much you love your M3.

 

[Beefy]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif I tend to shy away from direct comparisons these days - the MAX and the size of this thread have made us a bit of a target in certain circles.

Its a shame that you have to do that. Good comparisons are the best way for everyone to learn and make their designs better.

Quote:

Nevertheless, that's high praise, regardless - I know how much you love your M3.

Oi, I love both of my DIY amps!

I must say, I had more fun building my Max though. Partly because I knew a lot more than before starting my M^3, partly because picking boutique parts is much more interesting, and partly because the integration made casework a breeze. The Max certainly gets more comments with visitors too - there's just something about tubes......

 

[Iniamyen]

I just got my order from beezar and let me say wow. It was packed, organized and labeled really well. I figure it can't hurt to have another recommendation even if it's from a newb

As I'm waiting for my other parts from mouser/digikey (damn ups

) I guess the best thing to do is ask more questions to further avoid screwing things up when I start:

-I'm still a little confused about the heatsink assembly. I got the mounting kit from Beezar, the one that says it doesn't need thermal grease. However it seems as though everyone mentions using it. I have some arctic silver back from when I built my computer - would this work?

-I want to make sure I have the correct order down for the biasing step: Make sure DB bias is at minimum voltages, THEN bias PS, THEN bias tubes, THEN bias DB's. Can the DB's be damaged from sitting at their minimums? I assume the answer is no but I want to make sure.

-Grounding: I'm not sure what all is tied to case ground, or if I need to take precautions with the power supply. I notice that the power socket is isolated (so V+ and V- aren't tied to the case), but some of the notes about the RCA jacks used say that the MAX is negatively grounded. Can I tie the virtual ground to the case? Or will this cause problems? I'd rather not blindly assemble and then fry something.

Thanks!!!

 

[slowpogo]

Quote:

Originally Posted by Iniamyen /img/forum/go_quote.gif -I'm still a little confused about the heatsink assembly. I got the mounting kit from Beezar, the one that says it doesn't need thermal grease. However it seems as though everyone mentions using it. I have some arctic silver back from when I built my computer - would this work?

No, you don't need grease. The kit comes with a "thermal pad" that goes between the transistor and the heat sink; this serves the same function as the grease, that of heat transfer.

 

[Iniamyen]

Quote:

Originally Posted by slowpogo /img/forum/go_quote.gif No, you don't need grease. The kit comes with a "thermal pad" that goes between the transistor and the heat sink; this serves the same function as the grease, that of heat transfer.

That's my initial impression; however, mention is made of certain pad materials that you want to use grease for. I guess for this one you don't.

I'm probably just thinking about this too hard.

 

[slowpogo]

Quote:

Originally Posted by Iniamyen /img/forum/go_quote.gif That's my initial impression; however, mention is made of certain pad materials that you want to use grease for. I guess for this one you don't.

I didn't get my kits from Beezar, but I'm also using Bergquist thermal pads, and didn't use any grease. They are biased to 110 for a few months now so, so far so good.

 

[srserl]

Thanks for the info Tom and rhester. I still have plenty of time before I start my second one, so no hurry. I placed a mouser order today and included all items I might need for a mosfet max if that's what I decide to build. I read somewhere on the Millet Max site about using a small value resistor on the output...has anyone tried that on a mosfet max to see if it tames the highs a bit?
I plan to build a wood case for my next one, so I ordered the 1.5" heatsinks since there are no size restrictions for me. I also ordered a large heatsink that I will probably mount on the back of the case to help out. I'll place slots in the bottom and the back, plus the heatsink on the back.

Scott

 

[tomb]

Quote:

Originally Posted by Iniamyen /img/forum/go_quote.gif I just got my order from beezar and let me say wow. It was packed, organized and labeled really well. I figure it can't hurt to have another recommendation even if it's from a newb

Thanks for that kind remark.

Quote:

As I'm waiting for my other parts from mouser/digikey (damn ups ) I guess the best thing to do is ask more questions to further avoid screwing things up when I start: -I'm still a little confused about the heatsink assembly. I got the mounting kit from Beezar, the one that says it doesn't need thermal grease. However it seems as though everyone mentions using it. I have some arctic silver back from when I built my computer - would this work?

The other guys have answered this well. I'm pretty sure using grease will hurt the Bergquist pads, too.

Quote:

-I want to make sure I have the correct order down for the biasing step: Make sure DB bias is at minimum voltages, THEN bias PS, THEN bias tubes, THEN bias DB's. Can the DB's be damaged from sitting at their minimums? I assume the answer is no but I want to make sure.

Yep - sounds like a plan, and No - the DB's can't be damaged if everything is hooked up correctly and they're idling at low bias (<50mV).

Quote:

-Grounding: I'm not sure what all is tied to case ground, or if I need to take precautions with the power supply. I notice that the power socket is isolated (so V+ and V- aren't tied to the case), but some of the notes about the RCA jacks used say that the MAX is negatively grounded. Can I tie the virtual ground to the case? Or will this cause problems? I'd rather not blindly assemble and then fry something.p

Yes, the MAX and every Millett before it are negatively grounded. The difference is the MAX has a high-power/performance linear-regulated power supply on the board. That means the power input is AC - there is no V+ and V- at the power socket, in other words - it's 24VAC. You don't want that AC touching the case, because the negative DC power further down on the board can come into contact with the case very easily - either on-purpose or by accident. For instance, the recommended Beezar RCA jacks are not insulated/isolated. Their ground (same as the board's negative) touches the case.

Another way to look at it is to look at the board itself: there is no ground plane in the back where there's AC. Technically, the 24VAC runs only from the terminal block to the string of rectifiers on the very back of the board. It's DC after that, but still carries ripple until going through the power caps and the regulator. The filtered, fully-linear-regulated DC power makes its final connection to the ground plane at CR5 (also RR2 and DR1). Quote:

Thanks!!!

You're welcome!