Millett "Starving Student" hybrid amp
May 8, 2013 at 11:56 AM Post #6,511 of 7,265

Beftus

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Quote:
*Edit I just did a quick thread search, tons of heat is apparently normal. I'm seriously giddy over how awesome this thing sounds though.
 
 

 
It does indeed get hot, that's perfectly normal for this amp. How hot it gets is also a function of how large the heatsinks you used are. Bigger heatsinks cool things better.
 
May 11, 2013 at 5:55 AM Post #6,512 of 7,265

pingu turbo

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Well I managed to sort my 'mono' problem. After re wiring my whole amplifier to achieve the same result. :mad:

I have a faulty 'iDevices' phono lead. It's band new ***! I did learn a lot more about grounding and have improved any hum issues no end.

Casually enjoying some stereo sound now. :)
 
May 12, 2013 at 4:12 PM Post #6,513 of 7,265

jarscar

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I had to take my amp apart to fix an issue I was having with the paint job, upon putting it back together one of my MOSFETs is getting well over 40v from the source pin instead of ~13v
I didn't have to change any wiring when I took it apart, aside from having to disconnect the RCA jacks and the power plug because those disconnect from inside the case instead of outside, I've checked and re-checked 4 times, I guess I'll try replacing that MOSFET 
I took the tube out and left the other in, I'm getting a crackle on start up from the empty line, I'm assuming that's the capacitor reacting with the excessive voltage from the source. The other MOSFET works fine and the sound coming out is normal.
 
May 13, 2013 at 12:52 AM Post #6,516 of 7,265

KimLaroux

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You're getting 40 V at the source, but 34 V at the gate? Sounds like a burned MOSFET. Or you're messing up the source and the drain. 
 
You could try to disconnect R2/R8 of the misbehaving fet and see what it does. Just beware that if you disconnect the wrong resistor (R4/R10), I can guarantee you you'll blow a tube. Been there, done that.
 
If it's not a blown mosfet, then I'm guessing you have a cold joint on one of the pull-down resistor, R4 or R10.
 
May 13, 2013 at 12:09 PM Post #6,517 of 7,265

jarscar

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You're getting 40 V at the source, but 34 V at the gate? Sounds like a burned MOSFET. Or you're messing up the source and the drain. 
 
You could try to disconnect R2/R8 of the misbehaving fet and see what it does. Just beware that if you disconnect the wrong resistor (R4/R10), I can guarantee you you'll blow a tube. Been there, done that.
 
If it's not a blown mosfet, then I'm guessing you have a cold joint on one of the pull-down resistor, R4 or R10.

Thanks a ton, that tells me enough to play it safe and just rebuild that section of the amp, I've got enough spare parts so there's no reason not to
 
May 13, 2013 at 10:03 PM Post #6,518 of 7,265

jarscar

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You're getting 40 V at the source, but 34 V at the gate? Sounds like a burned MOSFET. Or you're messing up the source and the drain. 
 
You could try to disconnect R2/R8 of the misbehaving fet and see what it does. Just beware that if you disconnect the wrong resistor (R4/R10), I can guarantee you you'll blow a tube. Been there, done that.
 
If it's not a blown mosfet, then I'm guessing you have a cold joint on one of the pull-down resistor, R4 or R10.

Edit: I turned it off and walked away, I guess I shouldn't jump to conclusions, the voltages are all correct now, I guess I should have unplugged everything then tested again, but thank goodness it was just user error and not mechanical failure. Also I'm buying a new multimeter, and not one from Harbor Freight this time, I spent the longest time thinking my power supply was dead, turns out I had to solder the positive lead back on in my brand new multimeter, that should teach me to stop buying anything but a hammer from there.
 
Well I replaced the MOSFET and all the resistors for that side, I switched it on and was getting the correct 15v at pin 1, 48v at pin 2 however I was getting 18v at pin 3, went back to my work table and double checked things.
Then when I was double checking my measurements my multimeter probe slipped and connected pins 2 and 3 for a split second, now I'm getting 33v at pin 3, so there goes another MOSFET I guess.
 
May 14, 2013 at 6:22 AM Post #6,519 of 7,265

ProTofik

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I have a question to everyone that built a student.
Has anyone tried to measure a crosstalk? I am getting quite bad result:
 
 

 
The lowest my sound card is able to measure is about -75db so I don't think that it's a problem in here.
 
May 14, 2013 at 11:35 AM Post #6,520 of 7,265

Goobley

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What pot are you using? As far as I'm aware this part has the greatest impact on crosstalk. I know my starving student doesn't have any audible crosstalk. I'll measure it when I get home on Friday to compare.
 
May 19, 2013 at 11:44 AM Post #6,522 of 7,265

Martinus

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Hi guys.

I've been visiting for a while and keeping tabs on this thread. First I'd like to thank everyone who put time and effort into this project, not least Mr. Millett.

I prototyped my version on stripboard and put it all into an ugly little case pending my finding a more aesthetically pleasing enclosure. Whilst not exactly pleasing to the eye it worked first time. I haven't ever listened to music with an amp of this kind so my aim was to build it fast and cheaply to get a feel for how much effort I should put in. The amp has all of the additional parts suggested on the 12AU7 schematic. I didn't get a pair of matched valves (they were very cheap though).

I'm glad to say that I really enjoy the sound through my Sennheiser 595's fed from a Nokia N900. However, some minor grounding issues remain and the voltage at pin 4 is closer to 16V on one channel. I'm hoping there's an easy fix for this so I'm going to re-trawl the thread.

Many thanks.
 
May 25, 2013 at 2:41 PM Post #6,523 of 7,265

audi0lurker

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How's It going guys?  So I have recently embarked on this project and am stoked to see how it will turn out.  Having recently finished the one and only circuits class I will ever need to take, I am not the most astute electrical engineer, but one of the reasons I started this project was to learn more about circuits and theory and also I'm a bit of an audiophile and this is the perfect project for me considering I am indeed a starving student.  
 
SO my question regards the Tube LED's.  I have drilled out the tube sockets, easy, but I do not know where to attach the LED and led resistor in the circuit?   Could some one shed some light on this for me?
 
Also, I using the specs on a 3mm LED I have and this website I calculated I need a 1kohm resit or rated for 2.5 watts.
 
So ya thanks for the help Ill be posting build pictures along the way, I wight even construct a small bass boosting circuit as well who knows!!
 
May 25, 2013 at 8:41 PM Post #6,525 of 7,265

DingoSmuggler

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Quote:
Also, I using the specs on a 3mm LED I have and this website I calculated I need a 1kohm resit or rated for 2.5 watts.

That doesn't sound right, how much current you gonna be passing through it? A 1k ohm will give over 40mA and fry the average LED.
A typical LED has a rating of 20mA, but at higher temps (such as in a tube socket) you will need to de-rate this down to a lower value.
Also LED brightness is proportional to current, the resistor sets this current.
 
Most LEDs these days are fairly bright, you probably don't want to light your tube socket up so brightly as to completely overpower the tube glow.
I'd probably aim for a current ~2mA (22k 1/2W resistor). Anywhere between 10k (brighter) and 100k(dimmer) would also work.
 
Also, if you prefer, you can string both tube LEDs in series, saving yourself one resistor and a small amount of power draw from the supply.
48V(+) ---- (+)LED1(-) ---- (+)LED2(-) ---- Resistor ---- 0V/ground
The resistor can go anywhere in the series circuit you like, on the positive side, on the ground side, or in between the LEDs.
 

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