Millett "Starving Student" hybrid amp
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nullstring

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Quote:
EDIT: After reading nikongod's post I needed to clarify that I'm assuming a regular IRF510 / IRF610 MOSFET in which pin 1 is the gate, pin 2 the drain and pin 3 the source; as mentioned in this post from Dec 09.
 
Since you're talking about C2, and refering to this schematic, I assume it's V1 which doesn't light up.
 
By your description of your tests it seems to me that pin 1 of MOSFET Q1 is not at it's intended voltage level, which should be ~17 volts for a 12A_7 build and ~24 V for a 19J6 build. Since bypassing C2 makes the MOSFET turn on (which in turn lights up the tube), we can assume R3 is OK. Thus I can only think of R2 and R4 as the culprits, most likely R2.
 
Check the values of R2 and R4 (off circuit, of course; you can unsolder one end of each to check their values). Check their solder points. Check that the 'upper' (refering to the schematic) end of R2 does indeed sit at ~48V as the schematics point out. Check that the 'lower' end of R4 (again referring to the schematic) does indeed sit at ground ( 0 volts). If those previous conditions are true then the joint of R2, R4 and R3 must be at ~17 V. If this is true and yet the MOSFET does not turn on, then most likely the MOSFET is faulty.
 
BTW, about C2, it's purpose is precisely to isolate the MOSFETs pin 1 from the DC voltage and current present at the tube plate. Thus if it had somehow failed and became a "completely open circuit" that wouldn't affect the DC biasing of the MOSFET; you'd see the amp light and warm up correctly but no sound would come out of the Q1 channel, since the audio signal would not be able to cross C2 to get from the tube to the MOSFET.
 
On the other hand, if C2 had failed in other way and became a short circuit, then you'd see the tube light up 'as a Christmas tree' 
  as you yourself saw in your experiments.  That happens because with a shorted C2 the MOSFET gate sits at the DC voltage of the tube plate, ~27 V  if I remember correctly. Thus it's source pin is then forced to be at around 22 volts, which is the voltage that feeds the tube heater: 10 volts higher than it's intended voltage, making it light up brightly.
 
Since from your description we know that none of these last two things happen, we can pretty safely assume that C2 is OK.
 
I hope this helps you in understanding the circuit, but more importantly, to get your amp back up and running.

 
cheers!
 
As it turns out, the resistor connected to the positive voltage was cold soldered (I never reflowed this one, it was the best looking joint on the terminal strip >_<)
Thanks again for the help.
 
 
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post-6941205
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the_equalizer

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Quote:
 
As it turns out, the resistor connected to the positive voltage was cold soldered (I never reflowed this one, it was the best looking joint on the terminal strip >_<)
Thanks again for the help.
 

No problem! So it was indeed R2. Great to hear you got it back up and running again!
 
cheers!
 
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marcus76

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Hi there Head-Fi,
First post, my name is Marcus and i'm from germany, excuse my bad english.

After reading most of this thread (which took quite some time),
I decided to go for a build even though some parts got me confused.
I have some soldering experience and from the many great builds i've seen so far i thought it could be possible for me to do too.
 
There's only a problem with sourcing the parts here in germany since i don't want the shipping costs to eat up my budget that much by ordering at like 4 to 5 stores. Another problem would be which bom to use for an 12au7 build?
(Which of the modifications would be useful for my senn hd650 for example?)
I already checked some online stores, most of them only had some of the parts needed, were to pricey compared to the orig. bom, some parts not available at all, plus i can't decide which comparable parts would fit.
I'll make a list/bom at the weekend and post it here, maybe someone could check it and tell me if i chose the right parts,
before i place an order.
 
btw, already ordered parts are the power supply and some cheap nos tubes for 8 euro a pair.
I took a look at www.jacmusic.com/nos/ECC82.htm and got me two jan 6189W sylvania.
Hopefully these will do for my first try.
 
Thanks in advance for any help/info ,
Marcus
 
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post-6943425
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livewire

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@ Beftus - Nice clean looking build you got there!
And it works without problems? Good feeling, aint it?
Ha-Ha! I hear you about the bowl of spagetti inside, mine is like that too.
 
@ marcus76 - Your english is perfect! No worries there.
Beftus and the_equalizer are two of the 12AU7 experts around this thread.
(I'm not, mine is a 19J6 model, I havent taken the time to study other variants.)
I'm sure they will check in to help with your B.O.M. Good luck!
 
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post-6944042
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the_equalizer

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Quote:
Hi there Head-Fi,
First post, my name is Marcus and i'm from germany, excuse my bad english.

After reading most of this thread (which took quite some time),
I decided to go for a build even though some parts got me confused.
I have some soldering experience and from the many great builds i've seen so far i thought it could be possible for me to do too.
 
There's only a problem with sourcing the parts here in germany since i don't want the shipping costs to eat up my budget that much by ordering at like 4 to 5 stores. Another problem would be which bom to use for an 12au7 build?
(Which of the modifications would be useful for my senn hd650 for example?)
I already checked some online stores, most of them only had some of the parts needed, were to pricey compared to the orig. bom, some parts not available at all, plus i can't decide which comparable parts would fit.
I'll make a list/bom at the weekend and post it here, maybe someone could check it and tell me if i chose the right parts,
before i place an order.
 
btw, already ordered parts are the power supply and some cheap nos tubes for 8 euro a pair.
I took a look at www.jacmusic.com/nos/ECC82.htm and got me two jan 6189W sylvania.
Hopefully these will do for my first try.
 
Thanks in advance for any help/info ,
Marcus
 
Those tubes will certainly do fine for your build; you also need to get 9-pin mini tube sockets (like these ones).
 
As to which BOM to use, I believe there's no published BOM for P2P builds of a 12AU7 amp with Dsavitsk's improvements. You can either build using Pete Millett's original BOM (omit the tubes and tube sockets, naturally) and skip on Dsavitsk's improvements or use the diyforums.org BOM (again, omitting the tubes and tube sockets) and add three or four terminal strips (part number in Pete Millett's original BOM)
 
Finally, regarding where to source your parts from, I hope some of the builders in the thread that reside in Europe chime in and help you.
 
Good luck with your build and let us know how it goes; it's always nice hearing that a new SSMH has come to life.
 
cheers!
 
 
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post-6944087
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Ikarios

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Quote:
beezar doesn't sell pcbs/cases anymore, how would I get those?

If you want the original case, look for the Hammond model number. You'll have to drill holes yourself. You can use whatever case you want, but most likely you'll still have to drill holes yourself.
PCBs have been gone for a while; most builds since 19J6 stock ran out have been 12AU7, and point-to-point air-wired or done on a perfboard. It definitely takes more work, but you learn some more in the process because you basically have to understand the schematic.
 
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Beftus

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Quote:
 
There's only a problem with sourcing the parts here in germany since i don't want the shipping costs to eat up my budget that much by ordering at like 4 to 5 stores.
 
Have you tried Conrad.de? I order from Conrad.nl regularly, IME most of the stuff needed can be found @ Conrad. The parts needed to make one is pretty much bog standard stuff. Since German and Dutch Conrad part numbers are identical I can even help you find stuff that's difficult to locate in their catalog.
 
 
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Beftus

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Quote:
 
 
@ marcus76 - Your english is perfect! No worries there.
Beftus and the_equalizer are two of the 12AU7 experts around this thread.
 
 
Thanks for the praise, but I don't consider myself to be an expert. BTW I really like spaghetti 

 
 
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Secretninja

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Well, I got it finished but it doesn't work. One tube lights up, and produces sound, which I guess is a good thing.  It is however, very quiet.  The other tube fails to light up and the mosfet doesn't generate any heat.  Assuming I can figure out where I screwed up and fix the other channel, would removing the 50k resistors I put in between the rca jacks and the pot give it some more volume?
 
Thanks for any input.
 
Well, I tried re-flowing all the joints on the side that doesn't work, but no luck.  I'm hoping it isn't a problem where the mosfet is mounted, because that is epoxied in place.  Tomorrow I am going to pick up a piece of protoboard from radio shack and redo it on there.  I have more experience working with through holes, and I can check everything piece by piece.
 
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the_equalizer

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Low volume in the other channel is most likely an indicator of a miswiring in that channel too, if anything the SSMH is too loud, not too quiet. The only recommendation I can give you is to sit down and calmly trace your build against the schematic, wire by wire, pin by pin.
 
Good luck!
 
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