[the_equalizer]

Maybe you reversed the signal and ground connections for that channel at some point without noticing. Probably when wiring the headphone jack?

 

[Amatsu]

How do you reverse a signal? I also thought it had something to do with the headphone jack because if I disconnect the ground from it, the same hum appears in both channels. But I resoldered it a few times with no luck.

 

[Danthrax]

Quote:

How do you reverse a signal? I also thought it had something to do with the headphone jack because if I disconnect the ground from it, the same hum appears in both channels. But I resoldered it a few times with no luck.

Check all solder points, ALL seriously that was my issue. Also make sure you have the pin outs on the bottom of the tube isolated from each other (wrap them in electrical tape after you check the solder).

 

[the_equalizer]

What I meant is that you could have soldered the "hot" wire to the jack's ground lug and the ground wire to the jack's "hot" tab.  I't also been known to happen at the other end (the RCA input jacks).
 
cheers!

 

[Amatsu]

Thanks. I'm pretty sure the headphone jack is correct. I tried it again today connecting everything and realized that even with the semi-loud hum, there's music playing in both channels and it sounds very nice. At least I know there is nothing broken, that's a relief. There must be a serious ground issue since I also get pops and noises touching the other parts or the case itself, also putting the hand close to the tubes can cause the hum to get louder.
 
I'm thinking of starting again in a different case, maybe it wasn't a good idea doing it in a cardbox. I have a cookie case ready just in case, looks like aluminium: http://img.alibaba.com/photo/209300012/Royal_Dansk_Danish_Butter_Cookies.jpg
 
I also have a question about tube pins 4 and 5. Right now I have pin 5 alone to ground, I was wondering if maybe it was 4 goes through 5, and then to ground. I'll keep you guys updated.
 
 

 

[the_equalizer]

I thought you mentioned the loud hum was only in one channel, thus my suggestion of your possible wiring inversion. If the hum is in both channels then it most surely is indeed a grounding issue.
 
Since most tubes are not magnetically or electrically shielded, they'll hum when you put your hand close by; that is normal.
 
Metal enclosures definitely work better in my experience. You can't solder to aluminum so I strongly suggest you do as Mr. Millett and use a PCB blank or copper sheet as a ground plane for your circuit; look at the pics of his build on his site or this thread's very first post.
 
About pins 4 and 5, which are the tube heater: one of them must connect to the MOSFET and the other one to ground, it makes no difference which does what (the heater is not polarized) but when drawing the schematic a choice had to be made. Connecting them together will not only short out the tube heater (the heater won't light up and the tube won't work at all) but will also short the MOSFET source to ground.
 
cheers!

 

[Amatsu]

Good to know, thanks. The hum is often on the right channel, but sometimes after messing around with the ground wires it goes to center. I'll start on the new case tomorrow

 

[Beftus]

A 3-channel Starving Student. Has anyone built this too?
 
http://doesitpew.blogspot.com/2011/02/modified-and-refined-ss-tube-hybrid.html

 

[Danthrax]

Ok so today I noticed a volume imbalance between the left and right with left being significantly louder. I turned the volume to off (all the way down on the volume knob) and audio is still playing from the left channel. So I am wondering if there is a pot that is better than the standard 50k alps pot?

 

[tomb]

 
Quote:

Ok so today I noticed a volume imbalance between the left and right with left being significantly louder. I turned the volume to off (all the way down on the volume knob) and audio is still playing from the left channel. So I am wondering if there is a pot that is better than the standard 50k alps pot?

Let's be certain here before we go casting aspersions on brand names.  I've never had an ALPS RK27 do what you describe.  However, the Alpha pot originally spec'd for the Starving Student has had some known channel-balance issues, particularly at the bottom end of travel.  Quality control is not as good, either, so you may be able to buy several (they're pretty cheap), test them with a multimeter, and use the best one of the bunch.
 
One other method that we've recommended many times in this thread: select the input resistors so that most of your volume adjustment is in the middle of the pot's travel.  Even if there is a mis-match near the start of the pot turn, there should be plenty of room to allow for a complete shut-off at low volumes.
 

 

[Danthrax]

Actually I was using this one http://www.mouser.com/ProductDetail/Alpha-Taiwan/RV122F-20-15F-A50K/?qs=sGAEpiMZZMtxdMMi52izyq1dbvqSIaEoVagiCpW33UU%3d and I don't think that there should be any volume coming from any channel when the knob is fully off that is a defect regardless of if it has bad tracking, it should turn off. But I did manage to find a revision? or at least what looks like a newer model here: http://www.mouser.com/ProductDetail/Alpha-Taiwan/RV122F-20-15F-B50K/?qs=sGAEpiMZZMtxdMMi52izyq1dbvqSIaEoVMNCqk4w9sE%3d

 

[DarkTiger892]

Finally finished my 12AU7 build up with a lot of help from the_equalizer (thanks so much, you're a very patient and helpful guy).  Just wanted to share a few things I ran into that might help someone building one in the future.  I'm sure some of these things are super obvious to those who have built stuff like this before, but I know I struggled with them when I started building this.
 
A)
12AU7 Tube socket = 7/8" hole.  A step bit (even from harbor freight) does a really great in making perfectly sized holes in metal).  
 
B)
I had some buzzing issues, that were fixed by doing two things:
1) Make sure your wiring on the volume pot is correct.  I had my inputs flipped with my outputs and that induced a great deal of 60/120hz hum.
2) Make your output wires from the volume pot to the 12AU7 as short as possible.  The shorter they are, the less noise they pick up, and the less noise that gets amplified by first stage (tubes).
 
C) The 6mm knobs that are speced for the volume pot suck and don't fit on very well.  If you can find something better, get them.  They don't push on nearly far enough (even the deeper knob that mouser has in stock).  I had to bore out my knobs a bit, and then glue them to the pot to get them to go on far enough and look nice.
 
D) My biggest screw up was ordering 390 ohm resistors and not 390K ohm resistors for R2 and R8.  The difference between the incorrect 390 OHM and 220K OHM resistors makes the tube heater voltage very high (thanks the_equializer), the tubes very bright (and unhappy) and will quickly blow out the mosfets due to a higher than speced voltage on the gates.  
 
E)  I put a double pole switch on my outputs that when off isolates the headphone jack from the output of the amp.  I didn't want to have to keep plugging my headphones in and out, so I put a switch to disconnect.  This way I can turn the amp on, let it warm up, then flip the mute switch.  When I'm done, flip the mute switch off, flip the power switch off.  No thud, no voltage spikes on the output = happy headphones.
 
Here's some pictures.  Case is a rad shack project box.
 
http://img18.imageshack.us/i/finisheds.jpg
http://img145.imageshack.us/i/finisheddark.jpg/
 
edit:
F)  To anyone thinking about using these heatsinks: http://www.mouser.com/Search/ProductDetail.aspx?R=657-15ABPvirtualkey56720000virtualkey567-657-15ABP Those pins don't pull out.  They will shear off before they will pull out (speaking from experience).  They're nice heatsinks, but just be aware of this.  If you want to use them though, they must either be soldiered down (if they're on a material you can soldier to) or flip them over, and use the holes on the other side.  Don't even try to pull the pins out, you will just make a mess.
 
 
 
 

 

[tomb]

Quote:

Actually I was using this one http://www.mouser.com/ProductDetail/Alpha-Taiwan/RV122F-20-15F-A50K/?qs=sGAEpiMZZMtxdMMi52izyq1dbvqSIaEoVagiCpW33UU%3d and I don't think that there should be any volume coming from any channel when the knob is fully off that is a defect regardless of if it has bad tracking, it should turn off. But I did manage to find a revision? or at least what looks like a newer model here: http://www.mouser.com/ProductDetail/Alpha-Taiwan/RV122F-20-15F-B50K/?qs=sGAEpiMZZMtxdMMi52izyq1dbvqSIaEoVMNCqk4w9sE%3d

That's a linear taper pot and will not work for audio.  Yes, inexpensive pots will leak sound at their lowest setting.  There are probably other limitations you may notice with a Starving Student purchasing philosophy.

  I'm sorry about that disappointment, but I gave you some suggestions to help alleviate the issue.  My concern was that you incorrectly blamed ALPS, which has a sterling reputation around here, costs significantly more, and doesn't have the problems you're complaining about.
 
I have Walkmans and other stereo equipment that leak sound with very efficient buds and the volume racked to zero.  It's not an isolated issue.  You give up certain conveniences, depending on the design constraints of the product. 
 
 

 

[Danthrax]

Oh no I wasn't blaming alps I was just wondering if there was a better model.

 

[Amatsu]

Hey. I tried rebuilding the amp on my first wood case, the one where I failed twice. This time I managed to build it with fewer cables and better positioning. Well, even though there's still a quiet hum in both channels, I can finally listen to it. There's still something wrong with the ground because there are still some issues. For example, touching the volume pot generates a different kind of hum until I let it go. Also in some circumstances touching the case generates some pops.
 
I'll try to take some pics tomorrow, maybe you guys can see someting or give me some tips on the ground thing. It's messy though because I'm a newbie at these things and on top of it my soldering iron is kinda big for these projects.
 
I have spend around 40 mins listening to it, I almost cried when I have finally listened to the Beyer's with an amplifier, I've had these for around 2 years without amplification.