Millett "Starving Student" hybrid amp - Page 431

Mechanical Engineers rock...

I was looking at the different specs for tube sockets and wasn't enterly sure I got the right ones, thanks alot for clearing that up! :) I will probably mount them with the bracket on the opposite side witch leaves me with a problem. The longest distance the mounting holes will make is 25-25,5mm and even more if i countersink the screws. So maybe my best option is to make an adapter and use the 28mm on-center holes. It's going to be one small adapter though! (28-22,5)/2 = 2,75mm wont leave enough space for two screws so it has to be glued on. Maybe I should just glue it on without holes from the start.

nevermind, found good deal on tubes

Hoo kay. So my 12SR7 starving student is done. I wanted to post some pictures for you guys.

I would like to point out that this is my first ever tube amp and first ever DIY project of any kind so please be nice to me :)

So here is the case I decided to use for my amp. This is an aluminium case that my external HDD used to be in. 

Modding just the case took me a few hours.  I drilled few holes in top cover for pot, mosfets and tubes. I also repainted it. I am not happy with how the side panel looks like but I couldn't manage anything better with my skills. 

So I soldered everything together and... it didn't work. I checked everything once again and I found few stupid mistakes that I've made. I corrected them in less than few minutes and the amp started to work.

My cable manag... let's skip this part... I still need to work on that one.

Anyway, here is how it looks like finished:

The sound: 

I was listening to it just for a couple of hours but I think I can give you my brief impression. My setup was ODAC-Student-HD600

I don't have much experience with amps, especially tube ones (this one is first I've ever been listening to) so I will be comparing it to ODAC/O2 combo.

The sound is more fun on my HD600. There is definitely more bass that seems to have better punch (like HD650? maybe even better). I'm not a person that like bassy amps/dacs/headphones/anything but I must admit that this amp makes my HD600's bass very nice to listen to.

Midranges seems to be a bit quieter than with O2 but they still sounds quite good. 

Treble sounds like it's being a tiny bit emphasized but they still sounds very well. 

The overall clarity and amount of details through this amp is definitely worse than O2 but I still like it. "Normal" music such as Gorillaz, Eurythmics, Europe or Depeche Mode sounds very enjoyable and for this kind of music I prefer student over O2. This amp is definitely more forgivable for bad recordings than O2. However for classical music, metal or any kind of instrumental music I prefer O2. I hope you get what I mean :)

So that were my impressions for now. I hope you like it.

Now I have one technical question to you guys. Is it normal that the heatsinks are getting really warm? I don't want to sound paranoid but I am getting a bit worried...

Looking great. Well done.

One thing I noticed is the ground wire on the pot. It may be cleaner to run this wire directly to the RCA sockets, instead of having the signal return path go trough the enclosure. It is preferable to ground the enclosure at a single point, and keep the signal and power ground separated for as long as possible. 

And yes, the heat sinks do get hot. Just about everybody who built the amp write about it. I use a fanless, slot socket, Pentium II heat sink to keep mine cool. And even then... it's great to keep your tea warm! 

Funny thing about the genre of music you prefer... It's the other way around for me. I prefer Electronic trough my NFB-12 and classic, instrumental music trough the tubes. 

It's also normal that the amplifier is bassy at first. It's a well known phenomenon with tube amps. Apparently it has to do with capacitor burn in. 

Thanks. I will re-wire the pot ground cable soon.

If I would ever happen to build another student I will use larger heatsinks. Because my case is aluminium and there is a direct contact with heatsinks the case is getting quite warm but guess that it's a good thing.

I hope it would take a long time. I acually enjoy this bass.

I've heard that 12SW7 tubes sounds almost the same except that the mid-range is not muffled. Can anyone confirm?

I have one more question. What power supply you can recommend for my amp? For now I am using 3 different PSUs connected in series to get 48V  I was thinking of getting this one.

If you're asking me about a PSU... wrong person to ask. 

Conversely:

http://www.ebay.co.uk/itm/2A-SECONDARY-120VA-TRANSFORMER-WITH-30-0-30-VAC-OUPUTS-/250970010874

http://www.ebay.co.uk/itm/TEXAS-INSTRUMENTS-TL783CKCSE3-IC-ADJ-REGULATOR-HIGH-V-/170998399847

It's actually a good thing that your case is getting warm. It means the whole enclosure is being used as an heat-sink.

Protofic nice build man! Looks like it came out great. I think it looks pretty cool! Like that case a lot. Good job

I'm glad you guys like it.

I did a bit of cable managament. I got rid of most of the electric type and used hot glue instead. It's still not perfect but definitely better than before.

As KimLaroux suggested I rewired pot's ground to RCAs and the buzzing noise that I heard before is gone. The amp is almost dead silent now.

If you wonder why I changed C1 capacitor then here is the story.

I accidentaly connected the power supply incorrectly (swapped + with -) and it exploded...

Glad that was the issue. It's a common mistake when one begins in audio to think that "ground is just ground" and connect things to the closest ground point they can find. Turns out grounding is one of the most tricky, if not THE most tricky part of any audio design. Looking at your build, it looks like you're used to grounding stuff in cars. ;) There's a whole ideology in electronic audio where the enclosure should only serve as a shield, thus being grounded at a single point, and never ever carry any current. Some go as far as create separate "ground" rails for signal and power, and keep each of them as a mirror of the V+ it's referred from. They connect these together at a single point. It helps to keep return currents from "hijacking" each others.

Have you measured the resistance of the enclosure between the different ground points you're using? It may be less reliable than you think.

The black wire with a red line on it grounded to the lower right corner, is this the ground of the RCA? If yes then I'd recommend grounding it to the same screw as the DC input ground, or simply getting rid of it and adding a wire between the ground on the pot and the closest tube's pin 5 (assuming pins 5 are grounded, and not 4). You may want to try both these scheme and compare them. I know I did and the result wasn't the same.

What's the use of those 5W ceramic resistors?

I don't even have a car. I couldn't get used to anything because this was my first DIY electronic project ever.

I can do it but it wouldn't mean anything to me. I don't even know ohm's law.

I am able to read schematics and use soldering iron quite well but I have no idea how this stuff works. 

I didn't ground pins 4 and 5. I simply forgot to do it. I will rewire the black-red cable to DC jack's ground tomorrow and see how it goes.

When I asked my friend to check if this schematic is correct he suggested me to put these resistors in there because he was afraid that too much voltage can go through tube's heater.

With these resistors the heaters work on 11.5V. I tried run the amp without these resistors and the heaters were getting 12.5V which is ok but since it work on lower voltage I prefer to leave it this way (if it's safe). I don't know if it affects the sound anyhow. I haven't got the time to test it.

Here is my schematic: 

It's alright, we all start somewhere. I wasn't much more knowledgeable than you are now when I built my SS. And that was not long ago.

As for the heater's voltage, if your friend had read the datasheet for the 12SR7 he'd learned the heaters are rated for 12.6 V. So the 12.5 V you had was spot on. They also tell you it's safe within 10% of this voltage, so it'll be just fine with anything between 11.3 V and 13.9 V. These tubes were designed to be used in cars, so they have heaters able to work on a wide range. If my math is correct, your heaters should really be seeing 11.0 V. This may be too low.

And if your friend studied the schematic correctly, he would have learned that you can change the heater voltage by simply changing the voltage at the gate of the MOSFET. No need for a 5 W resistor when a normal 0.25 W would do. 

And about the pins 4 and 5... I forgot you built a 12SR7 version! In your amp, pin 5 (ground) becomes pin 8 and pin 4 (heater supply) becomes pin 7. In the 12SR7 pins 4 and 5 are diode plates.... Do not ground those pins! Leave them as they are!

Well, my friend told me that 6Ohm resistor would be perfect but I couldn't get one so I installed 10Ohm and it works.

I will measure what I am getting in there tomorrow, just to be sure.

My experience with PCs tells me that if something works on lower voltage it would overheat less and lasts for longer. Does it work like this for tubes as well?

Edit:

I soldered this black-red cable directly to DC socket and the buzzing is still there but even quieter than before. I am happy with it.

The voltage on heaters is 11.6VDC.

I just completed my amp but it won't start. When I turn it on my power led flashes periodically but does not stop even after 2min. I built the 19J6 version with the uppdated optional parts. I tried using one tube at the time and no tubes but it still flashes on and off.

The tab on the mosfets were isolated from ground before I soldered anything to them but now they are connected even when fully off from the casing. Is this normal?

I have checked the wiring but can't see anything wrong with it. The powersupply I am using is this one http://se.mouser.com/Search/ProductDetail.aspx?R=GS25A48-P1Jvirtualkey63430000virtualkey709-GS25A48-P1J which is rated for 0.52A so that should be enough for the amp + 1 led that i run from a 10k resistor. 

Edit: Picture added