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Millett "Starving Student" hybrid amp - Page 432

post #6466 of 6844
Quote:
Originally Posted by Grumus View Post

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?



Snip (Click to show)

 

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

 

 

The tab is connected to the drain, which is connected to V+ in the MSSH. So if there's conductivity between the tab and the case, it means your V+ rail is shorted to the case somewhere. What's confusing is the LED still works. For the LED to work, the short must not be a perfect 0 ohm short. A 0 ohm short would not allow the LED to turn on. What's the impedance between the tab of the mosfet and the case?

 

My first thought was maybe there's a short downstream of R13, but even a shorted R13 would not be able to overload the PSU. You need a short less than ~90 ohm to overload your PSU.

post #6467 of 6844
I have not found the short yet. The resistance between the tab and case seems to be 560 ohm. My ddm is very cheap but around there anyways.

When it's tuned on the led cycles with the psu fully on and fully off ever 1-2 seconds. When I then shut it off it stays on abit dim until the caps is discharged.


Edit: Solved the issue. I had connected pin 3 and 4 together last night... Well I am happy I found the problem, thanks for the help KimLaroux!
Pics coming later.
Edited by Grumus - 3/22/13 at 5:12am
post #6468 of 6844

This was one fun amp to build! Thanks for all the help and the vast amount of knowledge there is in this thread. I sure learned a thing or two building this amp and I am sure it will come in handy for future builds. dt880smile.png

So far it has paired up great with my HD598 and thanks to this thread it's dead silent. The gain switch is a hissbuster with an off position for turning the amp on and off. The text is etched in to the aluminum.

 

 

 

RCAs and 3.5mm input
 

On/Off Led is abit on the bright side with 10k resistor might add another resistor to dim it down some.
 

The tubes were alot brighter than I first thought!

post #6469 of 6844

Wow it looks great! I like what you did with the faceplate - except the blue LED. It just clash with the rest of the design. An orange LED would have been cleaner.

 

Glad you found the issue. It's always satisfying figuring things for yourself. Great way to learn.

 

You even countersunk the screws... nice attention to details.

post #6470 of 6844

What a beaut! Nice to see that 19J6's  are still showing glowing up.

post #6471 of 6844

Thanks Kim and livewire for the kind words! smile.gif I am with you on the led, it's too bright and blue does not fit in with the glow of the tubes. 

I was extremely lucky to find the tubes. They come from a radio museum 5 minutes walk from my collage, never thought I would find a source of 19J6 that close. Unfortunaly they only had 2 of the same and 3-4 odd ones, not a box full of them as I hoped. I asked them if they would know of another possible source here in sweden but they couldn't think of any.
 

post #6472 of 6844
Deleted.

(Had submitted a reply to Grumus, but realised KimLaroux and live wire had beaten me to it).
Edited by DudeMyCans - 3/23/13 at 2:20pm
post #6473 of 6844
I've been wondering if there'd be any value in building a 48v linear PSU for this amp rather than the standard switch mode unit. Has anybody tried it?
post #6474 of 6844

I did, but since I've never tried the SMPS, I can't tell if it's actually an improvement.

 

If done badly, as were my first couple of layouts, then it'll be worse.

 

If done correctly, as is my current version, then it's probably overkill.

 

Personally, I did it because I already owned a usable transformer. It's the whole reason why I built a Starving Student, actually. Building a vanilla SS using an SMPS seemed boring to me. Designing and building a linear power supply for it was the challenge that motivated me to build the thing.

 

If you were to buy parts to build a 48 V linear supply that would actually perform better then the SMPS, you'd probably spend more money on the power supply itself than the rest of the amplifier.

 

Not to mention the added complications of introducing AC inside the enclosure. You have to be very careful with your physical layout, and may be forced to use either a very large enclosure to keep things far apart, or place metal shields between the modules. I built everything in point-to-point, even the power supply. It's good to have a schematic, but a schematic is no guarantee of performance. The physical layout, the way you build the schematic, will have a significant impact on performances. It took me many revisions, in which I rebuilt everything, before the power supply performed to an acceptable level. I also had to build ferrous cages around the tubes to shield them from magnetic coupling from the transformer, and I'm using a shielded transformer.

 

I have the opinion that the Starving Student is the "Cmoy" of tube amplifiers. You can power a Cmoy using a Sigma 11, but it'll still just be a Cmoy. wink.gif

post #6475 of 6844
Fair enough, thanks Kim. Maybe I'll leave it for a while !
post #6476 of 6844

Hi all, I just got a huge assortment of tubes, and it made me want to really try building this amp. A lot of these tube boxes are mislabeled but just from what I've gone through I've got:

 

2 pair of 12SR7s, but they look a tad rusty but two look fine

3 12AH7-GTs (in so-so condition)

an assortment of 6SN7-GTs,

and 2 beautiful 12SN7-GTs.

 

The 12SN7-GTs are the nicest looking of the lot and are probably in the best shape, they look brand new. But I know next to nothing about tubes(so these might be terrible for this project for all I know) and I'm not confident enough to go modifying the 12AU7 schematic too much on my own. My question to anyone here who might know, which, if any of these would be best to use and how would I go about applying them to the existing circuit. Or should I just buy some 12AU7s and shelve all these? Thanks in advance!

post #6477 of 6844
Quote:
Originally Posted by jarscar View Post

Hi all, I just got a huge assortment of tubes, and it made me want to really try building this amp. A lot of these tube boxes are mislabeled but just from what I've gone through I've got:

 

2 pair of 12SR7s, but they look a tad rusty but two look fine

3 12AH7-GTs (in so-so condition)

an assortment of 6SN7-GTs,

and 2 beautiful 12SN7-GTs.

 

The 12SN7-GTs are the nicest looking of the lot and are probably in the best shape, they look brand new. But I know next to nothing about tubes(so these might be terrible for this project for all I know) and I'm not confident enough to go modifying the 12AU7 schematic too much on my own. My question to anyone here who might know, which, if any of these would be best to use and how would I go about applying them to the existing circuit. Or should I just buy some 12AU7s and shelve all these? Thanks in advance!

I believe there is a 12SR7 version of the MHSS floating around somewhere, but I might be getting confused.

 

EDIT: I wasn't getting confused, its back on page 189, courtesy of Logistic


Edited by DingoSmuggler - 4/9/13 at 12:54am
post #6478 of 6844

I made 12SR7 based student. The schematic for it is like 2-3 pages back.

post #6479 of 6844

How about PCC189 or 6H23p rocket tube russian tubes?

post #6480 of 6844

Hi I am very new to headphone amps and was going to try this build. i was wondering how hot the amp will get? the reason i am asking is to understand what kind of materials i would be able to use for the case/body. any help would be greatly appreciated, thanks everyone.

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