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Millett "Starving Student" hybrid amp - Page 433

post #6481 of 6757
Quote:
Originally Posted by DingoSmuggler View Post

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

 
Thanks, I'll most likely end up doing that build, I was kinda hoping these 12SN7-GTs would work because they're the best looking/condition tubes I've found in my pile, but for the sake of simplicity I'll probably just use the 12SR7s or even buy some 12AU7s 
post #6482 of 6757
Quote:
Originally Posted by jarscar View Post
...I was kinda hoping these 12SN7-GTs would work because they're the best looking/condition tubes I've found in my pile...

The tube parameters for the 12SN7  are quite similar to the 12AU7 BUT the heater current is double, being at 300mA, not the 150mA of the 12AU7 (or 12SR7). Because the heaters are used in the biasing of the mosfets, this makes the 12SN7 impractical.

post #6483 of 6757

Actually the MOSFET would not care, as they are biased using voltages and not currents. You'd just have to use very large heat-sinks. The problem is the PSU already has troubles with the 150 mA heaters, so doubling the load is out of the question in the default setup. You could do it with a larger PSU, say 1 A. But the thing would be dissipating so much heat you could make your breakfast on it.

 

I say go with the 12SR7. Not only will it be easier to build,  you won't need to add shields to the tubes. wink.gif

post #6484 of 6757

Nice!  Looks like a good weekend project.  Thanks Pete!  And thanks for all the great priceless technical books on your website!!!

post #6485 of 6757

Sorry for my silly question but can the MSSH modify to using only 1 tube like the other simple 12au7 hybrid amp?

post #6486 of 6757

I believe it does. It is is my plans to build a single-tube starving student next. If you know what you're doing and can do the maths, it should not be very complicated. But then the circuit would be so much different that you're probably better off just building the "super simple" amp. 

post #6487 of 6757
Just an FYI, but after personally testing thousands of dual-triode tubes with my years at Beezar, single-tube designs are a dead end.

The triodes in a dual-triode tube probably match within 10% about every 1 in 10 tubes. Finding one is the exception, not the rule. You can adjust somewhat with trimmers for the biases, but you'll never be able to fully compensate for the differing outputs.

EDIT: fixed the spelling from my lousy Droid.
Edited by tomb - 4/25/13 at 5:17pm
post #6488 of 6757
Quote:
Originally Posted by tomb View Post

Just an FYI, but after personally testing thousands of dual-triode tubes with my years at Beezar, single-tube designs are a dead end...

I built this Single tube Amp for grins, mostly because I had all the parts and a few pieces of copper clad left over, and it sounds OK...I have an ancient Mullard 12AU7 I have been running in it and it's not bad...the brand new JJ sounds like crap, it must be what you are talking about...

post #6489 of 6757
Quote:
Originally Posted by proid View Post

Sorry for my silly question but can the MSSH modify to using only 1 tube like the other simple 12au7 hybrid amp?

 

Quote:
Originally Posted by KimLaroux View Post

I believe it does. It is is my plans to build a single-tube starving student next. If you know what you're doing and can do the maths, it should not be very complicated. But then the circuit would be so much different that you're probably better off just building the "super simple" amp. 

 

I agree with the last part of KimLaroux's statement. A characteristic design trait of the SSMH is the use of the tube heaters as loads for the output MOSFETs. Using a single tube rules out that possibility and leads to a different design like the "super simple" amp pointed out by KimLaroux, which I believe is the same one which Vixr built. 

 

cheers!

post #6490 of 6757

Actually, you can still use the heaters the same way as they have a center tap on the 12AU7. Ground the center tap, and use each end for each channel. The problem, of course, is that you now only have 6 V bellow the MOSFET, which may not be enough. You'd have to add a resistor between the heater and the MOSFET to bias the MOSFET up.

post #6491 of 6757
Quote:
Originally Posted by KimLaroux View Post

Actually, you can still use the heaters the same way as they have a center tap on the 12AU7. Ground the center tap, and use each end for each channel. The problem, of course, is that you now only have 6 V bellow the MOSFET, which may not be enough. You'd have to add a resistor between the heater and the MOSFET to bias the MOSFET up.

I know about the center tap... I hadn't really dedicated much thought to this topology but now that I think about it I guess you're right. In the "normal" SSMH configuration, with one end of each of the two separate heaters connected to ground, it looks electrically equivalent to a single center tapped heater with the center tap connected to ground, yes.

 

About the 6 volts, well, you can always use a lower supply voltage for the MOSFETs, say, 24V. That would keep the voltage drop across the MOSFET at 18V and supply the 6V for the tube heater.  

 

You could even try 6J6 tubes that way, just scale the MOSFET heatsink for the 450mA heater current!!!

post #6492 of 6757

Ok I got everything built and done, put the tubes in and fired it up, clear as a bell out of the left side, right side I hear just a hint of sound, the MOSFETs seem to heat up and the tubes light up, I basically rebuilt the whole right side in case I had a bad connection or got a wire in the wrong place but same problem. I checked the voltage at the MOSFETS and both seem to be OK and I seem to be getting the same voltages for everywhere I'd think the voltages should match. 

An odd thing I noticed is that I was using earbuds to test it and they actually got warm, Is it normal to get 9 volts at the headphone jack? I consider myself fairly good at reading schematics and considering quite a few people have used this project as an entry into this kind of thing I'm bummed I'm having this much trouble with it. I regret using an enclosure on the smaller side as I basically have to undo certain parts just to work easily on other parts(all said and done it fits perfectly though). 

If anybody can shine a light on what my shortcomings may be I'd be forever in their debt. Thanks.

post #6493 of 6757
Quote:
Originally Posted by jarscar View Post

Ok I got everything built and done, put the tubes in and fired it up, clear as a bell out of the left side, right side I hear just a hint of sound, the MOSFETs seem to heat up and the tubes light up, I basically rebuilt the whole right side in case I had a bad connection or got a wire in the wrong place but same problem. I checked the voltage at the MOSFETS and both seem to be OK and I seem to be getting the same voltages for everywhere I'd think the voltages should match. 

An odd thing I noticed is that I was using earbuds to test it and they actually got warm, Is it normal to get 9 volts at the headphone jack? I consider myself fairly good at reading schematics and considering quite a few people have used this project as an entry into this kind of thing I'm bummed I'm having this much trouble with it. I regret using an enclosure on the smaller side as I basically have to undo certain parts just to work easily on other parts(all said and done it fits perfectly though). 

If anybody can shine a light on what my shortcomings may be I'd be forever in their debt. Thanks.

No! You shouldn't have any DC voltage at the headphone jack at all. 9V will cook your headphones in a fairly short space of time.

For anyone building any headphone amp, always measure the DC offset before connecting headphones, it should be less than 10mV for almost any design.

 

The output capacitors - C3 & C5 should block any DC from the output. Check that you haven't shorted out the leads of the ride side output cap, or it may even be internally shorted and need replacing.

 

If you did have a large DC current across the right side earphone, this will magnetically saturate the earphone, preventing the smaller AC signal (the music) from actually making any sound. So what I'm trying to say is, the output capacitor short might be the only thing wrong.

 

 

If you can't find the issue, how about some pictures.


Edited by DingoSmuggler - 4/29/13 at 1:35am
post #6494 of 6757
Quote:
Originally Posted by DingoSmuggler View Post

No! You shouldn't have any DC voltage at the headphone jack at all. 9V will cook your headphones in a fairly short space of time.

For anyone building any headphone amp, always measure the DC offset before connecting headphones, it should be less than 10mV for almost any design.

 

The output capacitors - C3 & C5 should block any DC from the output. Check that you haven't shorted out the leads of the ride side output cap, or it may even be internally shorted and need replacing.

 

If you did have a large DC current across the right side earphone, this will magnetically saturate the earphone, preventing the smaller AC signal (the music) from actually making any sound. So what I'm trying to say is, the output capacitor short might be the only thing wrong.

 

 

If you can't find the issue, how about some pictures.

Thanks, I'll look after work. If I had to be honest then I'd have to say that last part, with the parallel capacitors(one electrolytic, one not), threw me for a loop at first. I looked at the original P2P builds from earlier in this post and tried to match the wiring but I probably just have it wrong. I'll check those capacitors, make sure I didn't get the polarity backwards, I did it kind of late and I think for a while I began to go mad. Thank you for your help.

post #6495 of 6757

Yup, had my capacitor polarity off, works fine now, have a pretty ugly hum though, I've got a couple ideas what that is though, a little more tinkering and I think I've finally got a winner! Thank you so much for the help, I have only asked a couple questions but I can't help but me a little moved by how nice and helpful you guys have been, thank you.

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