Millett "Starving Student" hybrid amp
Dec 18, 2011 at 4:15 PM Post #5,971 of 7,258

capernicus

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I replaced the right MOSFET but not change in the observables.
 
I can't figure out why pin 1 / 6 voltages would be so different between L and R.  I measured the resistances of R1 and R7 and both are reading 33k.  Could my problem be a short in C2? - unfortunately I don't have any spares of those...
 
Dec 18, 2011 at 11:59 PM Post #5,973 of 7,258

capernicus

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Ok, so I have this thing mostly complete.  Its Purple Heart and Maple in a 12AU7 build
 

 
Unfortunately, as I mentioned in my previous posts I can't seem to get the Right side working correctly...
 
I took a picture of the internal bird nest below - I'm the first to admit I still have much to learn on how to do clean wiring and am also open for some pointers.
 
I picked up some nice aerospace grade wire scraps from work (silver tinned copper) and used it throughout except for the black wire which is when I ran out of the good stuff and had to dip into my crummy Depot stock.
 

 
Please let me know if you have any suggestions for my sick right channel...
 
 
Dec 19, 2011 at 9:24 AM Post #5,975 of 7,258

digger945

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Also, check for proper wiring of the jack itself. You could have one channel and ground reversed and that channel will still play, the other channel being only the difference between L and R.
 
Dec 19, 2011 at 6:11 PM Post #5,977 of 7,258

digger945

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Quote:
just studying the internal picture I see that I do have C8 installed backwards - I wonder if this could be contributing to my problem...
 
 


It should be fixed, but likely not the problem. 
 
In post #5970, you have an attachment with some of your operating points. You show 46volts at the gate of Q2, but 15.4volts on the other side of R9? With a good mosfet, you should read the same voltage on both sides of R9. If indeed you do show 46volts at the gate, then most likely you have a cold solder joint or bad connection to ground, at R10.
 
 
 
Dec 20, 2011 at 11:50 PM Post #5,978 of 7,258

CaptHowie

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Could anyone tell me where to find a mounting kit for the MOSFET on the heatsink? I live in Australia, so I'll be using Farnell/Element14 to order. At the moment I've found the heatsink, but the accompanying mounting kit is $9.85. Seems a little steep considering the heatsink is around $5.
 
Edit: Or would even this one work? Link
 
Dec 21, 2011 at 12:09 AM Post #5,979 of 7,258

Fred_fred2004

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I didn't use a kit on mine I insulated the whole heatsink from the circuit by useing an ABS case, if you are not useing a metal case you might not need to worry
 
Dec 21, 2011 at 12:10 AM Post #5,980 of 7,258

capernicus

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yes, i believe your e-bay link should work... you are correct, this should be an inexpensive purchase...
 
For comparison sake I purchased this one from Mouser: Link  I paid $1.65 US.  I have to think you should be able to get something like this for about the same cost in Australia.
 
 
Dec 21, 2011 at 3:11 AM Post #5,981 of 7,258

CaptHowie

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Quote:
I didn't use a kit on mine I insulated the whole heatsink from the circuit by useing an ABS case, if you are not useing a metal case you might not need to worry


I am using an ABS case. How exactly does this work?
 
Quote:
yes, i believe your e-bay link should work... you are correct, this should be an inexpensive purchase...
 
For comparison sake I purchased this one from Mouser: Link  I paid $1.65 US.  I have to think you should be able to get something like this for about the same cost in Australia.
 


I thought it was a tad expensive, especially for something as basic as a few screws and washers. Thanks for clarifying that. :)
 
 
Dec 21, 2011 at 3:42 AM Post #5,982 of 7,258

Fred_fred2004

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Here's some pics of the case I used, Its from Jaycars under $10 (from memory), as you can see the heatsinks are electrically isolated
 
 
 
cheers
FRED
 
 
 
Dec 21, 2011 at 11:13 AM Post #5,983 of 7,258

Ikarios

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Quote:
I am using an ABS case. How exactly does this work?


The reason that the heatsink mounting kit is used is because on the SSMH PCB version, the metal case was tied to ground, but the heatsinks/mosfets were NOT. Thus it was crucial to isolate the metal heatsink from the metal case, otherwise you would short the mosfet and burn it up. If you're using an ABS case, there's no concern, as long as your heatsinks are electrically connected only to the mosfets and not anything else.
 
 
Dec 21, 2011 at 1:44 PM Post #5,984 of 7,258

nikongod

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I would insulate the mosfets, and ground the heatsinks no mater what kind of case you used. 
 
The drain on the mosfet is connected to the mounting tab and accordingly there is +48V on the heat-sink tab of the mosfet. Certainly enough to give you a nasty shock if you touch the heatsinks & something that is grounded. Maybe it requires you to do a couple things at the same time, but they are not uncommon actions so its likely to happen. Its best not to leave things to luck once you have more than a few volts in a circuit.
 
Measure DC volts from the heatsinks to the ground in the amp. If its more than 0V, and it will almost certainly be, fix it. 
 
If we are talking about FRED's amp above, it will not be hard to attach a ground wire to the heat-sinks. Much better to blow up a mosfet or even a power supply than give someone a nasty shock. 
 
Dec 21, 2011 at 7:00 PM Post #5,985 of 7,258

Fred_fred2004

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