Millett "Starving Student" hybrid amp
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tomb

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Quote:

Originally Posted by amc /img/forum/go_quote.gif
I did install the washers but i really torqued down the heatsinks before reading about damaging the burquist pads.


Yep. Quote:

Indeed you are right. I read resistance between a scratch i put in the underside of the lid and both mosfet tabs.


Near zero resistance if they're shorting. Near infinite resistance if they're not. I'm not exactly sure what you mean by "I read resistance ..." Quote:

The mosfets will still be OK right? I just need new insulators?


Yes. Quote:


I got some at a local electronics store but they are not the same as the burquist pads you supplied but mica sheets. Can I use those? Should I clean off the buquist pads or just leave them in place...


The mica insulators are fine - you just need to use some heat sink grease between the mica and the MOSFET, and between the mica and the case. Quote:


Will work on it tonight and keep you guys posted. Thanks for your help so far.


 
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simwells

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Quote:

Originally Posted by simwells /img/forum/go_quote.gif
Wasn't planning on going the PCB route, but my lack of electronics means I think I'll stick with a normal switch! No idea how to recalculate other values.


Actually, how straight forward would changeing the resistor values to use a 10k pot be?
 
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tomb

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Quote:

Originally Posted by simwells /img/forum/go_quote.gif
Actually, how straight forward would changeing the resistor values to use a 10k pot be?


If you're talking the input resistors, they're sized at X times the pot impedance. So, if you're using a 10K pot, you'd want to try 10K, 20K, etc. - in those increments to find the volume range you want.
 
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simwells

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Quote:

Originally Posted by tomb /img/forum/go_quote.gif
If you're talking the input resistors, they're sized at X times the pot impedance. So, if you're using a 10K pot, you'd want to try 10K, 20K, etc. - in those increments to find the volume range you want.


So is it only the resistors shown here http://www.diyforums.org/SSMH/schema...MH-origPCB.gif as R16 and R17 that would have to be changed?
 
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tomb

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simwells

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Quote:

Originally Posted by tomb /img/forum/go_quote.gif
Yep.



Thanks for all your help! Almost there with ordering everything I need! Only thing I'm struggling to find is the power jack on farnell UK, any help with this? Think I must somehow searching wrong.
 
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tomb

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amc

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Quote:

Originally Posted by tomb /img/forum/go_quote.gif
Yep.Near zero resistance if they're shorting. Near infinite resistance if they're not. I'm not exactly sure what you mean by "I read resistance. mica insulators are fine - you just need to use some heat sink grease between the mica and the MOSFET, and between the mica and the case.


Hi Tomb / the_equalizer:

Should have been more specific sorry. After detaching the case lid I could see the case anodizing where the pads had been all but squished out close to the case holes. So I scraped off burquist pads and cleaned the lid with alcohol then had to make a trip to the computer store for another tube of heat sink compound. I have been extra careful re-attaching the mosfets to the case with grease on both sided of a mica sheet as an insulator. Here are 2 pictures of my handiwork. With no power to the board I measure 18Mohms on one mosfet and 22Mohms. High enough to be considered nice and isolated?

Left mosfet resistance reading
Attachment 24915

Right mosfet resistance reading
Attachment 24916

But alas I still have problems. Tubes dont heat up, and at R4 and R10 I only get ~=145mV across them. Any other ideas on where I should be looking next? Any closeup pictures that could help?

 
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tomb

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Quote:

Originally Posted by amc /img/forum/go_quote.gif
Hi Tomb / the_equalizer:

Should have been more specific sorry. After detaching the case lid I could see the case anodizing where the pads had been all but squished out close to the case holes. So I scraped off burquist pads and cleaned the lid with alcohol then had to make a trip to the computer store for another tube of heat sink compound. I have been extra careful re-attaching the mosfets to the case with grease on both sided of a mica sheet as an insulator. Here are 2 pictures of my handiwork. With no power to the board I measure 18Mohms on one mosfet and 22Mohms. High enough to be considered nice and isolated?

Left mosfet resistance reading
Attachment 24915

Right mosfet resistance reading
Attachment 24916

But alas I still have problems. Tubes dont heat up, and at R4 and R10 I only get ~=145mV across them. Any other ideas on where I should be looking next? Any closeup pictures that could help?



Those readings are fine. At this point, we need pics.
 
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bhjazz

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Got my P2P build running in tonight. And all four of my tubes sound a little different. I have two Delcos and two RCAs. One Delco has too-mellow high end, the other is good. The two RCAs are closer. I'll try one Delco and one RCA next to see if I can get these matched up good.

(edit: yes, one of each seems to be the trick!)

I've seen a few posts about people finding spares on eBay. Anyone having luck in other places? I'd sure like to get a few more in case mine are on their way out!
 
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simwells

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What's considered a reasonable/good price for 19J6 tubes now, have found them difficult to get hold of but have been quoted $10 per tube, I know they have gone up in value but is it by this much?
 
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tomb

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Quote:

Originally Posted by simwells /img/forum/go_quote.gif
What's considered a reasonable/good price for 19J6 tubes now, have found them difficult to get hold of but have been quoted $10 per tube, I know they have gone up in value but is it by this much?


YES.
 
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UKToecutter

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Yeah,

I got 4. All Tung-Sol and all the same batch number at $10 each.

 
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