Millett "Starving Student" hybrid amp - Page 427

Quote:

Originally Posted by Captn Dave 

There are something like 20 tubes in total. How many amps do you want to build this week?

The substitute tube, the 12au7 isn't free. The difference is the issue. Ten bucks? Well, tuition in the US has been trending upward since about the same year vacuum tube production was discontinued. I guess why its named for starving students.

20 is an optimistic estimate in that listing.  The 19J6's are out of stock in two of those listings.   Plus, there's only one in that listing that was below $13.49 and its shipping was $7.76.  You'd still have to buy another one from one of the other listings that are even more expensive.  Remember that buying NOS or used tubes is not like buying opamps - you may get 4 good ones out of 5 even if they claim "tested."  There are things that are not tested with most tube testers.

Just saying that posting, "There are plenty of them on eBay now." is a bit misleading.

Quote:

Originally Posted by Captn Dave 

I plan to build one of these amps. I'm putting together a mouser order now. It seems like a mistake to corrupt Mr. Millett's elegant design. The genius of his design is that it uses a very inexpensive power supply, with what was once an unloved tube with a very clever twist. Now the tube is better appreciated. Is that a reason to throw out the clever twist?

I have a poor opinion of the 12au7. It is not a very linear tube. There is one interesting outlier, the Conrad Johnson PV series of preamps. i was so surprised to learn that it uses the 12au7 that I once downloaded the characteristic curves, drew the load lines and operating points to see how they did it. That circuit operates with a high B+ and a very flat load line and about 2.4 ma current in order to operate in one of the very few linear regions. The modified design of this amp starts with a very low B+ and operates with a steeper load line and a fraction of the power. I just don't see it as sounding as good as the J6.

My thinking is that if one wishes to build a headphone amp with a 6 or 12 v tube, there are probably better ways to do that.

No, I completely  agree with everything you've just said.

My only issue was with telling everyone that ebay was a plentiful source for 19J6 tubes.

Quote:

Originally Posted by Captn Dave 

Okay, I guess that we can agree that their availability in somewhere in that narrow rage between extinct and plentiful. I'm going to hit the junk shops on Monday and see what I can find. I'll report back.

BTW, how many do you want?

Heh ... if you find 50 or more, shoot me a PM.

Btw I just placed an order at digikey... yet another for this project. Now I seriously need to find a name for this thing, because it'll not be a starving student any more after this upgrade.

Are you separating the valve and mosfet power supplies then? It really is a great up grade drop a heat point of view and allows you to push B+ higher :)

Yes, I'm going for ~30V for the MOSFETs and ~170V for the tubes. Though I'm not entirely sure what the B+ will end up at. I'll just build it and see what I can get. It should be between 150 and 200, so I'll just build the thing so it can operate within these values and tweak it after. I'll also build a tail CCS for each tube, and add global feedback. I think I'll aim for 3mA per triode, or 6mA per tube. I may tune this once I know where I can get the B+ at. It's kinda pointless to draw load lines when your B+ could be anywhere between 150 and 200... so I'm almost going in blind.

College is keeping me insanely busy. I may not have time to do all this before next summer, and even then...

Sorry for multiple posts, as I've just posted this in the 'post photos of your build', but here's my attempt. Built with 19J6's.

It's still unfinished as there's no sound in the right hand channel, but it worked ok when I first built it in a shoebox to test it, so hopefully it's nothing too serious. I'll post some better photos next weekend when I've finished it completely.

Hi. 

I am not able to get capacitors mentioned in the original design "539-150104K100AA". Would it be ok if I will get these?

In the original design it says "film capacitor 0.1uf - 0.22uF, 63V or higher". The one I want to buy seems to meet these criterias but they don't look like original ones. 

It is safe to use them? I am building 12SR7 version.

Thanks. 

Quote:

Originally Posted by ProTofik 

Hi. 

I am not able to get capacitors mentioned in the original design "539-150104K100AA". Would it be ok if I will get these?

In the original design it says "film capacitor 0.1uf - 0.22uF, 63V or higher". The one I want to buy seems to meet these criterias but they don't look like original ones. 

It is safe to use them? I am building 12SR7 version.

Thanks. 

Those replacements look just fine, I recommend the 220nF over 100nF if possible. Good luck with your build :)

Thanks for quick reply. 

What's the difference between 220nF and 100nF and why is it better to get 0.22uf? Sorry for stupid question but I have no experience with electronic.

And one last stupid question. It doesn't matter which direction I will install them, right? I can't see "+" sing beside C2 and C4.

The higher the input impedance of the current gain stage (in this case the mosfet) the smaller the capacitor (C2 or C4) needs to be, if the input stage has a lower input impedance the tube has to output more current , and in this case the lower frequency waves are limited by the size of the capacitor. I have personally found that the IRF510s can be quite variable in input impedance (before taking into account a mosfets high frequency roll off), so I very much advocate the decision that was made in the PCB version to bulk those coupling capacitors to 220nF, the cost difference is negligible at best and the larger values not going to have a noticeable sound colouring effect over the smaller ones, the difference just isn't great enough. All in all the caps keep the DC on both halves of the amp separated larger ones are more easily going to provide the best frequency response (no need to go over 220nF though).

And yes, these capacitors are non polar, you can install them either way around.

Cheers!

I will go for 0.22uF then.

Thanks.