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Millett "Starving Student" hybrid amp
Apr 16, 2009 at 11:44 PM Post #2,956 of 7,277
Hey Smeggy,

I did find it pretty useful to add the cathode bypass caps, but something to decide how you want to "tune" the amp is to throw a pot place of the cathode resistor and play with what you like. For my personal amp, I used 470uf Panasonic with 20k ohm resistor because basically I had only 100uf Muse or 470uf Panasonics in resonable voltages and the muse wasn't large enough to get a really nice and full sound. With my PSU, it goves me breathing room. In the others I have built 220uf is plenty, but I bumped the cathode resistor up to 5k or 10k. IMO without upping the value of the cathode resistor the sound can get too bassy.

A small gain switch for the input is uber useful. I have mine so that I can choose between 100ohm and 100k ohm with the high gain meant more for extreme case and the 100k as my normal with 32ohm headphones and because I don't listen very loud (even still listening right around of the middle of the pot).
In a two channel though, 20-40k is probably more in line depending on user preferences.

My personal amp is not the same as the BOM and uses a a higher current PSU which is modded and also used much larger power caps on the circuit to account for this, so YMMV and all that.

I will say that using a nicer PSU that can supply ample current not results in better sound, but the tubes "burn" less hot and with a bit less light. The fets also heat less. could possibly result in longer tube life, but I don't know.
 
Apr 17, 2009 at 1:37 AM Post #2,957 of 7,277
I have been reading up on this for the past couple days, and was disappointed when I found out how hard it is to find the 19j6's now. Are there any places left to purchase them on the 'starving student' budget?
 
Apr 17, 2009 at 1:43 AM Post #2,958 of 7,277
Quote:

Originally Posted by rush340 /img/forum/go_quote.gif
I have been reading up on this for the past couple days, and was disappointed when I found out how hard it is to find the 19j6's now. Are there any places left to purchase them on the 'starving student' budget?


I think TomB will have some for sale with the PCBs. Also, you can use the 12sr7 or 12sw7 tubes I posted about a few pages back.
 
Apr 17, 2009 at 4:19 AM Post #2,959 of 7,277
I'm gonna enjoy getting a board for the next one
smily_headphones1.gif
 
Apr 17, 2009 at 4:47 AM Post #2,961 of 7,277
Quote:

Originally Posted by -=Germania=- /img/forum/go_quote.gif
I will say that using a nicer PSU that can supply ample current not results in better sound, but the tubes "burn" less hot and with a bit less light. The fets also heat less. could possibly result in longer tube life, but I don't know.


Hi Germania,
I just searched through this thread to find your link to some power supplies...THANKS!!! I found your post and will probably try one of those since I can't seem to find the "standard" version around town for a decent price. Thanks!

Oops...just saw the caveat. Well, I'll keep diggin.
 
Apr 17, 2009 at 12:57 PM Post #2,962 of 7,277
Quote:

Originally Posted by -=Germania=- /img/forum/go_quote.gif
I will say that using a nicer PSU that can supply ample current not results in better sound, but the tubes "burn" less hot and with a bit less light. The fets also heat less. could possibly result in longer tube life, but I don't know.


Someone is going to have to provide a more technical explanation of this phenomenon since in my head, unless there's a pant load of ripple or something else I would think that if you measure the same operating points (19V at the heater for example) that the tubes would "burn" exactly the same way regardless of the power supply used. The fets heating less would indicate to me less current flowing through the output stage, would it not?

Germania, I'd suggest that you check the various operating points of your amp and see what you get vs. the intended operating points on the original schematic. If the changes you've made have altered them in any way I'd like to see more discussion of the technical side of the changes rather than just hearing you think "they sound good".
 
Apr 17, 2009 at 8:39 PM Post #2,963 of 7,277
Quote:

Originally Posted by -=Germania=- /img/forum/go_quote.gif
I used 470uf Panasonic with 20k ohm resistor because basically I had only 100uf Muse or 470uf Panasonics in resonable voltages and the muse wasn't large enough to get a really nice and full sound. With my PSU, it goves me breathing room. In the others I have built 220uf is plenty, but I bumped the cathode resistor up to 5k or 10k. IMO without upping the value of the cathode resistor the sound can get too bassy.


Um, you are severely limiting current across the tube which makes it run in an even less linear place. It also lowers its Gm which in turn reduces its ability to drive the gate capacitance on the mosfet, which rolls off high frequency. That is, you built a tone control to filter out bass and treble.

Don't you have the 3 channel deal? That design also most likely cuts out bass frequencies.

Anyhow, voltage for the cathode bypass cap needs to be on the order of 2.5V ... maybe 3V. I doubt you have a lot of 1V caps lying about.

Quote:

Originally Posted by -=Germania=- /img/forum/go_quote.gif
My personal amp is not the same as the BOM and uses a a higher current PSU


Current is drawn by the circuit, not pushed by the PS. Thus, the ability of your PS to supply more current shouldn't matter so long as the original PS's Zout is low, and it can supply sufficient current. I have not seen any reason to doubt that this is the case.

Quote:

Originally Posted by -=Germania=- /img/forum/go_quote.gif
I will say that using a nicer PSU that can supply ample current not results in better sound, but the tubes "burn" less hot and with a bit less light. The fets also heat less. could possibly result in longer tube life, but I don't know.


A different PS may very well change how the amp performs. However, the rest of this makes no sense -- both grammatically, and theoretically. Is that "not" supposed to negate that your PS results in better sound? Assuming you think no, the heaters will draw current based upon the voltage supplied. If they are drawing less current, it is either because you are supplying lower voltage on purpose, or because your power supply is sagging due to inability to supply the current that the circuit is attempting to draw. That is, you are simply running the thing at too low a voltage.

As to tube life, there are two issues. The first is heater life. There is some indication that running heaters slightly low (like 5% below rated voltage) may result in longer tube life. But, this is not because it keeps the heater filament from burning out. It has to do with cathode emissions. And this brings up the second issue. In fact, running them too low stresses their ability to emit electrons, and may strip the cathodes over time. The second issue is stress on the plate. Running tube very hard does this.

In the case of this amp, we are barely using these tubes. Cathode stripping is an issue at high currents with cold cathodes above 1000V. Not here. However, we are running these tubes so low that cathode poisoning may be an issue. This happens when you just run the heaters with no voltage on the plates -- the cathodes build up a layer of cruft which keeps them from emitting. Again, it is hardly a danger here, but it is the closest thing to one I can think of.

As for how hard we are running the tubes, and whether they age, these tubes can dissipate over 3W from their plates. In the stock setup, this amp dissipates around 1/40th of a watt. The tubes are built to drop 100V at 30mA, we are doing 24V at 1mA.


Quote:

Originally Posted by n_maher /img/forum/go_quote.gif
Germania, I'd suggest that you check the various operating points of your amp and see what you get vs. the intended operating points on the original schematic.


I agree.
 
Apr 17, 2009 at 8:47 PM Post #2,964 of 7,277
Ok, I fixed it. Both tubes light now. I made a stupid mistake I'd rather not explain. But, now there's another problem. Audio is at a whisper. When I turn the pot all the way up, then I can barely hear it. I know others have had this problem. What's most likely the cause?
 
Apr 17, 2009 at 9:16 PM Post #2,965 of 7,277
Cool, glad thats all sorted as it was confusing the hell out of me. Mine's running like a little trooper and even the ridiculous load of the woofdales didn't faze it much aside from the lower volume level and lack of overhead. Is there an easy way to beef the thing up to give it a little more power/gain?
 
Apr 17, 2009 at 9:36 PM Post #2,967 of 7,277
hey guys
I am having a problem with getting my amp to work, nearest I can tell the inputs and outputs are wired correctly, the signal is getting to the tubes but I have no sound whatsoever coming out of the headphones. No buzzing clicking - nothing. I have had a few people look over the schematic and we think that everything is in the right places, I did have a blown mosfet to start out with that is now replaced. All the DC voltages that are listed on the schematic match within about .2 v. The mosfets seem to be getting pretty hot but no sound.
 
Apr 17, 2009 at 9:40 PM Post #2,968 of 7,277
Quote:

Originally Posted by smeggy /img/forum/go_quote.gif
Is there an easy way to beef the thing up to give it a little more power/gain?


smeggy, what's the value of your pot? My two SS have differents pots, one has 50k the other has 100k. The one with 50k seems to have more gain, as in I can't turn the pot more than past halfway mark since its too loud, but both are ok with my Beyer.
 
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