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Millett "Starving Student" hybrid amp - Page 417

post #6241 of 6806
Quote:
Originally Posted by marcus76 View Post

Hi there,

beginner question again:

I'd like to put some switchable (dim/bright) tube led's and a power led into the circuit..

 

Need to check for the proper resistor values.

 

 

Consider using an on-off-on switch so you switch of the LEDs with the switch in its central position.

 

Need LED resitor values? Check this: http://led.linear1.org/1led.wiz

post #6242 of 6806
Quote:
Originally Posted by Beftus View Post

 

Consider using an on-off-on switch so you switch of the LEDs with the switch in its central position.

 

Need LED resitor values? Check this: http://led.linear1.org/1led.wiz

 

I already have a on/off/on switch with center off, where did you put this in your circuit?

Values for 48V with 5000mcd red 2V LED :

3,3K 2W for bright @ 15mA / 4,7K 1W for dimmed @ 10mA and 10K 1/2W @ 5mA for the power LED

Did your build have a power LED?

post #6243 of 6806

 

With all the information I gathered recently both here and there, it was time I open my amp again and try a few things. College have kept me busy, and today's the first day I actually had the time for it. So I spent the whole day on it. (okay, I got out of bed at 12... but I did spend the last 12 hours on this)

 

Here's what I did:

 

  • Shortened the cable from the RCA to the pot by a couple of inches. (the grey cable)
  • Connected a ground wire directly to the RCA, so that the shield of the cable is no longer carrying a signal.
  • Shortened the cables from the pot to the tubes. (the red cables)
  • Bypassed the terminal strip for the input of the tubes: The signal cables are connected directly to R14/R15, which in turn are soldered directly to the tubes. Leads of the resistors are as short as possible.
  • Grounded the red signal cables at the pot, with a single ground wire to the star ground. Again, the shield of those cables do not carry signal, there's just shields.
  • Shortened the output wires, so that they stay away from the pot.
  • Moved the MOSFET gate resistors as close as possible to the MOSFET, and cut the leads of the gates shorter.
  • Shortened the LED power wire.

 

I think that's it... Oh no, forgot. Before I opened the amp, I did a listening test with open RCA, just as a reference. Randomly, I touched both tubes, and noticed that the right tube hummed badly when touched, but not the left one. When I opened the amp, I saw that pin 6 of the bad tube was not connected. I guess I forgot to solder it, as I studied it and found no sign of a broken joint. Funny though that all this time only half the tube was used but I could hear no difference to the other channel.

 

So after all that, I fired it up for a test. There is no audible difference in noise level. It's just as bad, nothing's changed. Actually it's worse, now I get unstable noise too, as if someone was continuously moving the pot around. I think I damaged the pot with all this soldering. Now it pops to 100% at around 2%, in one channel. I tried to clean it, but it didn't help. I'll buy a new pot for my NFB-12, and transfer the Alps in the NFB-12 to my Starving Student, some day. I took care of keeping space in the enclosure for those big Alps. My current pot is really bad, so I'll place any further tweaks on hold until I can change it.

 

Could ferrous resistors and terminal strips be the source of the noise? I used a magnet and noticed that all my resistors are ferrous, and so are the tabs of the terminal strips. Since my enclosure is non-ferrous, magnetic fields can get trough and induce a current across the resistors and terminal strips. Possible?

 

Edit: Oh wow, 500th post. Could not ask better for a 500th post I suppose.

post #6244 of 6806
Quote:
Originally Posted by marcus76 View Post

 

I already have a on/off/on switch with center off, where did you put this in your circuit?

Values for 48V with 5000mcd red 2V LED :

3,3K 2W for bright @ 15mA / 4,7K 1W for dimmed @ 10mA and 10K 1/2W @ 5mA for the power LED

Did your build have a power LED?

 

My build didn't have a power LED, the tube LEDs double as a pair of power LEDs. I connected the on-off-on switch, LED resistors and both tube LEDs to the 48 volts right after the power switch. For the LED resistors I used two resistors in parallel to distribute the power dissipation load, this way I could get away with using normal size resistors.

post #6245 of 6806
Quote:
Originally Posted by the_equalizer View Post

 

The amp design is low voltage, and there are several tube options to implement it:

 

  • 19J6 - (if you can find any) this is the tube used in the original design by Mr Millett
  • 12SR7 - details posted by user Logistic in this same thread
  • 12A_7 (usually 12AU7) - details start with this post.
  • 17EW8 - same pinout as the 12AU7 build, only a couple resistor values change.

 

Any of them will do nicely, I recommend you pick the one that is the easiest/cheapest for you to get.

 

cheers!

 

Got hold of RCA 12AU7A on ebay. From the pics, looks like cleartop with black plates. logo says hammond and the number mentioned is 274-6026. Any idea how good or bad are these. Should work with this amp right.

post #6246 of 6806

Oh yeah, I wanted to mention. For those looking for tubes, especially Canadians, Parts ConneXion is having a sale on tubes and accessories. The sale lasts all of October. It's a good place to get both your tubes and socket, things Digi-key doesn't carry.

 

Ah and why not, while I'm posting... I noticed earlier today that the schematic lists a 50K pot. I'm using a 100k pot in my build. Since the background noise actually gets louder as I turn the volume up, I'm wondering if the higher impedance pot would be to blame. I mean, a higher value pot would place a higher resistance between the grid an ground, at the same volume level as a lower value pot. Would this have an effect on noise levels? Since the noise is proportional to the pot's position, something tells me it would. Another thing I'll have to try out. Is there a minimum value for this amp, or is it safe to use even a 10k pot?

post #6247 of 6806

This one looks like a winner. When I have some free time I'll design a PCB to have the circuit printed, although I'm thinking about changing some resistors in order to use more commonly used tubes since these are practically non-existent in Mexico. 

post #6248 of 6806
Quote:
Originally Posted by KimLaroux View Post

Oh yeah, I wanted to mention. For those looking for tubes, especially Canadians, Parts ConneXion is having a sale on tubes and accessories. The sale lasts all of October. It's a good place to get both your tubes and socket, things Digi-key doesn't carry.

 

Ah and why not, while I'm posting... I noticed earlier today that the schematic lists a 50K pot. I'm using a 100k pot in my build. Since the background noise actually gets louder as I turn the volume up, I'm wondering if the higher impedance pot would be to blame. I mean, a higher value pot would place a higher resistance between the grid an ground, at the same volume level as a lower value pot. Would this have an effect on noise levels? Since the noise is proportional to the pot's position, something tells me it would. Another thing I'll have to try out. Is there a minimum value for this amp, or is it safe to use even a 10k pot?

 

I've been wondering about that myself lately as I've been meaning to try a larger volume pot to get finer control. However, the pot in the SSHM also acts as the grid resistor, grounding the electric charge that would otherwise build up in the grid and drive the tube to cutoff. Since the 19J6 was designed to operate at low plate voltages I think there should be a considerable (compared to other tubes) grid current and that's probably why the value of the grid resistor could be critical.

 

I think, however that one might use a higher value pot in parallel with a 50K resistor, hence the grids impedance to ground is always at most 50k, while the input signal flows through a higher value pot. The parallel connection will most definitely affect the response curve of the volume pot but I think it would be usable.

 

Alas, only trying it out will give a definite yes or no answer.

 

 

Quote:
Originally Posted by Arkyle View Post

This one looks like a winner. When I have some free time I'll design a PCB to have the circuit printed, although I'm thinking about changing some resistors in order to use more commonly used tubes since these are practically non-existent in Mexico. 

 


Hey! A fellow country man!   This is a great little amp certainly, and a good platform to try different mods. You would have to be more specific as to which tubes it is you're talking about. I have recently posted all the different tube options available. The 12A_7 and the 12SR7 are certainly easy to get in Mexico.

 

Feel free to send a private message whenever you like. ¡Saludos!

 

cheers!

post #6249 of 6806

I have a wierd problem with my build (it's posted some 5 pages back), it refuses to power on after a day or so. I've reflowed the power switch and even chaged it to a different model, with no luck. It works okay after reflowing the solder at the switch but then stops.

 

Could it be the Cisco PSU? Got it off eBay like the most of us.

post #6250 of 6806
Quote:
Originally Posted by lehtinel View Post

I have a wierd problem with my build (it's posted some 5 pages back), it refuses to power on after a day or so. I've reflowed the power switch and even chaged it to a different model, with no luck. It works okay after reflowing the solder at the switch but then stops.

 

Could it be the Cisco PSU? Got it off eBay like the most of us.


What is the current capability of your PSU? Did you include extras like tube LEDs, power LED, etc?. 

 

If possible, try measuring the voltage output of the PSU while you turn on the amplifier. Do you see it "cycle"?

 

cheers!

post #6251 of 6806

I would be more concerned with the turn-on currents drawn by the tubes than by anything but the most tasteless of LED displays. 

 

A power supply rated for at least twice the design idle current of the amp is mandatory - not optional. 

post #6252 of 6806

Yes, though usually the 350 mA  PSU works alright. It does cycle quite a bit at startup but it ends up warming up the filaments and powering up the amp. That said I do use a 1A Cisco PSU :D

 

I guess if you're using the 350 mA PSU and add some 10mA for a LED here and there such a subtle change as the surface resistance of a new vs tarnished solder joint could tip the balance so that the PSU just cycles on and off forever.

 

It could also be just an über-hungry tube filament maybe?

post #6253 of 6806

Well an easy way to test that is to try powering it with only one tube in. Try once with only the left tube, and once with only the right tube. If it starts in both configuration, it's a clear sign your PSU is overloaded. If it doesn't start with one or the other, or both, then your problem is probably elsewhere.

 

I just find it weird that it works after reflowing the switch, even with a different switch. I have a feeling it's not a direct consequence. It's probably some other variable that happens to change when you reflow the switch, like time. What if you switch it on, wait, switch off, wait, and switch back on? Try it without waiting? It could be that your PSU isn't powerful enough to charge the capacitors and power the cold heaters at the same time. Isn't there a power LED on the PSU? The LED on a SMPS will usually flash when the unit cycles trough overload protection.

post #6254 of 6806
Quote:
Originally Posted by KimLaroux View Post

After a few hours of research, I came to the conclusion that not having at least a ground plane is a bad idea. I also found that the unshielded tubes will pickup the interference from wifi networks.

 

All this is leaving me very disappointed in my design. I expected the electronics to be more troublesome, but it turned out the casework is the thing that makes me scratch my head the most. confused_face.gif

 

Now I'm thinking just scraping the case and rebuilding the amplifier inside an Hammond 1444 enclosure. Not the sexiest thing, but possibly more functional. Edit: Just found out Hammond 1455 have slide out bottom. I'm so ordering one from my local supplier, today.

 

 

On a different note, I'm currently testing the NOS Sylvania 12AU7 I spent almost 50$ on. I can barely pick any difference between them and the new Sovtek 12AX7. Just for fun, I'm currently using one of each in different channel. Using software balance, I only have to move the balance by 7% to get the volume equal on both channel. That's far less significant that I was led to believe.

 

And I honestly could never tell which channel has which tube in a blind test. Which forces me to conclude that, for the SSMH, spending 50$ or more on the tubes alone is probably not a good idea.

 

Just had to chime in here about tubes and "tube rolling". The differences between tube TYPES (12AX7 vs 12AU7) are obviously real - although the effect will depend on the circuit.

(For example, when you use a cathode resistor with a capacitor across it, there is no cathode feedback; when you remove the capacitor, the resistor value acts as cathode feedback and reduces the gain. The gain reduction will be greater with a higher gain tube, so the net result is to reduce the difference between them in THAT circuit.)

 

HOWEVER, the differences between different BRANDS (or even plate styles) of the SAME tube number are largely nonexistent. In "the old days", all tubes of a given type were considered interchangeable; nobody expected a certain brand or plate color to sound different, and they all cost the same few dollars. Some vendors claimed that their "special formulation" of cathode coating or plate paint was a bit quieter, but nobody much believed them. There were also sometimes slight differences in things like inter-electrode capacitance between different brands, but it was also accepted that a well designed circuit would NOT be sensitive to those differences. A circuit that sounded best with a certain brand of tube would have been thought of like a car that only worked well with one brand of gas. It is only in recent times that this silliness about certain brands of tubes sounding better has appeared.... The reality is that, other than certain few Russian and Chinese brands actually sometimes having poor production quality and life spans, you are wasting your money paying extra for some specific brand of tube and expecting some significant difference in "sound".)

post #6255 of 6806

For the LED's and the stock PSU , I added 3 (switchable) at 7mA and 17mA, and the amp turns on just like before, takes like 2-3 seconds for the tubes to light up.

I did not do any measurements yet. Seems to work alright. I might look into the voltages before putting it into the case...

 

Nice info on the tube rolling thing, as i thought about buying some other "cheap" 12au7 to compare to mine but I'll use them for now.. cost:  8$ each. 

NOS Sylvania 6189w -> bought them because of this article I found before I placed an order  http://www.jacmusic.com/nos/ECC82.htm

For anyone looking for tubes in europe, take a look at http://www.nostubestore.com , some guy from turkey, the prices/shipping seem reasonable imo, quick delivery.

@ KimLaroux, what kind of sylvania did you buy?

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