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Millett "Starving Student" hybrid amp - Page 418

post #6256 of 6814

I got parts connexions # NOS-75306 which is described as "Sylvania JAN CHS 12AU7 / ECC82, Gold Logo, Short Black Ladder Plate, Medium-Mu Dual Triode, 9 pin, Original Box, USA". Though they really have green logo and markings. It also says 12AU7A on them. The ribbed black plates do look nice, and match my enclosure well. =P

 

Today I randomly noticed that I can hear humming from my transformer. If I place my hear directly on it, I can hear the same type of humming I often hear from step-down transformers on power lines. Is this normal, or a sign that it's overloaded? There's also another noise close to the mounts of the transformer, which is not quite the same as the white noise at the output. Wondering now what could be the cause of that. It may be current leaking trough the chassis.

post #6257 of 6814
Quote:
Originally Posted by the_equalizer View Post


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!

 

No leds in my build. I'll measure the output and see if it cycles, I'v got the 380mA version of the PSU.

post #6258 of 6814

I built the 19J6 version with no LEDs and the 12AU7 version with LEDs...the 19J6 cycled for a few seconds, the 12AU7 did not. Both use the 380mA cisco. When I say "cycled" I mean the tube heaters in the 19J6 would come on then go out a few times, then stay on...this event lasted about 3 seconds. I did not use a DMM to actually test it. The 12AU7 version heaters come on and stay on.


Edited by vixr - 10/18/12 at 9:02am
post #6259 of 6814

A year or two ago there was a builder in this thread who had a similar problem. His PSU's performance slooooowly degraded until it wouldn't power the amp at all. He got another one from ebay and that solved the problem.

 

cheers!

post #6260 of 6814

I just learned the hard way how to blow a tube in this amplifier: Turn it ON without R4 or R10.

 

I wanted to try higher bias at the MOSFET, as I thought mine was low. A few pages back it was recommended to add a 33K resistor in series with the 220K one to raise the bias, so I tried that today. I started by unsoldering one side of the 220K resistor on both channels. I then added a 33K on one side, and thought I should compare voltages with the other side. So I turned the amp on, forgetting the 220K resistor was still unconnected in one channel. The MOSFET went fully ON and blew up the tube heater on this side, within a couple of seconds. By the time I realized what was going on, it was too late. YAY!

 

frown.gif

 

So yeah, this amplifier can blow up tubes, be careful.

 

On a more positive note, I tried the amp with a 20K Alps RK27 and the noise level was significantly lower. It was so low that the noise at the RCA out of the NFB-12 drowned the noise from the SSMH. I thought the NFB-12 had a 10K pot, but it turned out to be a 20K. I really wanted to try the amp with a 10K, as those are easier to get. I'm debating whether to take a chance and just order a 10K one. There didn't seem to be a problem with 20K, but I'm really concerned 10K would be too low.

 

(yes I did take the pot out of my NFB-12 to try it in the SSMH.)

 

This seems to confirm my theory. A higher value pot will have the grid "floating" higher relative to ground, and will therefore be more sensitive to noise. I'd really like if anyone with noise problem give it a try and report back. Note that I originally had a 100K pot, so the difference between 100 and 20 may be more significant than 50 to 20. Anyone can try a 10K pot?

post #6261 of 6814
Quote:
Originally Posted by KimLaroux View Post

I just learned the hard way how to blow a tube in this amplifier: Turn it ON without R4 or R10....

 

Kim,

    thanks for posting this.

post #6262 of 6814

I just started building the 12au7 version of this amp after waiting a  couple weeks to get my hands on some IRF 510s. My question is, what does c6 do? and does it matter if c1 and c6 are 1000 uf?

post #6263 of 6814
C1 and C6 are power supply decoupling caps. Along with R13 they form a filter whose intent is to minimize the interaction between the input and output stages through their common line to the power supply. Wikipedia has a nice explanation here: http://en.wikipedia.org/wiki/Decoupling_capacitor

As to the value, theoretically speaking a greater value provides more power storage and hence increases the effectiveness of the decoupling. Real world that power must come from your power supply; it'll need to provide enough power at startup to charge those caps (along with lighting up the tube heaters, charging the output caps, providing current to the tube plates and MOSFET drains, etc.)

Will your specific supply be able to cope with the demands? I can't answer that. But, as I mentioned a few posts back, I bought a Cisco 1A supply to do away with all those concerns.

cheers!
post #6264 of 6814
Quote:
Originally Posted by n_maher View Post

If you have anything in the way of parts laying around (like I do) you can build one of these for under $50 including everything without breaking a sweat.

ooh Only read the first page of this but it seems interesting <3 I love getting DIY'r Amps :3 

post #6265 of 6814
Quote:
Originally Posted by the_equalizer View Post

C1 and C6 are power supply decoupling caps. Along with R13 they form a filter whose intent is to minimize the interaction between the input and output stages through their common line to the power supply. Wikipedia has a nice explanation here: http://en.wikipedia.org/wiki/Decoupling_capacitor
As to the value, theoretically speaking a greater value provides more power storage and hence increases the effectiveness of the decoupling. Real world that power must come from your power supply; it'll need to provide enough power at startup to charge those caps (along with lighting up the tube heaters, charging the output caps, providing current to the tube plates and MOSFET drains, etc.)
Will your specific supply be able to cope with the demands? I can't answer that. But, as I mentioned a few posts back, I bought a Cisco 1A supply to do away with all those concerns.
cheers!

+rep helps alot. I am using a .7A PSU I think this is more than the original one. hopefully I don't have any issues.


Edited by shamazo - 10/27/12 at 6:21pm
post #6266 of 6814
Quote:
Originally Posted by shamazo View Post

+rep helps alot. I am using a .7A PSU I think this is more than the original one. hopefully I don't have any issues.


Glad I can help. As others have posted they do quite fine with the 350ma or 380ma Cisco PSU so with the 700ma you won't be having any current supply problems. Good luck with your build.

 

cheers!

post #6267 of 6814
Quote:
Originally Posted by the_equalizer View Post


Glad I can help. As others have posted they do quite fine with the 350ma or 380ma Cisco PSU so with the 700ma you won't be having any current supply problems. Good luck with your build.

 

cheers!

So you know any one selling one of these

post #6268 of 6814
Quote:
Originally Posted by Mshenay View Post

So you know any one selling one of these

 

You'll have to go to the "For Sale/Trade" forums for that. From time to time one pops up on e-bay too. Good luck.

post #6269 of 6814
Quote:
Originally Posted by the_equalizer View Post

 

You'll have to go to the "For Sale/Trade" forums for that. From time to time one pops up on e-bay too. Good luck.

There's some fellow selling a kit on ebay, here's another question

 

Do these have enough power to drive a 250 ohm can? I'm sure the answer depends on who built it and what the used, but following the CHEAPEST build plana [in case I end up with that <.<] how would they do! 

post #6270 of 6814

it depends on the can. but millet states on the starving page that his hd600's are well powered with a standard starving student and thats a 300ohm can. ive never used anything high impedance with mine because im an ortho type of guy, but my ssmh poweres my hifiman he-4's well and they are usually difficult to power. generally any amp with tubes will have the required voltage to power high impedance voltage hungry cans.

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