Millett "Starving Student" hybrid amp
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Beftus

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My 12AU7 Mk II is alive. Finished it tonight, finally got around to putting it all together. Worked first time. 
 
some specs:
aluminium top panel also doubles as a bit of a heatsink
each triode now has it's own grid stopper resistor
no cathode resistor bypass caps as I want to try higher gain tubes
Elna RJH (C1,C6) and Silmic (C3,C5) caps, Silmic's are 50 volt type
MKP1837's as bypass and interstage caps
2.5" tall heatsinks (that still get pretty hot compared to 2" tall ones)
 
Still to do:
mount blue tube LEDs and wire up two resistors for bright-off-dimmed settings to be operated with a on-off-on switch (right one)
 
Nasty surprise: a cheap RCA jack spilled it's guts, a stark reminder to buy better quality parts. My Mk I is now missing one RCA jack...
 
How does it sound? Schweet! Doesn't play as loud as Mk I because it lacks C7 and C8. A picture to prove it happened. 
 

 
No pics of the wiring inside. Wanna know what it looks like? Imagine a bowl of spaghetti... 

 
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the_equalizer

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Nice job, congratulations! 
 
  BTW, what do you mean about not using cathode resistor bypass caps in order to try higher gain tubes?
 
cheers!
 
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Beftus

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The cathode bypass increases gain, combined with a higher gain tube the overall gain might become too high. Too much gain will limit the volume control to a small arc right at the beginning of the pot travel where the pot has less than ideal tracking.
 
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the_equalizer

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Quote:
The cathode bypass increases gain, combined with a higher gain tube the overall gain might become too high. Too much gain will limit the volume control to a small arc right at the beginning of the pot travel where the pot has less than ideal tracking.

Ah, OK, it's about staying out of the volume pot imbalance zone. So what other tubes are you planning to try?
 
cheers !
 
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Beftus

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I think I'm gonna buy NOS Philips JAN 12AT7WC tubes.  Not too expensive at €9.25 each.
 

 
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Ikarios

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Is increasing gain all the cathode bypass caps do? i.e. can I remove them with no ill effects? As it stands I can't really turn my amp below 9 o'clock and regularly get into my pot imbalance.
 
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Beftus

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Quote:
Is increasing gain all the cathode bypass caps do? i.e. can I remove them with no ill effects? As it stands I can't really turn my amp below 9 o'clock and regularly get into my pot imbalance.
 
I am not going to explain this because I can't. I did find this however: http://www.head-fi.org/forum/thread/319231/millett-starving-student-hybrid-amp/2115#post_5379295
 
This too: http://www.head-fi.org/forum/thread/319231/millett-starving-student-hybrid-amp/2385#post_5436249
 
Please note that Pete Millett didn't include the cathode bypass caps in the original 19J6 version. I can confirm a 12AU7 will happily play without them, just not that loud as with them. Listening now with volume pot @ 3 o'clock, couldn't do that with my Mk I
 
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the_equalizer

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Quote:
Is increasing gain all the cathode bypass caps do? i.e. can I remove them with no ill effects? As it stands I can't really turn my amp below 9 o'clock and regularly get into my pot imbalance.

Yes, that is basically all they do, you can remove them with no ill effects.
 
If you want a bit more detail...  some people prefer to have them since they do away with the local negative feedback generated by the cathode resistor (negative feedback is taboo to many audiophiles) and stabilize the quiescent point of each triode; others dislike them for the reason that they are in the signal path (capacitors, particularly electrolytics, in the signal path are taboo to many audiophiles); even further others dislike cathode resistor biasing for these previous reasons and use other means of biasing the tube.   In synthesis, it's pretty much a matter of personal preference if you want them there or not

 
 
cheers!
 
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ArtemF

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there are simpler ways to cope with high gain/9o'clock pot problem:
add a resistor in series with your pot and you'll easily go beyond 9 o'clock

 
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Ikarios

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Quote:
there are simpler ways to cope with high gain/9o'clock pot problem:
add a resistor in series with your pot and you'll easily go beyond 9 o'clock
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actually I do have a gain switch installed (50k/150k resistors in line with a 50k pot). It doesn't help too much, plus I'm afraid I'd be bumping the noise floor. it would be easier to snip off a couple capacitors... I'll have to think about this for a while.
 
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Ikarios

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Quote:
You can make the bypass caps switchable too...

have you seen my panels? 
there's like no space to put anything new, unless I wanted two gain switches on my front... I don't have access to my drill press either currently (nor the laser engraver) so it would just be easier to remove them. but you do make an interesting proposition. I guess I could just stuff it inside... I'm not sure if I have space for that, though.
I think I found a cathode resistor gain calculator here:
http://www.pentodepress.com/home/amplifier-calculators/cathode-capacitor/
plugged in my numbers (220uF for cathode bypass cap, 1k for grid resistor, 33k for plate resistor (what is this? whatever it is it doesn't change the numbers too much), and 2.2k for cathode resistor. It says the gain will be about 5.5dB across the board of frequencies. I have no idea what this means in relation to degrees of "turn" on my pot, though. I don't know anything about gain calculations yet.
 
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the_equalizer

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I wouldn't rely very much in that calculator; it's geared towards guitar amplifiers where the tubes are run at some 150 to 200 volts; with the much smaller voltages and currents in the SSMH the situation changes drastically.
 
Anyway, I think you're making this a bit more complicated than it is
;  I think you should simply remove the cathode resistor bypass caps and listen if that works for you in sound and volume pot tracking. If you find that for any reason you prefer the amp with the caps connected, you can always get something like these.
 
Finally, if you're really curious about what the plate resistor does, you can check this page out.
 
cheers!
 
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ArtemF

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it's interesting... I wonder how it can be that 150K resistor switched in series with 50K pot doesn't help.
It's 1:4 attenuation.
 
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calipilot227

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I'm debating whether to build this or the CMOY....screw it, I'm doing both! Can't wait to plug my Sennheiser HD280 pros into one of these bad boys!!! 

 
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