Aleph J illustrated build guide

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mpa

Member
Joined 2010
Hi chaps, just powering up my Aleph J for the first time! PSU ok on bulb tester and producing the expected voltage but when I attach an Aleph board, the bulb lights then dims and then brightens a bit again finally settling on what I would call quite bright but not as bright as a 100w bulb would normally be.. Both boards have the same result. Bias reads about 340mv and offset 140mv. Am I being too cautious in thinking there's a problem or should I just try without the bulb tester next??
 
mbrennwa

mbrennwa

Member
Joined 2005
audioroot.net
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mpa said:
Hi chaps, just powering up my Aleph J for the first time! PSU ok on bulb tester and producing the expected voltage but when I attach an Aleph board, the bulb lights then dims and then brightens a bit again finally settling on what I would call quite bright but not as bright as a 100w bulb would normally be.. Both boards have the same result. Bias reads about 340mv and offset 140mv. Am I being too cautious in thinking there's a problem or should I just try without the bulb tester next??
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Looks like the power supply draws a bit of current, which makes the bulb light up. This is how it's supposed to be! Someone else may know more about the right bias values, but I say if there is no bangs or smoke you're ready to go without the bulb tester. Then adjust the bias and offset, hook up a (cheap) loudspeaker and see if you get music!
 
6L6

6L6

Moderator
Joined 2010
Paid Member
Absolutely fantastic! Put the lid on (resting) let it warm up for 30min or so, set bias to 400mV and re-check offset. Let sit another 30 with the lid on, measure again, if stable, listen and enjoy!

The Aleph is a beautiful and beguiling amplifier, you will love it! Enjoy :yes:
 
Extreme_Boky

Extreme_Boky

Member
Joined 2003
Your output stage is floating, there's no current running through the output MOSFETs.

So, check the input JFETs and make sure the voltage drops are correct. This will generate a required voltage drop across R7, to "open" the "amplifying" MOSFETs into a conductive state.

The other, current-setting MOSFETs, are set into a conductive state by Q4 Vce. This also needs to generate 4.3V to let the current flow (through the output MOSFETs) from the positive rail.

See the 3 voltages in the attachment, measure yours, provide few photos...
 

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