MIC29502 voltage unadjustable

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thuanth43

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Joined 2016
#1
So I'm building an Aikido OTL headphone amp but got stuck with the heater supply. It is a MIC29502 circuit, 6.3VAC in and 6.3VDC out. Now the problem is no matter what I turned the trimpot, the voltage got stuck around 3.9-4vdc and the rectifiers got super hot. I tried to swap 3 different MIC29502s, same result. I'm quite cluless on what's happening. Any help would be much appreciated!

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#3
This is what I used:

gerrittube

Thread 'Get regulated 6.3 VDC from 6.3VAC'

I’ve been testing various options for creating a clean and stable 6.3 Volt DC from the 6.3 Volt AC heater transformer winding. Why DC? I discovered earlier that DC on the driver tube’s heaters did lower the residual hum and noise of my tube amplifier. Even though the hum was inaudible I could still measure some.

Therefore I use DC for two 6SN7 tubes per channel in my amplifier. This means each channel draws 1.2 A average heater current, with of course a current surge for cold tubes when switched on.

Of course this solution can be used for lots of other tubes with a 6.3 volt heater.

I...
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Regards, Gerrit
 
#5
Are you sure the bolt is isolated from the heatsink? Not familiar with this regulator so possibly my mistake.

BTW the potentiometre is connected wrong. The wiper should be connected to either connection in case the wiper does not make good contact.
 

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#6
Yes, the bolt is isolated, it's made of PEEK.

So I tried to pull out the 6SN7 + 6BL7 tubes and now I could adjust to 6.3vdc just fine, the rectifiers are really cool. But as soon as I got the tubes back in, same problem happens :-/ Bad tubes or my transformer doesn't supply enough current? I spec'd for 6.3vac/8A so really doubt it is the culprit.
 
#10
Ahem, rectified and filtered input voltage to the regulator. To solve your issue* such details are necessary. We usually measure and test PSUs with a load as that is the reason for their existence.

* I am very sure you used the wrong diodes. They have a minimum Uf of 1.66 and typical Uf of 2V with 10A load current. Even with a few Ampere load current Uf is already 1.3V. That times 2. Your PSU can not/never work OK with load. You need ultra low Uf Schottky diodes like the symbols in your schematic correctly indicate. Sloppy errors have consequences, certainly in “tight calculation” situations. Also please correct the wiper error.

Solved. The MIC29502 is perfectly adjustable so the thread title is off. “MIC29502 filament PSU failing under load”.
 
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