Millett Biasing Problem
Mar 30, 2006 at 11:57 AM Thread Starter Post #1 of 91

bperboy

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The Right side of my Millett biases fine, but the left is stuck at about 24vdc, so I would assume something is shorted out. I've swapped out tubes and also the buffers, but nothing works. Please help me solve this problem.
Bperboy
 
Mar 30, 2006 at 5:18 PM Post #4 of 91

n_maher

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I suspect you've got a cold or inadequate solder joint - I'd reflow the connctions on the bottom of the PCB (there aren't that man) paying careful attention to the trim pots.

Nate
 
Mar 30, 2006 at 5:21 PM Post #5 of 91

NeilR

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With the power off, measure the resistance between the tube grid and ground. Do the same with the cathode pin to ground.

Neither should be open. They should read somewhere between zero and the value of the volume pot/bias trim pot.

Measuring resistance cathode to ground, as above, the value should change as the trim pot screw is turned. If it does not, or it reads outside the range of the trim pot, something is wrong there.
 
Mar 30, 2006 at 6:00 PM Post #7 of 91

bperboy

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I reflowed the solder joints on the trimpot in question, and I also checked for resistance between the recommended tube pins, but the one side still reads 24 vdc for bias.
 
Mar 30, 2006 at 6:07 PM Post #8 of 91

n_maher

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Stupid question #2: are the tubes and buffers installed?

If so, try swapping the buffers and see if the problem follows it. Same with the tubes, see if the problem tracks with changing a component. Since you took the measurements at the cathode and grid what were the results? Did the resistance change as you adjusted the trimpot? Also, did you try measuring the trimpot to see if it is working properly?

Nate
 
Mar 30, 2006 at 6:28 PM Post #9 of 91

NeilR

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Do you also have 24V at the tube's plate pin?

If the grid has a connection to ground (via the pot) and the cathode has the trim pot resistance between it and ground, and you have 24V on the plate, there is not much left to go wrong. Just for laughs, check the voltage between R5 and D2, and then on the other side of R5 (on the DC in side).

As I understand it, if there are no voltage drops across D2 and R5, there is no current flowing at all through the tube. The most likely problem is the grid or cathode. You said you switched tubes and the problem stays in the same channel, eliminating a bad tube. That implies a problem in D2 or R5, most likely D2 since R5 doesn't do much.

I know this is a really stupid question, but are you sure you turned the trim pot enough to start current flowing? I say this because my 12FK6's read full voltage when I swap from 12AU6's, forcing me to do many turns on the trim pot. Just grasping at straws here. You could set the trim pot in the bad channel to the same value as the trim pot in the good channel, measuring resistance from cathode to ground.

One thing you could try is to replace D2 (which you probably don't have a spare) with a simple resistor. It should bias properly with a resistor replacing D2, the trick is to come up with the right value. I'm thinking 24K, for a 12V drop with 0.5ma, but not sure that it's that simple. Maybe someone has a more refined idea if it gets to this.
 
Mar 30, 2006 at 6:41 PM Post #11 of 91

NeilR

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Just checked Pete Millet's original article. He used R5=47K - 100K for his own tests. The idea here is to just replace a part and see if you get current flow, resulting in a voltage drop at the plate. Not trying to pick a "perfect" value, whatever that would be. It would also be very tube dependend but the idea is to get a voltage drop.
 
Mar 31, 2006 at 1:15 AM Post #14 of 91

Blooze

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I don't know if this is applicable, but I had one 12fk6 that when I put it in I could not adjust the bias on it at all. It stayed at max all the time. When I put a different one in or swapped tubes it adjusted fine. No idea why.
 

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