Millett Biasing Problem
Apr 1, 2006 at 4:31 PM Post #47 of 91
CCS (constant current source) = D2


25R plate to ground is WAY too low! Measure plate to ground in your other channel for reference. What does the good channel read?
 
Apr 1, 2006 at 4:48 PM Post #48 of 91
If you look at the schematic, each tube heater reads about 12.5R, for a total of 25R because they are in series.

The heaters are across the 24V supply line. Between the heater/supply line is R5 and D2. R5 and D2 measure 1KR each. I think you are using the J503 part????? I am using a 1n5291. Don't know if J503 has a different resistance. But that is irrelevent because even if it was zero, you should read 1K because of R5.

In effect, the heaters, D2 and R5 are in series between the tube plate and ground.

You apparently have a short between the plate and the other side of R5. Make sure you are not confusing 25R with 2.5R on the meter, which is why you should read plate to ground resistance on both channels to compare.
 
Apr 1, 2006 at 5:10 PM Post #50 of 91
I see on your image that your left buffer OPA551 jumper is WRONG. You are jumpering pins 3 and 7 (bypassing D2 and R5) instead of pins 2 and 6. I only noticed this by studying the board to see how you could have possibly shorted the opposite ends of the R5 and D2 node.

One or both of your OPA551's may now be suspect too- I didn't study that issue.
 
Apr 1, 2006 at 5:12 PM Post #52 of 91
Quote:

Originally Posted by NeilR
I see on your image that your left buffer OPA551 jumper is WRONG. You are jumpering pins 3 and 7 (bypassing D2 and R5) instead of pins 2 and 6. I only noticed this by studying the board to see how you could have possibly shorted the opposite ends of the R5 and D2 node.

One or both of your OPA551's may now be suspect too- I didn't study that issue.


The OPA551's are not a problem to replace, I got 5 from TI, but left, do you mean farthest from the pot? EDIT: never mind, I looked at the data sheet.
 
Apr 1, 2006 at 5:25 PM Post #54 of 91
YAY! SUCESS!!!!!!!!!! Thanks so much Neil for being patient and tryin to figure out what was wrong, even when I made such a big D"OH!
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YES, Millett goodness finally biased to about 12vdc on BOTH sides! WOOHOO!
 
Apr 1, 2006 at 5:33 PM Post #55 of 91
Okay, now that the bias is working properly, I'm only getting sound out of the left side, and there is a buzzing that is fairly loud. I've only had the thing running for about 10 minutes, so could this be the tubes just warming up. All the jacks have been double checked for correct wiring. The ground strap on the pot is wired.
 
Apr 1, 2006 at 5:34 PM Post #56 of 91
Your buffers are probably ok since the input should be able to handle +24V relative to ground since that is what would naturally fall on the input pin if the trim pot value is above about 2.5K, stopping current flow through a working amp.

I would check bias with the buffers removed, and if that is ok, try the original buffers and if you have any problems sub in some previously unused and abused buffers.

I would check resistance from buffer pins 6 (output) to ground, to make sure you have no shorts before putting in fresh buffers. You should have infinite resistance with the buffers removed.
 
Apr 1, 2006 at 5:36 PM Post #57 of 91
The tubes only take a minute or two to warm up. Is the buzzing coming from the left side or the right side?

First thing to do is swap tubes and buffers to see if the problem follows the parts.
 
Apr 1, 2006 at 5:44 PM Post #59 of 91
Swap tubes and buffers and see if the problem follows the parts.

Did it bias correctly with the buffers installed?

We've been through a lot of problem resolution logic here. Apply it to your current problem.
 
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