Help Needed with Bias Voltage

[samato]

AB763 Deluxe NoVerb Single

I finally finished putting this thing together and wiring everything.  I started my first time power up tests and found out the bias is not right. 

First, the bias voltage is positive not negative - I guess the diode is reversed?  Plus I'm only getting a range between about 7VDC - 14VDC while adjusting the trim pot.

I have so far only put the rectifier tube in and tested voltages.  I have my meter's black probe connected to the chassis and the red probe connected at the point where the (2) 220K resistors meet on the board.  I also tried directly on pin 5 of each 6v6 socket. 

I'm dying to put the rest of the tubes in and try it out but I know I shouldn't until the bias is right.  I could really use some help on this one.  I've attached my layout and some pics of the bias board.

[samato]

I realized I should be reading VAC, not VDC.  I'm getting from 0.8VAC - 1.8VAC so it's still way off and it's still positive.

Blue PT wire is giving me 53.8VAC so no problem there.

[PRR]

> I'm getting from 0.8VAC - 1.8VAC ....it's still positive.

How can you have "positive AC"?

Why would you bias an audio tube with AC? Isn't the 470u cap there to minimize AC?

Do you need a refresher on meter-reading?

> I guess the diode is reversed?

Compare it to the plan. The plan is right. In your photo, "I see" cap correct and diode wrong, but your eyes are a lot closer.

> at the point where the (2) 220K resistors meet

Fine, but also check at the top of the bias cap. That should be a full 70V or so, and NEGATIVE.

And if it has been powered-up WRONG polarity more than a few minutes, replace the bias cap. Reverse voltage on an electrolytic kills it.

[samato]

I don't think I need a refresher on meter reading, I need to actually learn the basics of meter reading and electronics for that matter - but I'm not gonna let that stop me.    Actually, that's not a joke - this is how I'm learning.  I do know how to discharge caps and I'm familiar with "chopsticking" and the one hand behind the back thing so hopefully the mistakes I make will only damage electronic parts and not body parts.

I'll try reversing the diode and maybe replacing the cap as it has been powered on wrong I guess for more than a few minutes.  I'll try checking voltage at the top of the bias cap first I guess.

[samato]

I'm reading the DC voltage at the top of the bias cap now before making any changes.

First I read 19V, then 17V, then 15V.  Maybe this is what PRR was talking about regarding reverse voltage killing the cap? 

I plan to reverse the diode and change the cap now.

[samato]

I reversed the diode and replaced the cap - life is good.  I can now go from -62VDC to -32VDC. 

That's a good range for this amp isn't it?

[tubeswell]

I reversed the diode and replaced the cap - life is good.  I can now go from -62VDC to -32VDC. 

That's a good range for this amp isn't it?

Mostly - depends what kind of tube(s) you are intending to run, how high the B+ is etc