Funny thing is happening with my bias circuit

[lpresnall]

Let me start off by saying that I barely know what I'm doing. Now, I've gotten some great help from another forum but I don't want to wear out my welcome there and I'd like some fresh eyes on this. I'm building this amp and I have current all the way to the "negative" board side of the left-most 220K in the bias circuit. It just stops there and I can't figure out why. I've replaced the resistor, the .1uF cap, re-flowed the solder connections in the entire bias circuit, and the pin 5 of the left-most power tube. I still can't get current thru the "positive" end of the left-most 220K. Any ideas?

[lpresnall]

Here is a shot of the issue from the other forum...I have no current at the left bottom circle:

[NickC]

Let me start off by saying that I barely know what I'm doing. Now, I've gotten some great help from another forum but I don't want to wear out my welcome there and I'd like some fresh eyes on this. I'm building this amp and I have current all the way to the "negative" board side of the left-most 220K in the bias circuit. It just stops there and I can't figure out why. I've replaced the resistor, the .1uF cap, re-flowed the solder connections in the entire bias circuit, and the pin 5 of the left-most power tube. I still can't get current thru the "positive" end of the left-most 220K. Any ideas?

I'm an amateur. But it looks like the cap in the bias circuit (coming off the pot wiper in the diagram you posted) is in the wrong place. The cap needs to be reverse biased with the positive connection to ground. That connection goes to a pot leg (NOT the wiper, which is the middle pot connection). The opposite pot leg should have a resistor to ground. The negative side of that cap joins with the negative side of the diode that goes to the pot leg. The middle pot leg (wiper) carries negative bias voltage to a pair of resistors and on to the output power tubes. There should not be a cap there.

I think all you need to do is move the negative leg of the cap off the middle wiper pot connection, to the right pot leg where the diode is hooked up. Leave the middle pot/wiper wire that is designated as under-board connection intact to the two 220K R's.

Take a look at the Fender AB763 schematic bias circuit attached. A picture is worth a thousand words.

[vibratoking]

You say you have current, but you mean voltage, correct?

The circuit should work properly as drawn in the Hoffman layout. It should also work properly in the AB763 circuit that Nick posted. They are slightly different, but result in the correct bias voltage.

The image you posted is correctly annotated in yellow. If your connections are as shown, then the circuit should work. I suggest you recheck your connections carefully. Also, could you post an image with better resolution so that we can read resistor values? I can't positively identify the resistors in the image. Also, post voltage readings at the various nodes of your circuit.

Make sure you are not debugging this with tubes in the sockets. A bad bias voltage could overbias your output tubes and destroy them.

[xtian]

With respect to Nick, Hoffman's layout is different from Fender's, and it's Hoffman's the OP is using (I like Hoffman's layout! Used it a couple of times).

It's tough to read the dark and blurry photo, but looks wired correctly.

Uh....vibratoking beat me to it.

[lpresnall]

Hi guys,

Thanks for the quick replies...I'm at work now and don't have a way to post a pic, but I can copy the values from the other forum and post here...the 220K's are Red, Red, Orange, verified....I did the easy stuff twice, and even replaced the left-most 220K with a fresh, verified one just in case. Still no voltage (I mis-spoke and said, "current" in original post)...at the tail end of that resistor or at pin 5 of the respective tube. And all of this is happening with no rec or tubes in place...

[lpresnall]

New pic:

[xtian]

In your photo the resistor just to the right of the bias cap does not appear to be soldered in.

[vibratoking]

For me, there is not enough resolution in that image to really identify anything for sure. I agree with xtian...that resistor doesn't appear to be soldered, but there are 5 or 6 places that don't look soldered either.

What about some voltages?

[lpresnall]

When I get back to town I'll be more forthcoming with pix and voltages...thanks for the input guys.

[Tillydog]

..the 220K's are Red, Red, Orange, verified....

I do hope you mean Red, Red, *Yellow* (or Red, Red, *Black*, Orange)

+1 for better photos.

[passfan]

What Tillydog said. Red red orange is 22K , not 220K.

[lpresnall]

Yes, red, red,yellow.

[passfan]

You've got a cold solder joint or bad 220k resistor or missed a jumper somewhere. You just haven't found it yet. Fortunately you don't have very much ground to look for it

[lpresnall]

I know you're right pasfan, but I"ve trod and re-trod that ground around the pair of 220K's...guess I'll re-flow everything once more...thanks for the input!