Newbie questions on AB763

[Willabe]

53.7 VAC in the blue and I noticed that on my layout, the pair of 220K's is connected by a jumper on the ground side...I see that in your picture these 220K's aren't connected....

In Doug's layout (which is what SG posted) they ARE connected look again.


                    Brad     

[SILVERGUN]

Yikes!!!      The layout I have must be off a bit...I remember getting a post from someone a while back that I cut the jumper somewhere...which I did...I'll go back and look.  I still get nothing on pin 5 of the power tube with the brown OT wire and -24.3 on the power tube with the blue OT wire...

Slow down Larry....and stay calm....sorry for the exclamation mark on my last post, but I need you to focus. 

1) Verify that the turrets in ALL 3 of those red circles are connected
2) Then figure out how to read -24VDC on both socket pin #5...if you don't get that reading, then one of those connections is still missing
3) Do nothing else until we are sure that everything is connected properly there

[sluckey]

Would you post your layout in this thread? Tubenit was the person that made that layout back in'08. Unfortunately the full size layout pic has disappeared.

[Willabe]

Red circle looks connected above the board, green circle does NOT look connected at least above the board, blue square does NOT look connected at least above the board.

You don't have to pull the board to check if their connected below the board, do a resistances check with your meter.

We also need a -dcv check for/at/in the blue square.


               Brad        

[lpresnall]

Point taken, Brad!  I guess I meant that there are two legs on the "tube" side of my board and one leg that connects with the 47 on the "ground" side....

[lpresnall]

Red circle looks connected above the board, green circle does NOT look connected at least above the board, blue square does NOT look connected at least above the board.

You don't have to pull the board to check if their connected below the board, do a resistances check with your meter.

We also need a -dcv check for/at/in the blue square.


               Brad     

Hi Willabe,


They were in the process of being connected while everyone was posting.  (Thanks all for the quick replies).  I get - 23.5VDC between the pair of 220K's.

[Willabe]

I get - 23.5VDC between the pair of 220K's.

I'm betting either the 22K R is not really 22K or the bias cap is bad.
The R colors are correct, I'd still measure it at this point with your meter.

The bias cap is 47uF. If it's leaking that would drop your -dc bias voltage.

I think the pots good because the -dcv drops right after the bias diode.

Your dropping to much -dcv.

The only other thing that's left that I can think off (if the 22K reads correct) is that the PI coupling caps are leaking the PI +plate dcv which would cancel out the -dc bias voltage.


                   Brad     

Edit; Some how I goofed up and posted under lpresnall. So I deleted it and added my post into this post.           

[lpresnall]

Would you post your layout in this thread? Tubenit was the person that made that layout back in'08. Unfortunately the full size layout pic has disappeared.

[lpresnall]

Ok Brad...if you ask me about jets I can tell you...but, I need the PI thing "Forrest Gumped" waaay down for me...where are these caps?  I'm gonna check the 22K and replace if need be right now...

[lpresnall]

I have another 47 cap from an old Peavey but it's HUUGE!  I'll try to temporarily wire that in and see what happens if the 22K tests out ok...

[Willabe]

I need the PI thing "Forrest Gumped" waaay down for me...where are these caps?  I'm gonna check the 22K and replace if need be right now...

PI = phase inverter, sorry. They block the dc plate voltage from the PI yet let the ac signal voltage pass to the next tube stages controll grid, in this case the power tubes.

They are the 0.1uF caps that connect to the 220K R's and the power tubes grids pin 5.


             Brad   

[SILVERGUN]

I think the pots good because the -dcv drops right after the bias diode.

Have we tried adjusting the bias pot from MIN to MAX and seeing what those voltages are?....I feel like I missed something 

[lpresnall]

Silver, the bias pot wasn't changing the values at all...that's what the initial issue was....the 22K tests good btw...on to the bias cap now....

[SILVERGUN]

Silver, the bias pot wasn't changing the values at all...that's what the initial issue was....the 22K tests good btw...on to the bias cap now....

Yes, but now that you made all of those other connections, the bias voltage has somewhere to go...

Try it again

I thought you were taking your original measurement of -.03VDC at a point that wasn't actually connected

[Willabe]

I've traced and re-traced my wiring against the Hoffman Lite layout but can find no mistake...Also, the bias pot still does nothing.

             Brad   

[Willabe]

With no rec, tubes, or speaker connected and the black meter probe to chassis ground I get the following:
(Top view of bias pot, starting with single leg and going clockwise)... - 24.5VDC, - 24.5VDC, - 37VDC

               Brad     

[SILVERGUN]

He ALWAYS HAD bias voltage present....

He was taking a reading at a point that wasn't connected

 

I'm talking about when he was reading -.03 VDC 

[Willabe]

No, you might be right.

I assumed he was still trying to turn the -bias pot when he posted -bias dcv at the 3 pot legs.


              Brad   

[SILVERGUN]

With no rec, tubes, or speaker connected and the black meter probe to chassis ground I get the following:
(Top view of bias pot, starting with single leg and going clockwise)... - 24.5VDC, - 24.5VDC, - 37VDC

OK, so the range of his pot should NOW read -24.5VDC at MIN and -37VDC at MAX

If he's reading -24.5VDC then he's got the pot set to MIN

[lpresnall]

When you say, "everything in red must be connected" do you mean to each other as in, underboard?

Inside red circles must be connected to each other....
And bias pot must be connected as marked..

You are missing some jumpers!

No problem on the exclamation point Silver!      I checked the red circles again and everything's connected.  The 22K tests good.  The meter still says, "no" on pin 5 of the first power tube but now I'm getting - 24VDC on pin 5 of the other tube and the bias pot does change it...

[lpresnall]

With no rec, tubes, or speaker connected and the black meter probe to chassis ground I get the following:
(Top view of bias pot, starting with single leg and going clockwise)... - 24.5VDC, - 24.5VDC, - 37VDC

OK, so the range of his pot should NOW read -24.5VDC at MIN and -37VDC at MAX

If he's reading -24.5VDC then he's got the pot set to MIN

I go from - 23 to - 64 on pin 5 of the second power tube by adjusting the bias pot.

[SILVERGUN]

The meter still says, "no" on pin 5 of the first power tube but now I'm getting - 24VDC on pin 5 of the other tube and the bias pot does change it...

OK,,,you still have one missing connection that is not allowing the bias voltage to get to pin 5 on the tube with no reading

The bias voltage gets "shared" by those 2 tubes.....if it's on one tube then you're just one connection away from the homestretch 

[SILVERGUN]

Work backwards from the pin 5 that has no voltage on it until you find where you're losing it...

It looks like it goes through the grid stopper (resistor from pin 5 to pin 6) to pin 6 - check there
Then go back to the board and check where the 220K and the 1uf come together....

Etc., etc,

[lpresnall]

I need the PI thing "Forrest Gumped" waaay down for me...where are these caps?  I'm gonna check the 22K and replace if need be right now...

PI = phase inverter, sorry. They block the dc plate voltage from the PI yet let the ac signal voltage pass to the next tube stages controll grid, in this case the power tubes.

They are the 0.1uF caps that connect to the 220K R's and the power tubes grids pin 5.


             Brad   

Should my next step be to swap the 47 cap at the bias pot?  Seems that if it was bad I wouldn't get voltage on pin 5 of the second tube.  The fact that pin 5 of the first tube is "0" makes me think I've got a bad solder somewhere since all the jumpers are in place...

[lpresnall]

Work backwards from the pin 5 that has no voltage on it until you find where you're losing it...

It looks like it goes through the grid stopper (resistor from pin 5 to pin 6) to pin 6 - check there
Then go back to the board and check where the 220K and the 1uf come together....

Etc., etc,

Sorry, posted that before reading your last post...will do it right now...

[Willabe]

Should my next step be to swap the 47 cap at the bias pot?  Seems that if it was bad I wouldn't get voltage on pin 5 of the second tube. 

No do like SG said.

I thought your max -dcv was only -37dcv. I thought you were giving max dcv.     


                   Brad     

[SILVERGUN]

If you've got it on one pin, then it's made it that far onto the board... you can forget about the pot, cap,,,etc., etc. for now

It comes into the board at the point where those two 220K resistors come together...
- Are those 2 turrets connected?

It then passes through those 2 resistors to the 2 points where each 220K meets up with it's companion .1uf cap
- Are the 220K resistors now connected to their corresponding .1uf cap?

Do you read -24VDC(ish) at both of those points?

[lpresnall]

Work backwards from the pin 5 that has no voltage on it until you find where you're losing it...

It looks like it goes through the grid stopper (resistor from pin 5 to pin 6) to pin 6 - check there
Then go back to the board and check where the 220K and the 1uf come together....

Etc., etc,

Ok, replaced the .1uF for good measure and re-flowed the connections around the entire bias debacle.  The last time I get good - VDC is at the positive side of the board at the .1uF-jumper-220K-pin 6 of the second power tube (farthest from bias pot).  The other half of that circuit (the positive side of the 220K, the other .1uF-pin 6 of the power tube closest to the bias pot) still reads "0"...am I getting closer or further away from an amp?

[lpresnall]

If you've got it on one pin, then it's made it that far onto the board... you can forget about the pot, cap,,,etc., etc. for now

It comes into the board at the point where those two 220K resistors come together...
- Are those 2 turrets connected?

It then passes through those 2 resistors to the 2 points where each 220K meets up with it's companion .1uf cap
- Are the 220K resistors now connected to their corresponding .1uf cap?

Do you read -24VDC(ish) at both of those points?

I have it on the top and bottom right sides but "0" on the bottom left.  (Sorry, I wish I knew how to notate on the pic like you gurus do).

[SILVERGUN]

"...am I getting closer or further away from an amp?

Look at my last post.....you're that close


BUT,,,all we are doing so far is making sure that your power tubes don't light up orange again.
With the problems that we have had getting to this point, there's just no tellin' what else might go wrong. 
Sorry Larry,,,but I'm not much for sugar-coating 

The bad news is, the bias circuit is one of the EASIEST things to get straightened out. 

[lpresnall]

Here's what I have now:

[lpresnall]

It's okay Silver, sugar-coating would likely only caramelize at the high heat I'm likely to get on power up, resulting only in a bad batch o' brittle....

[SILVERGUN]

This is the point where your own skill must take over and YOU must figure out how to get that voltage over to the other side.

IT IS just as easy as my last picture posted.

You WILL find which connection is not getting through.

Take a break, get some lunch, digest what we've said, and realize that's all you need is to split the same path that's getting through to the one tube....

YOU got this!
 

[lpresnall]

Thanks man!  And thanks for putting up with me, all of you... I'll look again in an hour... After all,  Rome wasn't burned in a day   

[lpresnall]

Ok, went back one last time for the evening and traced the bias circuitry.  I even replaced the 220K where I lose the signal and re-flowed the connections at the spot where the 220K, jumper, and .1uF meet.  Specifically, I lose the plot at the positive (side toward the tubes) end of the 220K closest to the bias pot (which is also the tube where I'm getting no voltage on pin 5.  I'm at a loss but I hope to give it a week and then get back at it again.  Thanks again to everyone who has posted here offering solutions.  This is a great forum and you are a good bunch of folks. 

[lpresnall]

Back in town and gave the amp an hour today...compared the layout to the actual board and re-secured and re-flowed connections.  Tried connecting the "good" 220K feed to the "bad" pin 5...of course, got voltage there.  When I put everything back still nothing...replaced 220K and .1uF on the "bad" side of the bias circuit...still nothing...as before, it's narrowed down to the south (tube) side of the 220K nearest the bias pot...I'm glad I don't do this for a living! 

[sluckey]

Measure resistance from pin 5 of each output tube to chassis ground. What do you get?

[lpresnall]

"0" and - 23 VDC

[lpresnall]

Here's the board...

[SILVERGUN]

Welcome back Larry...

Measure resistance from pin 5 of each output tube to chassis ground. What do you get?

"0" and - 23 VDC

See the problem with your answer?

[lpresnall]

Yep, on my way back upstairs! 

[lpresnall]

The problem pin reads 1491...the other -17.87

[SILVERGUN]

If you really do have everything connected properly, there is still a chance that there is an error in this area...

If those OT wires are touching the bottom of your board and you heated the turret up to the point that you melted through some insulation,,,that could be where you are losing your bias voltage.

Unbolt the board, lift it up and check for a short.

[SILVERGUN]

Or just check for continuity between those bias circuit turrets and your speaker jack connections if you don't want to lift the board....with power off

Bedtime for me Lar,,,,I think sluckeys still up 

Good luck sir

[sluckey]

The problem pin reads 1491...

That's kinda what I was expecting. Now check the resistance of the problem tube pin 6 to chassis. I am expecting about zero ohms. What do you measure?

I'm suspecting a short circuit that is killing the voltage for the problem tube. Over...

[sluckey]

Stick around for a few minutes and we'll have this thing sorted out.

[lpresnall]

Or just check for continuity between those bias circuit turrets and your speaker jack connections if you don't want to lift the board....with power off

Bedtime for me Lar,,,,I think sluckeys still up 

Good luck sir

Power off, problem turret on south side of 220K to speaker jack is 1 ohm more or less...on the "good" turret south of the 220K 216 ohms.

[lpresnall]

The problem pin reads 1491...

That's kinda what I was expecting. Now check the resistance of the problem tube pin 6 to chassis. I am expecting about zero ohms. What do you measure?

I'm suspecting a short circuit that is killing the voltage for the problem tube. Over...

1.1 ohms

[sluckey]

OK, that's what I thought. Now, unbolt the board and look to see what's shorting the bad turret. I'll stay on the board with you.

[lpresnall]

Ok, there's not a lot of room for movement with the caps in tight as they are...I'll give it a go...