Newbie questions on AB763

[SILVERGUN]

Rest of the pic look like it'll stay together when I power up?

I don't want to leave you hanging, but I also don't want to be the guy that "green-lights" your work...
In the end it comes down to your confidence in your own work.....
- Have you done your homework?
- Do you have a current limiter built, and plan on using it on initial startup?
- Do you know what failures could occur?

We all have our own techniques and systems for testing our builds to be safe and prevent damage to components on initial startup....
....my own builds still make me a little nervous, so I always make sure to check separate sections of the whole circuit as I build them (and that works for me)
I wire my heaters first and then install tubes and check that wiring first...
Then I wire my power xfmr secondary to my rectifier and test that separately...
Then I wire my power section, and when I'm ready to power up I use a variac to ramp up the input voltage to feed the new capacitors slowly.....etc, etc, etc

I am a bit of a control freak that way, but that method does take a lot of the mystery out of the inevitable troubleshooting...
Nobody plans on making mistakes,,,but to be honest, I have had at least one mistake in each amp that I have built so far that I have had to troubleshoot....and when you're excited to hear your new creation, that can be a "buzz-kill"...
But you will learn to love the whole experience and expect that your amp wont work when you first fire it up....one of these days I'm going to be pleasantly surprised to take the amp out of standby for the first time and actually have nothing to fix...

Here's a guide that was created to help you in this situation:
http://www.paulrubyamps.com/info.html

[Ed_Chambley]

For those who may not have a variac and are weird (like me) about want to insure filter caps form properly there is very good discussion on the Metro Forum Here:
http://forum.metroamp.com/viewtopic.php?f=4&t=8396

I believe most people using new filter caps do not concern themselves with this, but it is not a bad practice.

[sluckey]

I think you're ready for the smoke test. During all the following tests be ready to quickly kill the power at any sign of smoke, fire, arcing, explosions, unusual noises, etc. OK, let's go...

Unplug the rectifier and lay it aside for now. Plug the amp into the wall and turn it on. Verify that all tubes light up. Measure the negative voltage on pin 5 of EACH power tube. Verify that the bias pot changes that voltage on pin 5. Adjust for maximum negative voltage on pin 5 and leave it like that for now. If there is a problem with the filaments or the bias, FIX IT NOW. Otherwise, turn the power OFF and proceed...

Now pull both power tubes and lay aside. Plug in the rectifier and turn the power on. Measure the B+ voltages on each of the filter caps. Then measure the plate (and screen) voltages on all the tubes. Start with the power tubes and work your way to the preamp tubes. Voltages will be a bit high since the power tubes are not in the amp. If there's a problem with any of the B+ voltages, FIX IT NOW. Otherwise, turn the power OFF and proceed...

Plug in the output tubes and connect a speaker. Don't connect a guitar yet. Turn all controls to zero. Turn the power on. Look, listen, and smell for any problems. Turn it off immediately if you have any problems. If all is well, plug in a guitar and see if it makes a joyful noise.

There's a 50/50 chance that the NFB will be reverse phase. Squealing or howling are big indicators of this. If so, turn the power off and swap the OT primary leads that connect to pin 3 of the output tubes.

[lpresnall]

Thanks guys!  I'll get to try all this out today, hopefully.  I did build a current limiter and this is the first amp that I haven't taken to a pro to actually power up - hence the apprehension!  I'll let you know how it goes.

[lpresnall]

I think you're ready for the smoke test. During all the following tests be ready to quickly kill the power at any sign of smoke, fire, arcing, explosions, unusual noises, etc. OK, let's go...

Unplug the rectifier and lay it aside for now. Plug the amp into the wall and turn it on. Verify that all tubes light up. Measure the negative voltage on pin 5 of EACH power tube. Verify that the bias pot changes that voltage on pin 5. Adjust for maximum negative voltage on pin 5 and leave it like that for now. If there is a problem with the filaments or the bias, FIX IT NOW. Otherwise, turn the power OFF and proceed...

Now pull both power tubes and lay aside. Plug in the rectifier and turn the power on. Measure the B+ voltages on each of the filter caps. Then measure the plate (and screen) voltages on all the tubes. Start with the power tubes and work your way to the preamp tubes. Voltages will be a bit high since the power tubes are not in the amp. If there's a problem with any of the B+ voltages, FIX IT NOW. Otherwise, turn the power OFF and proceed...

Plug in the output tubes and connect a speaker. Don't connect a guitar yet. Turn all controls to zero. Turn the power on. Look, listen, and smell for any problems. Turn it off immediately if you have any problems. If all is well, plug in a guitar and see if it makes a joyful noise.

There's a 50/50 chance that the NFB will be reverse phase. Squealing or howling are big indicators of this. If so, turn the power off and swap the OT primary leads that connect to pin 3 of the output tubes.

Ok, turned the power on just now.  No bangs, booms, pops, or smoke!  That's the good part.  Now, the preamp tubes glow but nothing on the power tubes.  This is with no rectifier and no speaker load.  I made a quick check of the layout against my pin wiring and so far don't see anything out of place.  I did however see that my layout has a heavy dot at pin 2 of the rectifier but no wire?  I may be misinterpreting what that means, but I have nothing wired into pin 2 there, so....anyway, I'm assuming (perhaps dangerously) that the problem is in the wiring between the 2nd preamp tube and the first power tube?  I did see a couple of solder connections at board components that could be better while I was tracing the wiring from the tubes.

[SILVERGUN]

Ok, turned the power on just now.  No bangs, booms, pops, or smoke!  That's the good part.  Now, the preamp tubes glow but nothing on the power tubes.  This is with no rectifier and no speaker load. 

I'm assuming that you mean the filaments aren't lighting up....check for 6-7VAC by holding your meter leads from pin 2 to pin 7 on the power tube sockets....

without the rectifier tube in there,,,not much else is going on

[SILVERGUN]

Pin 2 on the rectifier is critical...:

when you get that far 

[lpresnall]

Ok, turned the power on just now.  No bangs, booms, pops, or smoke!  That's the good part.  Now, the preamp tubes glow but nothing on the power tubes.  This is with no rectifier and no speaker load. 

I'm assuming that you mean the filaments aren't lighting up....check for 6-7VAC by holding your meter leads from pin 2 to pin 7 on the power tube sockets....

without the rectifier tube in there,,,not much else is going on

Ok, I get 6.92VAC on checking pins 2 and 8 together on the power tubes.  Not sure what I'm looking at on the rectifier diagram, but that's my lack of understanding...I am using a SS rectifier.

[SILVERGUN]

Ok, I get 6.92VAC on checking pins 2 and 8 together on the power tubes.  Not sure what I'm looking at on the rectifier diagram, but that's my lack of understanding...I am using a SS rectifier.

I hope you mean pins 2 and 7

If it is 2 and 8, that would explain why they are not lighting up

[lpresnall]

Ok, I get 6.92VAC on checking pins 2 and 8 together on the power tubes.  Not sure what I'm looking at on the rectifier diagram, but that's my lack of understanding...I am using a SS rectifier.

I hope you mean pins 2 and 7

If it is 2 and 8, that would explain why they are not lighting up

No, I meant 2 and 7...sorry

[lpresnall]

Looking at your rectifier diagram again...are we saying that pins 2 and 8 are jumpered together?

[SILVERGUN]

Looking at your rectifier diagram again...are we saying that pins 2 and 8 are jumpered together?

Sorry Larry,
I'm very busy tonight and wont be able to hold up my end of the deal.....

Pin 2 doesn't get used if you are using a SS rect

One leg of the 5v supply would get connected to it if you were using a tube rect

[lpresnall]

Gotcha!  I understand life happens!  I appreciate all of your input.  I leave for 2 weeks tomorrow to train on a new aircraft so I'll be back to the amp and on the board after then.  Be well!

[lpresnall]

Ok, it's been months and I'm back on this amp.  I'm rushing (not a good thing) trying to get it running before I leave again.  I get all the way to plugging the SS rectifier in and the fuse blows.  I did get a noise thru the speaker before the fuse blew tho...I'm wondering if my rec is ''wired" wrong...by "wired" I mean the pairs of I4007N diodes from pins 8-6 and 8-3...any ideas or am I not forthcoming enough?  I can get more readings if need be.

[Willabe]

I'm wondering if my rec is ''wired" wrong...by "wired" I mean the pairs of I4007N diodes from pins 8-6 and 8-3...

Yes.

Pin 6 to pin 8 for 1 diode (or pair in series) and pin 4 to pin 8 for the other diode (or pair in series). Both/all with the band going to pin 8.


                Brad   

[lpresnall]

Thanks!  I will re-do it in the morning!

[lpresnall]

Ok.  Wired diodes into SS rec properly and it didn't blow the fuse this time.  Went thru the rest of the procedure recording voltages without power tubes, then with everything in place.   Light glow to all tubes,  no noise, smoke, etc.  Connected speaker, turned power on and went looking for guitar.  Returned to VERY bright tubes and then fuse blew.  B+ values are 483/483/483/483/467.  Pin 1 on the preamp tubes 300 and 476.  Pin 3 on power tubes 478/478.  Pin 4 478/480.  Pin 5 Neg. .05.   Was kind of scary seeing all the brightness in the power tubes and I think there was a like amount of heat as well.  I don't know where to start looking for the trouble now.  Any wisdom from the gurus out there?

[Willabe]

I'm not a guru, but a couple of things;

1. From the voltages you posted it sounds like the tubes aren't drawing any current. (No voltage drop.)

Do you have a light bulb limiter? Any time on a new build for 1st power up you should use it to check for shorts in the B+ power supply (PSU).

Pin 5 Neg. .05.

2. With all the tubes out you need to check for negative DC voltage on pin 5 of the output tubes.

With the amp off and the B+ filter caps discharged take your meter and use your meter probe clips. Clip the negative to chassis ground and clip the positive probe to pin 5, set meter for DC voltage. Now turn on the amp and take a reading. If your amp has adjustable -bias turn the -bias pot kind of slowly from full counter clockwise to full clockwise watching for -bias dcv. Write down both extremes. Then do the same thing for the other power tube socket.


                             Brad     


Edit; added underline.       

[sluckey]

Number 1 priority is to fix the negative bias voltage on pin 5 of the output tubes. You say you have neg .05. You should have -30 to -50. Don't put the output tubes in the amp until this is resolved.

[Willabe]

Look at reply #52 from Sluckey, he explained how to 1st start up the amp and to check for -bias dcv.


                  Brad     

[lpresnall]

Number 1 priority is to fix the negative bias voltage on pin 5 of the output tubes. You say you have neg .05. You should have -30 to -50. Don't put the output tubes in the amp until this is resolved.

Ok, I feel kind of stupid asking, but if I'm not getting the voltages I need, how do I proceed?  The bias pot didn't seem to change anything, but this was with the preamp tubes in.  I may've misinterpreted your earlier advice...I'll go back and re-check the readings and report back.

[lpresnall]

I'm not a guru, but a couple of things;

1. From the voltages you posted it sounds like the tubes aren't drawing any current. (No voltage drop.)

Do you have a light bulb limiter? Any time on a new build for 1st power up you should use it to check for shorts in the B+ power supply (PSU).

Pin 5 Neg. .05.

2. With all the tubes out you need to check for negative DC voltage on pin 5 of the output tubes.

With the amp off and the B+ filter caps discharged take your meter and use your meter probe clips. Clip the negative to chassis ground and clip the positive probe to pin 5, set meter for DC voltage. Now turn on the amp and take a reading. If your amp has adjustable -bias turn the -bias pot kind of slowly from full counter clockwise to full clockwise watching for -bias dcv. Write down both extremes. Then do the same thing for the other power tube socket.


                             Brad     


Edit; added underline.       

Yep, I have the limiter...I'll check the setup again...I had preamp tubes in when I checked the first time.

[Willabe]

Yep, I have the limiter...I'll check the setup again...I had preamp tubes in when I checked the first time.

Number 1 priority is to fix the negative bias voltage on pin 5 of the output tubes. You say you have neg .05. You should have -30 to -50. Don't put the output tubes in the amp until this is resolved.

Ok, I feel kind of stupid asking, but if I'm not getting the voltages I need, how do I proceed?  The bias pot didn't seem to change anything, but this was with the preamp tubes in.  I may've misinterpreted your earlier advice...I'll go back and re-check the readings and report back.

No need now, do what Sluckey said, fix the adjustable -bias 1st.


                  Brad     

[lpresnall]

Ok Brad.  Thanks!  Just have to figure out why the bias pot isn't having an effect...all of the soldering and connections to and from look proper.

[Willabe]

Just have to figure out why the bias pot isn't having an effect...all of the soldering and connections to and from look proper.

Something's not wired correctly in that circuit.


               Brad   

[lpresnall]

Ok, with no tubes or rec pin 5 on the 6L6's is now "nothing".  The 2nd one (blue wire from OT to pin 3) will start at  - .030 VDC but then count down to "0" in a couple of seconds.  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.  Thinking I may've omitted the under board wire from the pot to the 220K resistor I jumpered the points above board, still to no effect.  What could be going on here?  I'm (as usual) at a loss.  Looks like this amp won't be making the trip with me on the 10th.

[sluckey]

Measure the voltage directly across the bias cap. How much?

It's probably time to post some hi rez pics.

[SILVERGUN]

Make sure you didn't put a 470K resistor where you only need 470 ohms

[lpresnall]

Ok, I'll look in the morning and get a pic or two posted.  Thanks for the replies!

[lpresnall]

Well, I had hoped for a simple value mistake on the 470 but alas, it's 470 ohms.  I'll post a pic or two so you guys can have a look...don't laugh!   

[lpresnall]

Board and wiring

[lpresnall]

SS rec

[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

[SILVERGUN]

The board and wiring pic is way too small to see anything clearly....

But, let me ask you this:
Do you have a wire going from the bias pot to the correct spot on the board?...so that the bias voltage is making it to pin 5 on both power tubes?
See [12] on the bias circuit pic I posted above (coming off of the bias pot)...

[SILVERGUN]

I'm not sure what layout you were using but if you were using one that had vibrato,,,,and didn't adjust for removing the vibrato circuit,,,,then that's hao you would have missed this connection...
I have adjusted this layout to show you where that [12] connection needs to be:

[lpresnall]

Ok, I'll repost the pic...I got carried away shrinking the size after getting a few "file size" errors when posting.  I'll also re-check the bias circuit.  There's one under board wire that I thought I could've missed but jumping the connection above board changed nothing.  I'm using the layout from Sluckey's thread "no verb/no vibe".

[lpresnall]

Bigger pic

[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

What voltage do you get here for the bias tap in the red circle. Will be AC voltage, should be ~60acv.


                Brad     

[SILVERGUN]

Everything in red must be connected....

[Willabe]

And what AC voltage do you read in the blue square?


             Brad     

[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

What voltage do you get here for the bias tap in the red circle. Will be AC voltage, should be ~60acv.


                Brad   

54.5VAC

[lpresnall]

53.7VAC 0n the middle (connecting wire) of the blue square.

[lpresnall]

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

[Willabe]

54.5VAC

Ok, that's pretty good.

What about the blue square now.


             Brad   

[SILVERGUN]

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!

[lpresnall]

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....

[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!

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...

[lpresnall]

The iron's heating up for the missing jumpers....just to be sure, there needs to be a direct connection from the single leg of the bias pot to the first 220K and the pair of 220K's should be jumped together as well?

[lpresnall]

Ok, the  jumper that Sluckey advised me to remove was at the choke...nothing to do with our thing...

[Willabe]

there needs to be a direct connection from the single leg of the bias pot to the first 220K and the pair of 220K's should be jumped together as well?

Yes.

There is no single leg on a pot, they have 2 ends, 1 full counter clock wise (CCW), 1 full clock wise (CW) and 1 wiper which is the center leg.


               Brad