First time build with AB763 1 Chnl.

[Richs1979]

Here are the numbers
Rectifier pins 8 and 2 = 5.4 vac
Rectifier pins 4 and 6 both @164.5vdc
Nodes.....
A = 439 vdc
B = 433 vdc
C = 425 vdc
D = 409 vdc
I'm going to have to quit for now. But I'll be at it in the morning. Appreciate all the time you spent on this with me.

[Willabe]

You don't have the amp still plugged into the LBL do you? You don't need that any more. It will throw off ALL your measurements.

You sure you have a GZ34/5AR4?

We need the acv on pins 4 and 6 of the rectifier tube and the dcv on pin 8 and pin 2.

Measure each of those 1 at a time, red probe to the pin, black probe to ground.

You did them backwards.

Need ACV on pin 4, ACV pin 6. (See below.)

I forgot; You have protection diodes on the rectifier tube socket. So What I'd like to see is the ACV from the PT's red wires.

DCV on pin 2, DCV pin 8.

[Willabe]

Measured pin 3 on both V6 and V7
V6 = 418v
V7 = 417v
At 1 ohm resistor
V6 = 31.0mv
V7 = 31.0mv
Pin 5
V6 = -49
67 = -49

Nodes.....
A = 439 vdc
B = 433 vdc
C = 425 vdc
D = 409 vdc

Somethings wrong here, 20+ dcv difference between Node A and pins 3 on V6/V7.

It should be the higher dcv, that would seem correct for that PT.

You have to get these measurements right, so you can bias the amp correctly.   

[Richs1979]

Will take those measurements again later today. In the midst of cleaning up around here with the ice storm we got hit with. What should the voltage be for that PT.? It says 355 vdc on the sheet.

[Willabe]

What should the voltage be for that PT.? It says 355 vdc on the sheet.

Don't know. Spec sheet number is unloaded, it will be less fully loaded, with all the tubes in.

 

[Latole]

What should the voltage be for that PT.? It says 355 vdc on the sheet.

Don't know. Spec sheet number is unloaded, it will be less fully loaded, with all the tubes in.

Answer #1

[Richs1979]

I had to stop the cleanup, now it's pouring rain.  Anyway I checked these points again.
Rectifier = both yellow are 5.4 ac
Pin 4 = 160 dcv max
Pin 6 = 160 dcv max
Could those 2 diodes be messing something up. The voltages dropped a few times down to 142 vdc before stabilizing at 160. Stupid question but shouldn't this voltage be around 175 dcv with thos ht secondaries rated for 355 v?

Filter caps nodes
A = 426 (2 in parallel 22uf 500v)
B = 418 22uf 500v
C = 408 22 uf 500v
D = 385 22 uf 500v
These are F&T caps that Mr. Hoffman put in this kit

[SEL49]

Pin 4 = 160 dcv max
Pin 6 = 160 dcv max
Could those 2 diodes be messing something up.

Yes they are. The AC voltage Willabe wants you to check is actually on pins 5 and 7 (the pins where the PT red wires are connected).

[Richs1979]

Ok will do. Be back in a little while. Trying to clean up here b4 the deep freeze 2night.

[Richs1979]

No read on 5 and 7. Meter just jumps around. Should I just pull the diodes out and rewire to pins 4 and 6?

[Willabe]

No read on 5 and 7. Meter just jumps around. Should I just pull the diodes out and rewire to pins 4 and 6?

No, leave the diodes.

Set your meter for AC voltage.

[Willabe]

We need DC voltage on pin 2, DC voltage pin 8.

[Willabe]

A = 439 vdc
B = 433 vdc
C = 425 vdc
D = 409 vdc

A = 426 (2 in parallel 22uf 500v)
B = 418 22uf 500v
C = 408 22 uf 500v
D = 385 22 uf 500v

These reading shouldn't be that different.

from now on, measure your AC voltage at the right wall before you take any more power supply voltage readings and post that 3 too.

Is your meters battery old?

[Richs1979]

I was ready to check the wall outlet when I read this post. I did remove the diodes and rewired to pins 4 and 6. I get a  max of -19 on both and then it just jumps around. The dc reading on pin 8 is 428 vdc. I will check my battery and outlets right now. Thank you again.

[Willabe]

Do you know if your wall ac voltage goes up and down a lot?

[Willabe]

I get a  max of -19 on both and then it just jumps around.

Is that   -   as in negative?

19 what?

AC? DC?

[Richs1979]

Yes That was -19 on both 4 and 6 after I pulled the diodes out. Battery reads 9.4 volts but wall outlet reads 118

[Richs1979]

Just wanted to add that we've had brown outs lately due to icing. And who knows with the new smart reader they put in our bill went thru the roof. Nother subject sorry. Yes it does fluctuate. I had a read last month of 123 volts in one if my basement outlets.

[SEL49]

Post a pic of your meter.

[Richs1979]

I measured new outlets in basement and they're all 120. Here's my old meter.

[SEL49]

You either have a faulty meter or operator error. But really, there's no reason to get hung up on measuring the AC voltage on pins 4 and 6 of the rectifier tube because the rectifier is putting out the correct DC B+ voltage. You're way past all that. Just set the bias and call it done.

[Willabe]

You either have a faulty meter or operator error.

He's given 3 different B+ dcv, 22dcv difference.

With 355-0-355 @ 300mA, with a GZ34, the readings he's giving look a little low.

I was trying to find out what was going on. 

[Willabe]

Yes That was -19 ...

-19 what?

You need to post AC or DC on any voltages you post. (Not that it matters with that reading it's wrong any way.)

On your meter, looking at the pic you posted, going clock wise from the off setting, 1st is DC voltage, 2nd is AC voltage. DC is the straight line, AC is the wavy line.

[Willabe]

So, measure the plate DCV of the power tubes, pin 3, then put that number into the bias calculator.

Use the 6L6GC 30w tube setting.

Set the bias at the 1 ohm bias R in dcmV, then re-measure the plate dcv, pin 3, then plug that number into the bias calculator and re-calculate the bias. Then re-set the bias, should be good then.

[Richs1979]

Doides back in measured 414 dcv at pins 3 on 6l6gc. Set bias to 36.7 mA at 1ohm fed from pin 1and 8 power tube

[Willabe]

So, how does it sound?   

[Richs1979]

Sounds strong and clean. But the hum is still there. When you turn up the volume the hum goes away. There's no ground buzz from the guitar and when I let go there's no noise.its the hum when the volume is turned all the way down. Can I post a sound file or video on this forum so you can hear it.

[Willabe]

Yes, if you want.

[Willabe]

There's no ground buzz from the guitar and when I let go there's no noise.

So, it humms only when your touching the guitar with the amps volume all the way down/off?

[Richs1979]

It hums when the volume is completely off. No guitar hooked up. Just the amp on with volume completely down. When raising the volume even slightly, the hum dissipates. I'll be back in a while. Gotta start dinner.,,, Thanks ...

[Richs1979]

I turned on the amplifier again and I noticed that if I put my hand above a certain part of the amp, the hum subsides. You don't have to touch it, just put a part of your anatomy over the top of that section. I'm wondering if it just needs some type of a faraday shield. Most of the Fenders I've seen have some aluminum glued or stapled to the cab where the amp sits. I have a piece of thin stock I'll try placing over the top to see if that works.

[Calboy]

Can you take  new pin5 power tube measurements and post them.

[Richs1979]

Pin 5 is -41.3 volts

[Richs1979]

I was right about the shielding. Put some foil over cardboard and the hum is acceptable now. Sounds like my 69 SF.  Thank you to everyone especially Willabe who stuck through this newbies 1st build. Thank you again. Time to build a cab.

[Richs1979]

The finished AB763 build in it's new cabinet.