Ab763~ ish build no sound

[silat]

My light limiter of course gets bright then dims a little and stays on. No sound... I used the Weber layout 6A20 head and combo. Gz34-n 4-6V6 Etc.

[silat]

More pics.

[silat]

And these....

[PRR]

Any voltages in there?

[Paul1453]

If your voltages are all good, then these silent amp builds often have input/output jack wiring issues.   

[silat]

                                                                     p1     p2    p3     p4     p5  p6     p7  p8      p9

Something wired wrong or something. Gz34          224           -63         -61          224
                                                         
                                                         6v6     -41          224           -41
                                                            "       -40          225           -40
                                                            "       -41          225           -41
                                                            "       -40          224           -40
                                                           12---  130    6    24                            127   7     24
                                                                     182   -23                                   155  -23
                                                                     104                                            93
                                                                     221            3                              221   3
                                                                    101                                              96
                                                                     106                                             117

[silat]

heater voltage is low Ac  2 on pins 9 and 4 and 5 on the pre-amps

[sluckey]

That voltage table looks like gibberish! How are we supposed to make any sense of that?

[silat]

   Well Sluckey  that is what the "ish" for  as in my  title heading of this thread post: Ab763~ish as in Gibberish. I will try and fix it where it can be read.

[sluckey]

If you want to make a table using the 'space' key and have it present properly you'll have to use fixed width fonts like Courier. Even then, preview before posting to be sure it looks like you expect it to look. If you just use the default font your table will look like gibberish.

Otherwise figure out another way of listing tube voltages. Maybe one line for each tube pin. Kinda like...

V1-1 = +150V
V1-2 = 0V
V1-3 = +1.5V
etc, etc, etc.

Or, better yet, use Doug's tube voltage chart and post a link to the info.

[sluckey]

Your PT is wired wrong! That red/yellow wire that is connected to your bias board should be connected to chassis. Then you need to connect a wire from the rectifier tube socket pin 4 to the bias board ***AND*** replace that 470Ω resistor with a 100K/3W resistor. Then recheck some voltages with no tubes except the rectifier tube. I'm particularly interested in the output tubes pins 3, 4, and 5.

[silat]

 V11- pin 2 =425
 V11- pin 8= 425 No readings on the other pins.
V10-pin 3=426
 V10-pin 4=0
V10 -pin 5=-10
V9 pin 3= 426
V9 pin 4=0
V9 pin 5=-10
V8 pin 3=425
 V8 pin4=0
 V8 pin 5=-10
 V7 pin3=425
 V7 pin 4=0
 V7 pin 5 =-10


 

[Willabe]

You have no screen B+ dcv.

Pin #4 is the screen grid, should be a little less than pin #3 (plate).

[silat]

You have no screen B+ dcv.

Pin #4 is the screen grid, should be a little less than pin #3 (plate).

 So that makes 5 the control grid? Then why nothing at pin4? what did I miss?

[PRR]

> what did I miss?

this

[sluckey]

I guess you corrected the PT wiring? A little feedback from you please.

Low heater voltage... Measure AC voltage directly on a 6V6 socket with one probe on pin 2 and the other probe on pin 7. What do you have?

No voltage on 6V6 pin 4 (screen)... Pin 4 is an input. You have to connect a voltage to it. That weber layout is not the easiest to read but all the info is there to show you how to get voltage from filter cap B, to 6V6s pin 6, through those 470Ω resistors, and finally to pin 4. You've missed a wire somewhere in that chain.

Pin 5=-10v... Don't plug those 6V6s in until you get this voltage up to about -30v or -35v. May have to lower that 100K/3W resistor a bit.

[silat]

 I have 6 volts Ac on all of the 6V6 pins 2 and 7 but lost the -10 on pin 5... Yes I corrected the  bias resistor to 100k 3 watt. Still 0 on pin 4.

[sluckey]

Read reply #10 again.

DID YOU DO ALL THE STUFF I SUGGESTED?

[silat]

Yeah you are right I did not take any of the cap B to pin 6 to 4. I am doing that now. I  tried to do everything you said.

[silat]

I have 426 on pin 3  about 427  on pin 4. Nothing on pin 5

[sluckey]

Read reply #10 again.

[silat]

I read your post # 10 over and over and have done every single thing you have said in it.  What specifically have I not done?

[silat]

 Sluckey wrote"Your PT is wired wrong! That red/yellow wire that is connected to your bias board should be connected to chassis. Then you need to connect a wire from the rectifier tube socket pin 4 to the bias board ***AND*** replace that 470Ω resistor with a 100K/3W resistor. Then recheck some voltages with no tubes except the rectifier tube. I'm particularly interested in the output tubes pins 3, 4, and 5."
I did every bit of this now I have nothing on pin 5 of the 6v6's. It is like my bias circuit went out. The 3 and 4 pins I am getting voltage on now but five has nothing.
My light limiter stayed on all the time but it is now off and does not even come on when I first start it up.

[sluckey]

***DON'T PLUG IN THE 6V6S OR RECTIFIER TUBE UNTIL YOU HAVE THE PIN 5 BIAS VOLTAGE FIXED***

The bias circuit is very simple. I suggest you download the schematic for this layout and learn how to read it. I've attached a pic of the bias circuit that you should be able to follow and fix the pin 5 no voltage issue. When you make all the corrections in that pic you should be able to adjust the bias voltage on pin 5 from about -20vdc to about -40vdc.

You can do this.

[silat]

 Sluckey, In that drawing I have tried to tell you I have done every bit of it. I added the wire from the rectifier pin 4 to the bias board to which I also changed the 470 k 1 watt to a 100K 3 watt.... I also figured out after you telling me that I had nothing going to pin 6 that goes over to pin 4 on the 6v6's and remedied that by adding the wires . You show that on the layout marked in yellow.(Everything you have marked in yellow I have done) I pulled all of my tubes... I have this all done but I am showing no reading on pin 5 unless I burned a resister somewhere or a wire came loose. I am wondering if maybe the 220k resistors are bad in the bias circuit?
I had reading of -10  on pin five and it was wired just like it shows in the highlighted yellow but it is now gone. My light limiter is not not showing anything.

[HotBluePlates]

I wonder if the bias components were damaged earlier when the high-voltage center-tap was connected to the bias board.

Do you have a.c. volts on pin 5 of the output tubes (I understand it says 0vdc now)? If so, the bias cap or rectifier diode was probably damaged.

[silat]

Oh ok! But I do not have another cap that size. that is the 50Uf 150 volts.

[silat]

yes I am showing AC volts in the 200's

[sluckey]

When you do everything I suggested in the pic I posted the bias supply will be fixed.

You don't need the lamp limiter to work on the bias supply so just plug the amp straight into the wall. You also don't need any tubes plugged in until the bias circuit is fixed. I've said this several times now. I hope they are not plugged in.

Probably the only components in the bias circuit that may have been damaged are the diode and filter cap. Those resistors should be fine but it certainly won't hurt to check them.

[silat]

Just to be clear I did take the tubes out.

[Paul1453]

If you have AC on pin 5 your bias diode is likely shorted, and that has also likely killed the E-cap in your bias circuit.   

[silat]

I replaced the diode but I had to order another cap from Doug. I am waiting for it to come.

[John]

I replaced the diode but I had to order another cap from Doug. I am waiting for it to come.

I think for right now you can use any electrolytic you have (100+ volts rated). Just tack it in since you'll be taking it back out. It'll still let you measure your neg. voltage. Don't forget that positive actually goes to ground.

[Paul1453]

200+VAC does seem a bit high for the input to your bias circuit.
Double check your connections and resistors.
As John said you can use almost any E-cap of 100V or more for now.

You don't need or want -250VDC for your bias voltage. 

[sluckey]

There will be about 300VAC input to this bias circuit.

[silat]

I have -5 volts DC and that is all I can get after tacking in a 150 uf 100volt cap which is the lowest I have. There is still something wrong I did not change those two 220k resisters but I do not have anymore of those either. I ordered some from Doug though.

[Paul1453]

You are right, something is still wrong.

But you are making progress in the right direction.   

No smoking or very hot resistors in that circuit?

Something still seems to be shorted, or the wrong value in there.  IMO

[HotBluePlates]

You are right, something is still wrong.
...
Something still seems to be shorted, or the wrong value in there.  IMO

Current drawn by that bias supply will be no more than the input a.c. divided by the resistance of the bias pot & resistor to ground (so 12mA for 300vac input and bias pot set for max resistance). It will be less than that by the drop across the 100kΩ resistor from H.V. winding to the diode, but that drop will be non-linear.

Silat is getting the huge a.c. because the bias cap is shot (probably from having too much voltage across it when the center-tap was connected to the bias board). Once he gets a functioning cap in there, the supply will likely land back on "near-normal" d.c. voltage.

[Paul1453]

Right HBP.

When that bias circuit is functioning properly there will be little current running through it.
Tubes should still be out now.

He said he put in a new E-cap(you took the old one out right), so I figured there must still be something dragging his voltage down.

Maybe making something hot?  Was hoping to use the hot part as an indication of where the problem remains?   

[silat]

I put a cap just to see if I could get close but it is nowhere near close. The only thing I did not change were the 220k resistors in that bias circuit. I am waiting on those and the right cap to com.

[sluckey]

Look at the pic I posted. See the note at the very top? How much voltage do you measure at that point on the bias board?

[silat]

Look at the pic I posted. See the note at the very top? How much voltage do you measure at that point on the bias board?

-4 vdc... I cannot get much else.

[sluckey]

How much AC voltage do you have on each side of that 100K resistor on the bias board?

[silat]

  Right 259 and  Left side 316 AC

[sluckey]

How much dc voltage on the right side of the diode?

[silat]

-5 VDC.

[sluckey]

Turn the amp off. Measure resistance from right side of diode to chassis.

[silat]

20

[sluckey]

20 WHAT?????

[silat]

20 K ohms is all I have.