Is this right ?

[hebaton]

In a Super Reverb presumably AB763, There is a resistor from cathode to ground even tough ther is a bias supply going to the grids ( pin 6 )
Is this correct ?

[sluckey]

Pin 6 is not a grid.

[hebaton]

Correct, this makes no sense. See atached

[sluckey]

Correct, this makes no sense. See atached

Makes perfect sense. That's how Fender made every AB763 amp back in the '60s. The two resistors on the sockets are original. The blue resistors were added to facilitate bias checks.

Have you never looked at a Super Reverb layout? Well, take a look...

https://el34world.com/charts/Schematics ... _schem.pdf

[Stevem]

The cathode of a 6L6 or 5881 is pin 8.

I see the amp still has the death cap in there.

[hebaton]

THe death cap is there but nt cnected to anything. a 3 prong cable is installed.
So, what is that blue resistor for ?
And what is going to what seems to be pin 6 wich is unused in that tube ???

Let me see if I am getting this.
Bias supply is coming in on pin 5 trough the 1.5K resistor across pin 1 ?
HIgh tention is going to the plates from the OT B + is coming trough the 470K resistor on pin 4 ????

THere is no place I can read bias current dirctly, SO I want to use Voltage drop / R but my multimeer is not precise enough. I don't get decimal when reading plat V OR Center tap V

[Ten Over]

THe death cap is there but nt cnected to anything. a 3 prong cable is installed.
So, what is that blue resistor for ?
And what is going to what seems to be pin 6 wich is unused in that tube ???

Let me see if I am getting this.
Bias supply is coming in on pin 5 trough the 1.5K resistor across pin 1 ?
HIgh tention is going to the plates from the OT B + is coming trough the 470K resistor on pin 4 ????

THere is no place I can read bias current dirctly, SO I want to use Voltage drop / R but my multimeer is not precise enough. I don't get decimal when reading plat V OR Center tap V

Why don't you measure the voltage across that blue 1 Ohm resistor? mA = mV because the resistor is 1 Ohm, then subtract out the screen current.

The amp looks like it might have been an AB568 model. Can you adjust the bias voltage or just balance it?

[hebaton]

YOu are correct it was an AB568 but turned into an AB763
Bias is adjustable. SO tha's a 1 ohm and I can read the value right there ?
IF I try to measure that resistor Iget an unusable reading keeps jumping up and down...

and where do I measure screan curent ?

Thanks !

[Ten Over]

YOu are correct it was an AB568 but turned into an AB763
Bias is adjustable. SO tha's a 1 ohm and I can read the value right there ?
IF I try to measure that resistor Iget an unusable reading keeps jumping up and down...

and where do I measure screan curent ?

Thanks !

Yeah, read the voltage across the blue 1 Ohm resistor. That gives you the plate current plus the screen current. The screen current is something like 5% of the plate current from the data sheet, so divide the cathode mA by 1.05 to get the plate current. You could get the screen current by measuring the voltage drop across the 470 Ohm screen resistor and then dividing it by 470, but 5% of plate current is close enough and much easier.

[TUBEDUDE]

YOu are correct it was an AB568 but turned into an AB763
Bias is adjustable. SO tha's a 1 ohm and I can read the value right there ?
IF I try to measure that resistor Iget an unusable reading keeps jumping up and down...

and where do I measure screan curent ?
.

Thanks !

You're measuring a small voltage. It's imperative you make good contact to get a stable reading. Try cleaning the probe tips and resistor ieads. Or sharpen the lead tips and stab the solder puddle at ground, and the socket tab where the resistor is connected.