Adjust Bias Voltage: Super Reverb ?

[Greg_M]

I just finished putting in new pots, caps and tubes in the '66 Super and I am trying to figure where to measure the voltage (it says -52v on the schematic) for the bias. I don't want to just go poking around with the Fluke meter. Where is that measurement taken?

 

[johnk_10]

it's taken from pin 5 of the output tubes referenced to ground.
it is just a ballpark bias voltage. i find it better/easier to install a 2 watt 1 ohm resistor from pin 8 to ground( in place of the hardwired jumper wire), then with your meter set on DC volts measure across the 1 ohm resistor with the amp on and adjust the bias to read 28-35 mv. measure it on both output tubes and you'll also be able to determine how matched they actually are in the cicuit.

 

[Greg_M]

Thanks a lot!

 

[Greg_M]

it's taken from pin 5 of the output tubes referenced to ground.
it is just a ballpark bias voltage. i find it better/easier to install a 2 watt 1 ohm resistor from pin 8 to ground( in place of the hardwired jumper wire), then with your meter set on DC volts measure across the 1 ohm resistor with the amp on and adjust the bias to read 28-35 mv. measure it on both output tubes and you'll also be able to determine how matched they actually are in the cicuit.

By the way...
Do you remove the resistors after the test or just leave them in place?

 

[beans-on-toast]

Just leave the resistors in place.

Also, I like to match the resistors as close a possible. Buy 1% tolerance if possible.

 

[Greg_M]

Thank you

 

[johnk_10]

By the way...
Do you remove the resistors after the test or just leave them in place?

yep, leave them in, as they can also work as a 'protection' device.

 

[Greg_M]

I was finally able to find a little package of them at Fry's. They are 5% tolerance, but I can go through them and pick out two that are similar/close (hopefully)

 

[Tapp]

Or you could shunt across the output transformer with your ammeter and then have some real fun!! No JohnK's instructions are the safer bet for sure.

 

[johnk_10]

I was finally able to find a little package of them at Fry's. They are 5% tolerance, but I can go through them and pick out two that are similar/close (hopefully)

those will work just fine if you match them as close as you can.

 

[BassmanPaul]

Fender usually indicates setting the bias voltage at the junction of the pair of 220KΩ grid leak resistors. This is where I do my initial setup before installing the tubes. After that it's the same as John.

Paul