Blues Junior Bad bias

[Peter61911]

I have a blues junior I'm looking at for a friend. Only need to fix the reverb which is done now. While I was there I thought I'd check the bias.

See sheet below for figures. Also when I probe the plate on one side its really hard to get a voltage drop reading here that stabilises. Also on the side where there's almost no voltage drop theres big pop and squelch from this side and not the other.

Any suggstions to what might be wrong here. I've checked screen resistors. Plate resistors. Negative bias supply. It's a fixed bias circuit. 2 x el84s. Most things in the amp I've checked. Nothing audibly too bad with the amp apart from low hum. I'm investigating more tomorrow but for now is this a very obvious problem to someone. Bad tube...

 

[Peter61911]

Essentialy one tube is dra

I have a blues junior I'm looking at for a friend. Only need to fix the reverb which is done now. While I was there I thought I'd check the bias.

See sheet below for figures. Also when I probe the plate on one side its really hard to get a voltage drop reading here that stabilises. Also on the side where there's almost no voltage drop theres big pop and squelch from this side and not the other.

Any suggstions to what might be wrong here. I've checked screen resistors. Plate resistors. Negative bias supply. It's a fixed bias circuit. 2 x el84s. Most things in the amp I've checked. Nothing audibly too bad with the amp apart from low hum. I'm investigating more tomorrow but for now is this a very obvious problem to someone. Bad tube...

I have a blues junior I'm looking at for a friend. Only need to fix the reverb which is done now. While I was there I thought I'd check the bias.

See sheet below for figures. Also when I probe the plate on one side its really hard to get a voltage drop reading here that stabilises. Also on the side where there's almost no voltage drop theres big pop and squelch from this side and not the other.

Any suggstions to what might be wrong here. I've checked screen resistors. Plate resistors. Negative bias supply. It's a fixed bias circuit. 2 x el84s. Most things in the amp I've checked. Nothing audibly too bad with the amp apart from low hum. I'm investigating more tomorrow but for now is this a very obvious problem to someone. Bad tube...

View attachment 1125923

Essentially one tube is at almost 100 percent plate dissapation and the other is almost nothing. Could I be getting a bad reading here.

 

[Lowerleftcoast]

BJr is known for cracks etc around the PCB tube socket soldering points. They also may have cracked wire inside the ribbon wires, or bad connections from ribbons to PCB.
Check for heater voltage to the tube that is not functioning.

 

[dan40]

Check to see if pin 8 has a good connection to ground. Just set your meter to the ohms scale and check from pin 8 to ground. The reading should be very close to zero if pin 8 is properly grounded. A reading of "OL" would indicate a broken ground connection.

Edit: You want to check the socket that is not drawing current.