Blues Jr: v2b not drawing current....not sure why

[wizard333]

Bj version1 green board. Someone Mickey moused a bunch of "Bill M" mods, I was told it was Bill M himself, and its signed, but I don't see him being that sloppy.

Power tube sockets were replaced by someone who burned off every trace and did a crap job of bypassing with flying wires. Found V5 plate was getting no voltage, fixed that. Still no sound.

V2b: No voltage drop across plate resistor. Plenty of signal at the grid. I thought, Aha, cathode resistor issue, but no, cathode resistor reads connected from pin 8 to the resistor, the resistor reads correctly, and the other side reads connected to ground. Nothing appears lose, tried multiple tubes, no love.

Any idea what I'm missing?

[dmeek]

All the Blues Junior schematics I have show fixed bias. I guess that was one of the Mods. Check for high voltage at the screen grids EL84 pin 9

[g1]

Did you check V2B plate resistor?

[wizard333]

All the Blues Junior schematics I have show fixed bias. I guess that was one of the Mods. Check for high voltage at the screen grids EL84 pin 9

? Fixed bias is intact, plates are ok now on the 84s... not sure what you're getting at?

[Mick Bailey]

Check continuity from the socket pins to their connected circuit with the tube removed, to eliminate the possibility of an internal fracture. If all is well the contacts with the tube pins may not be good - could need the socket re-tensioning.

[wizard333]

Did you check V2B plate resistor?

Yes 97.8k

[wizard333]

Check continuity from the socket pins to their connected circuit with the tube removed, to eliminate the possibility of an internal fracture. If all is well the contacts with the tube pins may not be good - could need the socket re-tensioning.

Checked and checked, no joy.

[Mick Bailey]

A good tube with a connected plate and cathode resistor, with the cathode resistor grounded and B+ applied to the plate resistor has to conduct unless there's a break in the circuit. The cathode resistor may read correctly on a DMM, but could be going open under circuit conditions.

[mozz]

Those short ribbon cables are garbage. Check tube socket pins for correct voltages and gnd. Swap tube.

[wizard333]

Those short ribbon cables are garbage. Check tube socket pins for correct voltages and gnd. Swap tube.

Did that, see OP. No voltage at cathode under operating conditions, but shows continuity once off and drained, as noted in OP.

[wizard333]

A good tube with a connected plate and cathode resistor, with the cathode resistor grounded and B+ applied to the plate resistor has to conduct unless there's a break in the circuit. The cathode resistor may read correctly on a DMM, but could be going open under circuit conditions.

You're not wrong, that's why I'm baffled. I show plate voltage and grid signal when on, no cathode voltage, but continuity from the tube pins to the resistors when off and all resistors measure fine, no stray grounds. I even gave it mechanical stress to see if I could cause a fault or intermittent connection, nothing.

Do you have a suggestion for testing that, short of pulling the board and shotgunning replacements hoping for a random event?

[Helmholtz]

Schematic?
What is V2b?
Is V2 heating?

[wizard333]

Schematic?
What is V2b?
Is V2 heating?

Doesn't everyone have blues junior schematics?

V2 is heating; all tubes heating, parallel heater system.

[Helmholtz]

Doesn't everyone have blues junior schematics?

I don't.
And I don't consider it my job to find it.
It's not me asking for help.

[g1]

The heater system is parallel, but the sockets are soldered to board. You must verify both sides heater of V2 visually. Also can you verify plate voltage at V2 pin6 at the tube itself?
And the grid needs to get it's ground reference via tone stack. I suppose if it did not, the tube might be able to go into cut-off.

There are quite a few versions of Blues Jr., so it's better if you post the exact one, although they may not have changed much.
BJr III schematic attached.