need bias help with Mark IV

Went to bias my Mesa mark IV and experienced an issue.

The multimeter had no reading on the two inside tubes (JJ 6l6's). I threw in some Mesa, and other JJ 6l6's, but still no readings. It has to be a socket problem, right? Where Do I start?

Update: I switched to Simul-Class to run all the tubes, and one of the tubes now has a reading... BUT still one tube reads 0. Any ideas on where to start looking?

1st.. You can't bias a Mark IV since its fixed Bias. You can mod it to do so but that's not what we are talking about here. Does a tube glow in the socket? How are reading the tube? Bias Probe? With the probe between the socket and the tube?

i had a bad plate resistor cause this on the one side of the PI tube

stephenphilp said:

Went to bias my Mesa mark IV and experienced an issue.

The multimeter had no reading on the two inside tubes (JJ 6l6's). I threw in some Mesa, and other JJ 6l6's, but still no readings. It has to be a socket problem, right? Where Do I start?

Click to expand...

You can't adjust the bias circuit. Not without modifying it anyway.
The power supply for the bias is shared by other circuits.
If you change the bias voltage you might disable the other circuits.

But as for the bias voltage on pin 5, of the two inner tubes:
It's not the sockets.
The bias comes from (two) 2.2 K resistors that feed each pin 5 on the inner tubes.
It is not normal for those resistors to fail. So, it's probably a bad circuit track or broken connection.
Trace the bias voltage up to and thru those two resistors. I am thinking the circuit track is broken.

schematic link:
http://schems.com/manu/mesaboogie/boogie_mkiv.pdf

One other thing that makes the bias fail:
The 2 coupling capacitors (.1uF) from the phase inverter can short out. I have seen this happen.
The original caps were 400 volt. Replace them with 600 volt rated caps.

Now look at point "E" on the schematic. Is the bias voltage at that point?
If not, the circuit tracks on the board may have failed.
Or the power transformer winding that supplies bias voltage may have burned out.
I have also seen those tracks fail, and I have seen the windings for the power transformer fail also.

There is no substitute for the transformer...