Mark IIB - stray 1M resistor

[sdbthai41]

I hope you guys can help me.

I am restoring a Mark IIB that has had some significant modifications and was in a very bad way when I purchased it. I am restoring it to its original condition. I am carefully going through the circuit against what I see on populated on the PCB and I am finding a few anomalies. The main one is a 1 meg resistor located between V1B and V2A and I have had no luck locating it against the circuit.

Below is a photo that shows the 1 Meg resistor on the right hand side near /under the capacitor.

I have indicated on the circuit below where I believe the resistor would be located on the circuit.

Can anyone tell me if this is original of if it has been added later or if I should remove it or leave it. I'm just about the fire up the amp for the first time and I don't want any problems!

Thanks for your help

 

[b00g13mk2B]

It looks like it is on my MK IIA board. Not on the schematic. My amp has not been modified to my knowledge. So would seem to not cause a problem. It's hanging off the tone stack, not in any bias circuit.

 

[sdbthai41]

Thanks b00g13mk2B,

Many thanks for this observation. It seems like the mesa schematics are not very reliable sometimes. I have also seen some anomalies on the bias circuit of my amp when compared to the schematics.

Can anyone else confirm this observation of the additional 1 meg resistor on the tone stack? Importantly, could someone suggest what it does?

 

[b00g13mk2B]

It is not in the tone stack, but just past the tone stack with V1B buffering what follows. this is where signal is sent to the reverb driver. In our case, there is no reverb driver. A guess is the 1M resistor may look like the load of the 150K resistor and reverb driver input. My amp does not have reverb, am I correct to assume yours does not?

If you do have reverb and the 1M, then seems like an incorrect schematic. It's been awhile since I looked at tube schematics, so can't tell exactly why is should be there.

Would be interesting to know if others have this resistor.

 

[sdbthai41]

Interesting. I definitely have reverb on this amp so the mystery continues. I wonder what others have on their IIA or IIB's?

Does anyone know how accurate these schematics are? To help me with my rebuild I've actually plotted out the complete layout of the amp and have tried to relate everything back to the schematics. One day I will post the layout on this forum and get others to check it for accuracy. I was actually going to send it to Mesa to see if they can verify the layout.

I've finally conceded that I have bitten off more than I can chew with this amp! I normally work on old fender tweeds or blackface. This has been a huge learning curve for me.

Grateful for any help.