Mark IV A V2 Heater Mod (+ pics)

[_arch]

Hi everybody

I'm working on a Mark IV A (Built august 92) which needs serious servicing (recapping, new vactrols, new tube socket and so on).
On my PCB, there is a difference on preamp tubes heater connections.
On the schematics, it's written : V1 12 VDC on pin 5, ground on pin 4 and V2-V5 6.3 VAC on pin 4+5 and 9.
On my PCB, V1 and V2 are 12 VDC on pin 5, ground on pin 4 and V3-V5 6.3 VAC on pin 4+5 and 9.
Has anybody here saw that mod done on other MkIVA?
If so, is this an official mesa mod or a cutom one (unfortunately i'm not the original owner of this amp and on't know what has been done on it)
I wonder if this mod has been done either to reduce noise issues or current draw issues, any thought on this?
Thanks for your help

here are two pics of this mod :
On the PCB track side : note the grinding on the tracks that goes to Pin 4 and pin 9

On the component side : note the wires from pin 4 and pin 5, and also the grinding on the track that joins pin 4 and pin 5

I made a comparison between A and B revision of the markIV's schematics and V2 heaters are not on 12VDC on any version.

 

[Restless Rocks]

I've seen this done to a MkIII a couple times, but never a IV. Many III's have a hum issue, which I've connected to certain brands of tubes and especially vintage long plates in the VI and II spots. The solder looks well done but I don't recall a mesa bulletin on it. I'd guess a custom job.

 

[_arch]

Thanks for your answer.
It confirms my thoughts. Moreover, when i had this amp, it was fitted with Philips miniwatt ECC83 in V1, V2 and V3, which are vintage, but not long plate as far as i know.