Tube Diagrom for SOB -- What is V3 ? 12AX7 or 12AT7 ?

[mbmesa]

I was doing some work on my bandmates Boogie SOB and noticed that he has three 12AX7s in all three preamp positions. On the chassis it is written with a marker that V3 is a 12AT7. First, what is the correct tube for V3 and second, what effect on the amp has or can occur with a 12AX7 in V3 ?

Anybody have the manual ? All I've seen are hand scribbled schematics on the web .

Thanks

 

[MrMarkIII]

I'm guessing that, since "12AT7" was written on the chassis at the factory, that perhaps a 12AT7 is indeed the proper tube for V3.
The 12AT7 is often the reverb driver in earlier Mesa designs.
Hope this helps

 

[mbmesa]

This amp has no reverb though. There are three preamp tubes only. The amp was working with the three 12AX7s in there but didn't sound all that great. The chassis itself is clearly multi purpose as it has a hole for the 4th tube but it has no socket in it. That is why I am not sure I can trust the writing in marker although it is also written SOB with the same marker. This is the 60 watt combo , not the 60/100 watt head.

 

[MrMarkIII]

Just guessing, again, but since the chassis is multi-purpose, perhaps, just perhaps, the marker is meant to indicate the actual position of the correct tube.
After all, the folks at the Mesa factory did the marking - one might assume they know what they're doing.
I forgot SOBs have no reverb (duh!). V3 is the phase inverter.

 

[stokes]

V3 is the PI and a 12AT7 is specified,but a 12AX7 will work.The AX7 will give it more drive to the power tubes,not necessarilly a good thing in this position.Try a comparison and see which you like better.

 

[Geiri]

I have an early mk4 and there is also written with a marker on the chassis 12at7 for the reverb driver. I read somewhere that the 12at7 was used to lower the noise in the preamp. It should be ok to use a 12ax7 but you might get added noise with that one.