MK I A/B Transition

[Boogiebabies]

http://cgi.ebay.com/1993-MESA-BOOGIE-MARK-IV-GUITAR-AMP-AMPLIFIER-HEAD-NR_W0QQitemZ200244055178QQihZ010QQcategoryZ10171QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Interesting MK IV here. It has the IVB effects loop and reverb footswitch, but no third relay to switch the two power amp voicing caps. With the 6/93 date, it could not have been long before the circuit boards were changed. I wonder if the lead voice was A or B and if the reverb was changed to a 12AX7 ?

 

[igfraso]

Boogiebabies:
what is the missing third relay you are mentioning that makes it different from the B version? Where should we look at?

I guess I have the same "in-between" Mark IV.
Regards

 

[Boogiebabies]

The third relay for the B was to the left of the three 30uf/500V caps on the main board. It switches between two ceramics disc caps to voice the power amp differently for R1/R2 and Lead.

Is the reverb on your IV A/B a 12AT7 ?

 

[igfraso]

Mike told me my amp is a B. All the preamp tubes are 12AX7.
Regards

 

[Boogiebabies]

Interesting. I do believe we may have found the transitional MK IV B version 1.

 

[igfraso]

I sent you a pm a couple of days ago.
regards

 

[igfraso]

Just for your reference, my Mark IV (lead voice B) has a 3/93 date in the chassis and the serial number is 61XX.
Regards