Please school me on my Mark III Blue Stripe

[4406cuda]

I have a Mark III Blue Stripe Long Head. It has the 4 switches on the front. One of the switches is for half power.

How do I know if this head is a simulclass head that can run EL34s in the outer sockets?

 

[Boogiebabies]

Get a flashlight, pull the outer two tubes. Look in the socket hole and if you see a blue resistor in both center holes, it's Simul-Class.
Sometimes, both sides of the transformer bells were stamped with the part number. It would read 562003 or 562004R1
for Simul. Most long head chassis had a Simul-Class sticker below the extra power outlet on the back, but not all.