Mainly due to the output transformer primary inductance with respect to plate resistance..The 6146 is much lower in transconductance and is less sensitive to drive signal, thus needs a bigger voltage swing to drive than a 6550...The second version of a vintage SVT is what is called the "black line" SVT.The lines and text are black instead of blue, and like the later run "blue lines" use 6550 power vacuum tubes instead of 6146Bs. James of Von Saint Design is a bass lover like the rest of us, but even deeper his passion lies in vintage Ampeg SVT gear.He sent us Parts 1 and 2 of a 4-part You Tube series he made about the classic equipment.The SVT has been through many design changes over the years but is still in production today.
The 6550A version of SVT amps tends to have a more rounder deeper bass sound..
The time for this voltage to come back to normal is based on the time constant of the 150K mixer resistors and the coupling cap..this time it is too late, the bias is pushed way positive and the current gets slammed through the 6146 and they tend to blow the tubes.
Some users have an electronics technician re-wire the 12BH7 feed same as on the later 6550 heads.
Diodes were later used to by-pass the 22ohm screen resistors.
In the event of a tube short failure or simply a transient overload condition the diode will conduct once the current in the 22ohm screen resistor reaches 30m A and beyond, preventing further burning of the PCBA.