sentences:  The time for this voltage to come back to normal is based on the time constant of the 150 kΩ mixer resistors and the coupling cap - but by this time it is too late, the bias is pushed too far into the positive and the current gets pushed through the 6146, resulting in blown tubes.Some users have an electronics technician re-wire the 12BH7 feed the same as on the later 6550 heads.By adding a 1K and filter cap feeding from the 220 V screen supply to the front voltage amp of the 12BH7, a 6146B tube runs reliably with far fewer issues.