sentences:  So when the follower grid is driven well over its own plate voltage, it saturates on the positive half of the signal and thus takes over the BIAS voltage, forcing it very positive.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.