Hi,

positive DC biasing the heater winding of a tube amp is occasionally used to eliminate H-K hum in the pre-amp - a clever alternative to using a well filtered DC supply for the same job as done in many guitar amps.

However, it means the heater circuit is no longer firmly grounded and could acquire a high voltage under certain fault conditions.

Marshall in their infinitesimal wisdom have done this on their latest models
- the JVM 400 series. The 6.3V heater winding is floated to about +70V DC using a voltage divider off the HT.

Now, output tubes like the EL34s used are prone to failures that send their cathode to a high voltage or short pin 3 to pin 2 ( anode to heater arc over). When that happens, the heater winding rises to meet the B+ supply. None of the other tubes will like that - max rated H-K voltage is about 100V.

Maybe the idea is *too* clever.


..... Phil