Well, we have a CCS (sink), common for both tubes (diff. phase splitter/driver). DC voltage between the cathodes (connected together and "CCS-ed"), and the ground is only a few volts, and for good work CCS "needs" more, at least 20V. We must use a "negative rail", or -20V referred to ground. Heatsink must "deal" with P=U*I dissipation, and if we use large neg. voltage, then we apparently need large heatsink:-)