26-07-2016, 08:34 AM
Looking again at the circuit, Lawrence, the control grid of the first valve is returned to a tap on thefilter resistor (R1), via a 2Meg resistor(R8), but as R1 is only 1260 ohms, by using a 2M2 resistor for R8, and returning it's 'earthy' end direct to the negative line. The control grid of the o/p valve is returned to the centre tap* of the mains Tx secondary via R19(640k), R14(320k), R13(2Meg), & R7 (1Meg), so the answer is, yes GB for the 1st and 4th. valves is derived via the filter circuit. (* i.e. to the negative terminal of the reservoir capacitor)
