enjoythemusic's t-rex amp: where is the +70VDC tap?

[jarthel]

i have looked several times at both the psu and amp schematics and I cannot find the +70VDC tap. do you know where it is?

thanks for help

[Martin Hayes]

The purpose of the +70V, according to the scarily brief notes to the construction of this amp, is: "Finally, note that the filaments of the 5687 are referenced to a DC voltage of +70V to maintain the heater-cathode voltage differential for the cascode's upper triode within its 100 V rating."

The +70V is drawn from the branch of the B+ rail that supplies the 5687 cascaded driver stage. The path is via R12, whose value is 300K (fig. 1).

This point is connected both to the centre tap of the secondary winding of the traffo that supplies the 5687 filaments, and to the virtual center of the filament supply of the power supply tubes (KT88 and 6922), formed by resistors R13 and R14 (fig. 2).

[jarthel]

Originally posted by Martin Hayes
The purpose of the +70V, according to the scarily brief notes to the construction of this amp, is: "Finally, note that the filaments of the 5687 are referenced to a DC voltage of +70V to maintain the heater-cathode voltage differential for the cascode's upper triode within its 100 V rating."

The +70V is drawn from the branch of the B+ rail that supplies the 5687 cascaded driver stage. The path is via R12, whose value is 300K (fig. 1).

This point is connected both to the centre tap of the secondary winding of the traffo that supplies the 5687 filaments, and to the virtual center of the filament supply of the power supply tubes (KT88 and 6922), formed by resistors R13 and R14 (fig. 2).

thanks for that. I thought that +70V label on Figure 1 meant that has to be connected to the +70V bias.

thank you.