glip22
Well-known member
IMO and one others there are two possible reasons you can throw the 70% rule out the window with the CCV and do as Mark reccommends. One the PT is designed with a lower impedance across the primaries. Not sure of all the specifics on that. The other is that the amp has been designed with lower screen current than one would expect. With higher plate voltage, the screen grid voltage can be decreased, allowing slightly larger AC voltage on the plate without excessive screen grid current. This again leads to improved tube efficiency (at a higher load impedance) so you can set the bias much higher than you would think. Maybe the screen resistors are larger. A power tube failing is usually at the screens not the plates. It would be great to know the actual average screen current in the CCV. It seems from what has been posted the plate voltage is at or near 550 give or take. Biasing this amp past 70% would be asking for trouble if the screen current was average to the plate voltage as is the case with most amps. I am banking on this scenario. There has to be a reason or tubes would already have failed. The tubes are not even getting hot and glowing much with the amp dimed. The question is how high can one bias the amp without risking damage if a tube fails and arcs. Arcs are not fun. I took out two output transformers when my fuse (correctly rated) did not do it's job.
. Dragging 2-Bogner 412's with Scums in it. Sounds phenomenal.
