Have you or can you or your tech determine the failure mode that occurs, i.e., whether it is a short or an open on the primary, or on the secondary winding that serves the 5Z3, or on the secondary winding that serves the 6AU4? Perhaps that can be determined with a simple multimeter, and it might prove to be useful information.
Also, do you know if the 5Z3PAT and the 6AU4 are the same versions as were originally supplied with the amp, or for example might the previous owner have substituted the 5Z3PAT for a 5Z3, or a 6AU4-GT for a 6AU4-GTA?
Finally, when you referred to the amp as the 845 91-05, after looking at their website I think you probably meant 845-05.
In any event, good luck. Regards,
-- Al
Also, do you know if the 5Z3PAT and the 6AU4 are the same versions as were originally supplied with the amp, or for example might the previous owner have substituted the 5Z3PAT for a 5Z3, or a 6AU4-GT for a 6AU4-GTA?
Does anyone know what the spec would be to the plate of a 6au4?FWIW, here is the datasheet for the 6AU4-GTA. The curve of plate voltage vs. plate current is shown at the bottom of the second page. Not sure how helpful this will be, though, as we don't know anything about how the amp is operating the tube.
Finally, when you referred to the amp as the 845 91-05, after looking at their website I think you probably meant 845-05.
In any event, good luck. Regards,
-- Al