question about input coupling caps

[ke4mcl]

i posted this on an antique radio board as well. amp is on the bench so i'm hoping to get a speedy response.

i'm working on a davis 106 mono tube pa amp. tube line up is 6eu7, 2x6bq5, rectifier.

i noticed this amp doesn't have caps on either the mic in or the line in. must have been a cost cutting measure. im assuming this means if something goes really wrong in the 6eu7, i can end up with b+ on a mic or source device?

can i just add in an 0.1mfd or so on those inputs?

[Skywavebe]

ke4mcl,
It all depends on the input impedance of the tube circuit. If it is high then the small values should work. If not then a larger value Electrolytic might be needed. The only way your tube will put voltage into the input circuit is if the plate shorts physically over to the grid or whatever element the tube uses for input. That is very rare that this would happen. Most tubes fail due to burned out elements such as grid and plate or the typical open filament. Most of my tube experience is in high power transmitters where 450 Amps can be input into the Filament and plate voltages as high as 9500 Volts. but the same principal applies. Otherwise I stick with the easy solid state stuff.