What is the minimum voltage I need to provide my tube to be able to produce some sound? I am trying to debug my first tube amp, but I can only power it with a 11,8V supply instead of the recommended 13,3V.(PCL86)

Could it be that the triode and pentode doesn't open because of this? Or in an other way: What filament voltage do I need relative to the recommended one to be able to use a tube?

  • \$\begingroup\$ Where is the schematic? \$\endgroup\$ Apr 4, 2015 at 19:58
  • \$\begingroup\$ @LeonHeller This is a question simply about a not official property of a electrical component. I don't think a schematic is needed. \$\endgroup\$
    – akaltar
    Apr 4, 2015 at 20:01
  • \$\begingroup\$ This is only about 10% low so I wouldn't expect it to affect performance very significantly - it certainly shouldn't stop it functioning. \$\endgroup\$ Apr 4, 2015 at 22:19
  • \$\begingroup\$ @KevinWhite Are you sure? Because it is said that filaments are quite sensitive... But if you are, or could get some refs, then please do so in an answer, so I can accept that. \$\endgroup\$
    – akaltar
    Apr 4, 2015 at 22:21
  • 1
    \$\begingroup\$ It will work. Emission vs voltage is highly nonlinear so don't be surprised if you only get half the anode current you expect but that won't stop function altogether. Such variations were expected in battery-operated radios, didn't stop them being used. (You can reduce the negative grid bias to compensate, to some extent) \$\endgroup\$
    – user16324
    Apr 4, 2015 at 22:53

1 Answer 1


The RCA receiving tube manual RC-23 is quite vague about the amount of permissible variations, however it does warn that insufficient filament voltage can lead to degraded performance and reduced tube life. Of course excessive filament voltage can also shorten tube life.

Since tubes were normally operated from unregulated mains transformers, I would think your -11% would be acceptable, but perhaps not if that nominal voltage also has a large tolerance and/or you're working the tube close to the maximum ratings.


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