Split Cathode Bias: Safe to run one tube?

[Cygnus X1]

30 watt amp running a pair of 6L6GC's, one of my own builds.
I have split, independent cathode biasing using 10 watt 500R ceramics and 25uf caps.

I've built three of these and no problems so far.
Was breaking in my latest, just running it with radio input signal and after a while
I noticed the volume had dropped.
One of the 6L6's had lost filament, so it was running on one tube.
Safe or not?

I think it's no problem, might even be able to run it that
way for a half-power mode.
If it is safe then does the impedance change?

Running 8 ohm, the only tap available.

Thanks in advance!

[roberto]

What's the configuration? PP? PSE?
Whats the Raa of the OT primary?

[martin manning]

Turning off one side of a PP amp means the remaining side will run at the same load impedance it runs in class AB. If it sounds ok now with both sides working, it probably is. The separate cathode resistors protected the remaining tube and OT since the bias point remained the same. Its not a good idea to use this as a half-power mode. The operating point would have to be shifted and transformer is not designed for class A.

[Cygnus X1]

PP config, 4K5 primary into 8 ohms.
Class AB in normal operation.

Good thoughts Martin.

[Structo]

Hey, one time on my D'lite I had removed one 6L6 for some reason or another.

The next day I wanted to play some guitar so I turned the amp on and started playing.
Something didn't quite sound right, lower volume, no balls.

Then I remembered and looked in the back, sure enough one power tube.
I was surprised it worked at all.