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!

[Tom Phillips]

Since you have separate cathode resistors biasing each 6L6 it would not hurt anything when one of the 6L6s turn off.
However, this is not the way to implement half power mode because you are turning off one side of the push pull circuit. You will end up with a big DC imbalance in the output transformer, probably lots of hum and a huge amount of bad sounding crossover distortion. The half power mode you are thinking of is the method that shuts off two tubes (one on each side) of a four tube output stage.

[Cygnus X1]

Thanks Tom.
I kind of thought so, wasn't 100% sure.

[Tom Phillips]

Your Welcome.
There is a discussion about running a push pull circuit in single ended mode at http://music-electronics-forum.com/t25816/ . Even if you don't plan to build such a thing the discussing includes some good information about what's going on with the circuit.
Cheers,
Tom

[Cygnus X1]

Useful to me!
I am known for salvaging old iron.
Using one side of a PP OT isn't out of the question!

[Cygnus X1]

A little surprised this thread isn't kicking up more dust.
Seems split cathode biasing is found more in hifi amps, and the guitar amp community just doesn't bother with it much.
Don't get me wrong...they exist, but not very often.

Unless I'm missing something, if the circuit's tone supports it, it would be the ideal situation for safety and ease of use.
You could throw in any pair of tubes, matched or not...and all is fine.

Or did I miss a class in 101?

[Steve Conner]

What sort of dust are you wanting? If it sounds good, use it.

If your tubes are biased hot (near to Class-A) then you won't have a major problem with crossover distortion, but you will still have the DC imbalance saturating the transformer and making it sound weak. The cure for this of course is to only remove the signal from the other power tube and leave it conducting its DC idle current. I believe Mesa have the patent on this.

[Steve Conner]

What sort of dust are you wanting? If it sounds good, use it.

If your tubes are biased hot (near to Class-A) then you won't have a major problem with crossover distortion, but you will still have the DC imbalance saturating the transformer and making it sound weak. The cure for this of course is to only remove the signal from the other power tube and leave it conducting its DC idle current. I believe Mesa have the patent on this.

[kleuck]

Indeed, that's the way to turn a PP in a SE amp.

[Cygnus X1]

Mmmm not a good way, but it's a way.

[kleuck]

If you know another, sell it to Mesa.

[Cygnus X1]

If you know another, sell it to Mesa.

LOL!
No, I'll just patent it.

Oh, wait...another badly conceived thought.