The AM Forum

Does using only one triode in a dual triode tube cause the cathode on the OTHER triode to become poisoned if there are no voltages applied to any of the elements?  I had never seen this mentioned in the tube material I have read, but it was published on some website somewhere.

Not a big problem, but just curious ya know.

If the cathode of the unused triode section is lit and up to operating temperature, then there is no problem.  If it is off, it could collect collect and adsorb tube gasses that would reduce its emission.

Chuck...

That's what I thought.  This fellow was saying the opposite. That the hot cathode would absorb gases and get poisoned. and that you should connect dual voltage dual triodes (12AT7 or sim) such that only the cathode you are using is lit, or put a smal voltage on the plate to keep things moving.

or in a lot of applications, use both sections in parallel.  - full fil. and plate voltage that way and twice the dissapation.

Rick,

Sometimes the tubes are de-rated when you use both sections and the power dissipation is not doubled. Take the 6CG7 dual triode which is like a miniature version of a 6SN7. The good old RCA manual says that either plate can do 4 Watts but if both plates are used, the maximum is 5.7 Watts.

When you are putting dual triodes in parallel like in a 6080 push pull triode audio amplifier, I have seen some information that suggests that you use one half up top and one half down below, no matter how many tubes you add. I would imagine that this promotes overall gain and resting current balance.

Mike WU2D   

yes, it's true that the envelop package can only dissapate so much heat.

- interesting in your note of using up/down symmetry.  Makes sense for symmetrical power distribution

Also interelectrode capacitances are doubled with the triodes in parallel too, not so great in vhf.
I was thinking more in the line of audio driver stages in some old boat anchors.