What to do with unused triodes?

[Bill Mountain]

If I only needed one triode out of a 12A_7 tube, what should I do with the other one?  I know that I could simply make another gain stage, a buffer, run both triodes in parallel, or some sort of cascode stage but I'm more interested in what to do if I don't use it at all?  Should I treat it like unused inverters and tie the inputs to ground?  Do I need to use both heaters (when wiring the heaters in series at 12V how would I just use one heater anyway)?

[merlinb]

You can leave grid/anode/cathode unconnected, or connect them to ground; it doesn't matter either way.

You don't have to use both heaters. You can run just one heater by using pins 4 and 9, or 5 and 9 on a 6.3V supply. On a 12V supply you are forced to use both heaters, or wire up only one and put in a series dropping  resistor to get 6.3V across the one you are using.

[wavley]

The 6v6 version of the Traynor Bass Mate uses one half of a 12au7, some guys like it that way, some use it as an extra gain stage, some put it in parallel.  Nothing says you have to use it if that's all you want.

And, what merlinb said.

http://www.0rigami.com/vb/traynor_bassmate_yba2.pdf

[Bill Mountain]

Thanks guys.  I had searched a little bit and kept coming up with stuff about cathode stripping, poisoning, etc.  I just imagine that all those things the hifi-ers say can happen to tubes is not really a concern when running at such low voltage.  To me, the appeal of starved plates is that everyone tells you it won't work but we have 100+ pages of Valvecaster innovation that says otherwise.

Also, that's a cool link.  I love simple designs like that.  As soon as I nurse my V4 back to health, I'm going to look into scratch building a simple tube amp.

[iccaros]

According to the RCA receiving tube manual cathode striping only happens in transmitter tubes of high voltage, not standard receiving tubes like used in a Guitar amp. Merlin I believe in one write up spoke of Cathode Poisoning when the heaters are on for long periods and no current is flowing. Since you can use only one heater this is not an issue, plus I think you would have to leave the tube heated for days at a time to have this happen. I also have have some single triode tubes I got from an organ I can send you if you want to try those.   I have seen where some people will wire a switch to change which triode they are using, so they have a on-board spare so to say.

[Bill Mountain]

According to the RCA receiving tube manual cathode striping only happens in transmitter tubes of high voltage, not standard receiving tubes like used in a Guitar amp. Merlin I believe in one write up spoke of Cathode Poisoning when the heaters are on for long periods and no current is flowing. Since you can use only one heater this is not an issue, plus I think you would have to leave the tube heated for days at a time to have this happen. I also have have some single triode tubes I got from an organ I can send you if you want to try those.   I have seen where some people will wire a switch to change which triode they are using, so they have a on-board spare so to say.

That's a generous offer but I have plenty of tubes to experiment with.  I do like the switch idea.

[R.G.]

Thanks guys.  I had searched a little bit and kept coming up with stuff about cathode stripping, poisoning, etc.  I just imagine that all those things the hifi-ers say can happen to tubes is not really a concern when running at such low voltage. 

Cathode stripping is really not an issue under about 600V on the plates. There is a syndrome called (no, I'm not kidding) "sleeping sickness" that was found in computer duotriodes where tubes were held off continuously. Had to do with migration of high-resistance compounds on the oxide coatings, etc. Happened at 100V +/-.  But that's not really much of an issue here.