Twin triodes but only using one of them

[JoeAlders]

Tried to search the forum before posting this thread but failed to find an answer on this question:


I am using only one triode of a twin as an input stage of an amplifier. What is best practice, simply leave the pins of the one not used open or shall I connect all three pins to earth?

 

[petertub]

Tried to search the forum before posting this thread but failed to find an answer on this question:


I am using only one triode of a twin as an input stage of an amplifier. What is best practice, simply leave the pins of the one not used open or shall I connect all three pins to earth?

Connect them to ground. Floating elements are never good. If possible disconect the filament for the unused triode.

 

[Ray Waters]

Joe,

You might also consider connecting the two triode sections in parallel. This has certain benefits as described in this thread:

Parallel operation of triodes.

 

[egellings]

Connecting the two triodes in parallel could upset the circuit design. What I did was to make 2 socket adaptors for the 1st 12AX7 in my Mac MC-30's, which use only one half of the tube. One adaptor lights up & uses 1/2 the tube, and when that wears out, I change the socket adaptor to the other one that uses the other half of the 12AX7. Works like a charm, and I have a fresh, unused triode when I switch to the 2nd one.

 

[Junm]

I had done repair on a 6SL7 driver using only 1 triode. the other triode is not used. when i check the tube with my mutual conductance tester, the used one still reads good but the un-used part reads low. Is this a result of cathode poisoning as they say?

 

[petertub]

I had done repair on a 6SL7 driver using only 1 triode. the other triode is not used. when i check the tube with my mutual conductance tester, the used one still reads good but the un-used part reads low. Is this a result of cathode poisoning as they say?

Yes. That's why unconnecting the unused filament is needed.

 

[artosalo]

You might also consider connecting the two triode sections in parallel.

I agree.

Connecting the two triodes in parallel could upset the circuit design.

In such case the circuit should be designed for parallel operation.

 

[OldHector]

Another option could be to load up on cheap NOS EABC80, since the triode part is half a 12AX7, and better quality according to this link ...
EABC80_T

 

[SemperFi]

Yes. That's why unconnecting the unused filament is needed.

Very few tubes have this option. AFAIK only some battery directly heated types have connections to each of the separate cathode filament. ?

 

[SemperFi]

I had done repair on a 6SL7 driver using only 1 triode. the other triode is not used. when i check the tube with my mutual conductance tester, the used one still reads good but the un-used part reads low. Is this a result of cathode poisoning as they say?

Intersting. I had a 6AS7 amp using only one section per channel, but using two tubes. I did this b/c I needed to push the plates slightly above max rating of 13W but did not want to parallel. I used pins 1,2, and 3 on one tube, and pins 4, 5, and 6 on the other tube. Then used to swap them every now and then.

I was pleased with this and felt it prolonged the tubes life...

But perhaps I was not? I was pushing one side too hard while poisoning the other at the same time?

What to do with the unused half? I guess connect all free elements to GND. I think floating is worse. Hmm, so many things to worry about....

 

[petertub]

Very few tubes have this option. AFAIK only some battery directly heated types have connections to each of the separate cathode filament. ?

I think the author used a ECC83, here one can disconnect one filament.

 

[SemperFi]

Ah yes of course. My bad...

 

[56085]

Another option could be to load up on cheap NOS EABC80, since the triode part is half a 12AX7, and better quality according to this link ...
EABC80_T

I'm a bit surprised that the site you linked to says that the triode section of the EABC80 (and the 5751) is identical to a triode section of a 12AX7 because there are some differences (most important one: μ = 70 for the EABC80 against μ = 100 for the 12AX7). But it does confirm what one can deduct from the distortion figures in Philips datasheets for the EABC80: It's an overlooked sleeper.

 

[JMFahey]

Very few tubes have this option. AFAIK only some battery directly heated types have connections to each of the separate cathode filament. ?

Any 12A*7 dual triode has separate filaments which allows you to leave one unconnected/cold.

 

[56085]

The 12AH7-GT, a dual triode with an Octal base, has no centretap on its heater. But since it looks like there never was an all metal version of the 12AH7-GT, your statement still holds.

 

[JoeAlders]

Thanks guys for all your responses here. Much appreciated.
Apart from the possibility to use only one half of the filament connections (like ECC83 and ECC99),
I am puzzled about the way how that unused triode can give any crosstalk with the one that is actually being in use.
Neither the signal grid nor the cathode, nor the plate is running any voltage connected to it.

 

[artosalo]

From where the unused halve could get the signal to cause crosstalk?

 

[JoeAlders]

From where the unused halve could get the signal to cause crosstalk?

Exactly. So why connecting those pins to ground? Just leave them floating.
And with crosstalk I mean from the inactive triode to the active one.

 

[jgf]

Floating == Antenna. Why not just ground them? No down side.

 

[JoeAlders]

Floating == Antenna. Why not just ground them? No down side.

You are right, I can simply ground them. But will it function as an antenna? Just a bunch of floating metal.
I can imagine that this is of importance when operating the active triode in a circuit as for instance an HF oscillator/ (tuned)amplifier. But in the audio frequency range?