unused 12AX7 triode - ground unused pins?

[HTH]

I've added an extra 12AX7 gain stage to the front of a 2203 Marshall and was wondering if it matters much if I leave the grid/plate/cathode of the unused triode unconnected or connect them together and ground them.

What's best for hiss/hum/noise? (or doesn't it really matter)

Thanks, Ian

[jrfrond]

Unused sections must be grounded.

[Jag]

Why is that?

[HTH]

I'm running it right now with the unused triode unconnected and the hum is a little louder than I'd like, but I think there are other contributing factors to that.

So to get this right, I'm cool to connect pins 1, 2 and 3 together and ground them, yes?

[Don Moose]

So to get this right, I'm cool to connect pins 1, 2 and 3 together and ground them, yes?

Sure, or connect the two triodes in parallel - that actually lowers noise ... somehow.

Search on parallel triode or parallel 12AX7 for details.

Hope this helps!

[Enzo]

Certainly there are Fender amps out there with an unused triode left unwired.

[CitizenCain]

Sure, or connect the two triodes in parallel - that actually lowers noise ... somehow.

Search on parallel triode or parallel 12AX7 for details.

Hope this helps!

If you do that, do the plate and cathode resistors need to change values, or can you just keep the same setup and run the tube paralleled?

[jrfrond]

It is always considered to be good electrical practice to connect unused tube electrode and even IC opamps and inverters to ground because they can become antennae for noise.

Paralleled triodes would work too. The advantages of paralleled triodes are increased second-harmonic distortion products, as well as lower noise, which is due to the effect of the paralleled plates halving the effective plate resistance. The plate and cathode resistors remain the same value. One fine eample of paralleled triodes is in the Matchless Chieftain, which I think is one asskickin' amp.

[Enzo]

I don't disagree that it is good practice, but the solid state stuff is different in that the two sections share the substrate. The tube sections are separate.

[Bruce / Mission Amps]

I don't disagree that it is good practice, but the solid state stuff is different in that the two sections share the substrate. The tube sections are separate.

And if the unused tube elements are not connected to anything, they are not energized or active so under any near normal condition, how could they generate any noise and therefore transfer it to anywhere?
I say just let them float.

[chocopower]

And if the unused tube elements are not connected to anything, they are not energized or active so under any near normal condition, how could they generate any noise and therefore transfer it to anywhere?
I say just let them float.

Maybe the proximity to the used triode could generate some microfonic noise?¿?¿?

[mooreamps]

Unused sections must be grounded.

oh boy.... Must Be, huh ?? Well, I'll tell you what. If I had an extra gain availible, it would either be as a parallel gain stage, or an extra series gain stage, or wired up as a constant-current bootstrapped cathode follower for a tone stack.. . . .

Secondly, probably no such thing as proximity noise.



-g

[jrfrond]

oh boy.... Must Be, huh ?? Well, I'll tell you what. If I had an extra gain availible, it would either be as a parallel gain stage, or an extra series gain stage, or wired up as a constant-current bootstrapped cathode follower for a tone stack.. . . .

Secondly, probably no such thing as proximity noise.



-g

OK, but that might not be everybody's idea for their circuit design, and why use the rarer single-triode tube types?

As long as the cathode is heated, there is electron emission, which will mean a space charge within the tube, with no where to go because the plate is open and there is no electron flow. Any noise picked up by the open electrodes could generate noise, via the space charge conduit. THIS is why unused sections need to be grounded. What is the big deal anyway? It's just piece of buss wire and pins 1, 2, 3 or 6, 7, 8! I say that, if you are going to be so meticulous about a build, then do what is electrically correct and ground unused sections.

[Enzo]

Let me say that this discussion is acedemic, any disagreement is in the interest of science. It is not like either way is terribly wrong somehow.

I'm with John on using a common 12AX7 when you only need one triode instead of mounting some esoteric less common single triode tube. And if you have a tried and true design that works as is, and you find it in an ampthat winds up with an extra triode section, I don't see the point in cooking up some way to add it to the circuit. Cathode follower to drive the tone stack might be a great idea, but for Pete's sake, Fenders have sounded just fine over the last 50 years without them.

The space charges are extremely local around the two cathodes. The vacuum in the tube is not filled with electrons. The two sides of a 12A?7 are not going to interect very much if at all.

WHY is it a big deal to run a piece of wire across three pins and ground it? It's not. I've seen commercial amps done both ways, and in neither case did I detect a noise problem. One could easily do it, or not. My surmise is that either way it will make very little difference.

[Albert Kreuzer]

Just to add to the discussion - I have seen unconnected triode sections go bad over time (cathode poisoning, I guess). So even if leaving the pins open will not affect the circuit in a negative way, it can be detrimental to the tube.

Cheers,
Albert