unused 12AX7 triode - ground unused pins?

[HTH]

thanks for all the replies, my question was more based on how sound a practce it is from an engineering standpoint, not that I'm too lazy to do it (though I am bloody idle, ha ha)

I'm heating up the iron as I type this, so I'll post back and report if there is any difference to the hum levels by doing this.

as for the CF idea - the amp is a Marshall 2203 so it's already got a CF-driven tonestack.

if I had more time I'd play around with the extra stage and use it for even more gain, however it's taken months of playing around to get the extra single triode sounding the way I want. I'll try two extra gain stages in a 2203 some other time.

my gut feeling is that adding two additional gain stages to a Marshall 2203 would be best executed by having one stage into the existing frontend and the other extra stage before the CF (i.e. similar to a SLO100). I reckon that could be an absolute scorcher of an amp.

[Enzo]

Maybe you could add a trem...

[dehughes]

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

I'd heard of "cathode poisoning" as well, so I just wired up the unused triode in my amp and ran a 1M to ground off the grid. So, it is just happily idling away until I can think of something productive to do with it.

[Chuck H]

I'm going to second the idea of a parallel gain stage. Just half the plate and cathode Rs. IIRC you should also double the bypass cap if your using one.

The advantage with this is slightly lower noise (although noise from other sources will probably make it undetectable) and LOWER IMPEDANCE. The lower the impedance of a preamp gain stage the better. Not sure exactly why, but it probably has to do with reactance. AFAIK it's always better to run a lower impedance to a higher impedance. And the bigger the impedance difference, between the source and the input, the better. I just know from listening tests that you'll get less buzz and fizz from THAT stage if you lower it's impedance by running parallel. The triode is just sitting there anyway. It almost certainly wouldn't detrement the tone to do it. So why not?

Chuck

[CitizenCain]

Paralleled triodes would work too... The plate and cathode resistors remain the same value.

I'm going to second the idea of a parallel gain stage. Just half the plate and cathode Rs. IIRC you should also double the bypass cap if your using one.

OK, so do you keep the plate and cathode resistance the same, or halve it?

Double the bypass cap, yes/no?

I'm going to parallel the first tube in my Heathkit conversion rather than try and work a switchable gain stage into it. I need the 411!

[Bruce / Mission Amps]

I'd heard of "cathode poisoning" as well, so I just wired up the unused triode in my amp and ran a 1M to ground off the grid. So, it is just happily idling away until I can think of something productive to do with it.

A waste of a 1m resistor, wire and time IMHO.

[mooreamps]

OK, so do you keep the plate and cathode resistance the same, or halve it?

Double the bypass cap, yes/no?

For two triodes in parallel, keep everything the same.... You can trust me on this one.. . . . .

-g

[HTH]

I can confirm that grounding the unused triode did result in less hum/noise, only slightly, but it does seem better now.

I've not seriously considered parallelling the two triodes as when I've done this in the past it made the amp sound a bit blurred and gave a little bit more gain too. That wouldn't be a good thing in this situation, however for the sake of soldering three short wires in place, I'm gonna try it - never seen a Marshall hotrod with a parallelled extra gain stage.

The DC heater board is nearly ready too. Be interesting to see how much hum can be killed with this method.

[CitizenCain]

For two triodes in parallel, keep everything the same.... You can trust me on this one.. . . . .

-g

[Bruce / Mission Amps]

I can confirm that grounding the unused triode did result in less hum/noise, only slightly, but it does seem better now.

I've not seriously considered parallelling the two triodes as when I've done this in the past it made the amp sound a bit blurred and gave a little bit more gain too. That wouldn't be a good thing in this situation, however for the sake of soldering three short wires in place, I'm gonna try it - never seen a Marshall hotrod with a parallelled extra gain stage. ...

I would bet it had more to do with your choice of 12AX7 or the wiring then... I really have never heard that happen by grounding the unused elements of a dual triode.
I have seen lots of production amps with unused triodes and floating lugs.
Next time I get one in for a repair I'll set it up on my scope and see if anything changes by grounding the remaining lugs and pins.

[HTH]

I would bet it had more to do with your choice of 12AX7 or the wiring then... I really have never heard that happen by grounding the unused elements of a dual triode.
I have seen lots of production amps with unused triodes and floating lugs.
Next time I get one in for a repair I'll set it up on my scope and see if anything changes by grounding the remaining lugs and pins.

everything was the same Bruce, same 12AX7, same wiring, just a single wire to ground the unused triode.

like I said, it wasn't night & day difference, but it was noticeable certainly.