Shared 12AX7 cathodes...but only using one triode...?

[dehughes]

I have an idea...(one of many, mostly lame...)

In the on-going newbie troubleshooting fest that is my 1st build I have another tube-related question:

I'm only using one triode in the 1st 12AX7 (gain stage off the input jacks), but in order to facilitate a cleaner layout I've run the cathodes of both these triodes together and connected them to a 25uf/3k3 resistor to ground. This lets me get the cap/resistor pair in a more convenient position on the board, and able to isolate that ground point from the other stages. The plate and grid of the unused triode are not connected to anything...only the two cathodes are connected.

Now, is there any chance that this might be contributing to the hissy/crackly preamp I'm suffering from, at present? That is, are there any potential negative effects to this somewhat odd wiring? For exampe, hypothetically (as I don't know enough to be certain), say, heater noise getting drawn to the unused-but-connected cathode and transferred via the shared cathode wiring into the circuit as noise via the triode that is being used?

I've seen unused triodes before in various amps/circuits, but never ones that have been "half-wired" like in my case. My thought was that sharing the cathodes wouldn't cause trouble as the unused triode has no grid or plate connection to the circuit, but am I wrong in this thinking? Can there indeed be some kind of signal/noise transfer in this situation?

[David Root]

Hissy, crackly, could be a bad ground connection or a bad tube, probably the former unless you're using NOS or used tubes.

Just my $0.02 worth, but why not use the whole tube? You could parallel the two triodes, gets you a bit more intrinsic gain, lower noise, and possibly higher actual gain transfer to the next stage because you halve the output impedance.

Or try a self-biased cascode, see a related post in this forum, for much more gain.

Another possibility is an SRPP (Series Regulated Push Pull) taking the output off the upper cathode, gives you more gain and much lower output impedance, getting close to that of a cathode follower.

Or just use the extra triode as a cathode follower, if you're going into the tonestack from this stage that should also get you more voltage left at the other end of the tonestack.

All of the above will give a bit different tone to a single triode grounded cathode line stage, but thay all work.

[dehughes]

Thanks. Drz400 said it was probably a bad ground, and I think he's right....as I'm inclined to say the input jacks need a re-working. The parts should be here Monday for that...

As for the tube, I'm all out of real estate for any other tinkerings. I have an EF86 channel (I) and a 12AX7 channel (II) that use either sides of the PI. The 12AX7 channel is a TMB with a cut and a Master volume, and uses 1/2 of the 1st 12AX7 and then another full tube for the cathode follower circuit. I REALLY like the tonality of it, so I'm inclined to leave it as-is...

Also, what is cathode poisoning? A really knowledgeable person on another forum mentioned this as a possibility if I don't remove the heater connection from the unused triode... I'll go search via Google, but figured I'd ask here, too...

[David Root]

Cathode poisoning is where what little gas in the tube gets ionized and attracted to the cathode. These ions react chemically with the cathode coating, tieing up those metals and reducing the cathode emissions to the point where the tube won't function properly, or even at all.

[dehughes]

Cathode poisoning is where what little gas in the tube gets ionized and attracted to the cathode. These ions react chemically with the cathode coating, tieing up those metals and reducing the cathode emissions to the point where the tube won't function properly, or even at all.

Right. Very good. Thanks. So, if I just want to use one triode, I should disconnect the heater connection to the unused triode, yes? If I'm using the triode connected to pins 6, 7, and 8, I should then remove all connections from pins 1, 2, 3, and 4, right? Should I then leave pin 9 connected as-is, and just move the wire from pin 4 over to pin 5?

[unklmickey]

hi dehughes,

a couple of thoughts come to mind.

since cathode poisoning would take a long time to occur, replacing the suspect tube, with a known good one will tell you either this is a possible cause of your problem (the known good works fine), or definitely not (you still have hissy/crackly with the known good).

you originally chose to connect the 2 cathodes together for convenience in layout. but now you are concerned about the connection of the unused cathode.

hindsight is 20-20, but i'm beginning to think you should have just added a 1 Meg to the grid to reference it to ground, and a plate resistor sized the same as the one on the other section.

that would have allowed the unused section to just happily idle along, and you wouldn't have even needed to change the cathode resistor in the first place.

well at least we'll know to do that if we ever are faced with this situation again, eh?

[dehughes]

Hmm...you're right, but what I think I'll do is just TOTALLY disconnect that 1st triode and just wire the other cathode directly to the resistor/cap and be done with it.

Oh, and the amp is only a couple of months old, so it doesn't yet suffer from cathode poisoning. Swapping out tubes does nothing for the hiss/static, so I'm still looking at input jack grounds as the suspect...


david

[Mickey_C]

When the cathode-bias resistor is shared by both triodes, if one triode is not conducting its share of current, the other triode (depending upon the value of Rk) could bias to a point that is above its plate dissipation rating, causing the crackling sound (high current causes crackling noise).

Usually when both triodes are connected to voltages, and sharing the current, we look for an intermittant connection at the cathodes to one.

In your case, I think you've already stated what's going on.

Mickey

[dehughes]

When the cathode-bias resistor is shared by both triodes, if one triode is not conducting its share of current, the other triode (depending upon the value of Rk) could bias to a point that is above its plate dissipation rating, causing the crackling sound (high current causes crackling noise).

Usually when both triodes are connected to voltages, and sharing the current, we look for an intermittant connection at the cathodes to one.

In your case, I think you've already stated what's going on.

Mickey

Now THAT'S cause for me to do some re-wiring....

THANKS MICKEY!!!

[Mickey_C]

WRONG ANSWER - Don't you know an unused triode is a Sin?

That was like commandment 11: THOU SHALT NOT WASTE TRIODES.

I wish I could get a spare triode... guarantee you I'd find a use for it quick.

[dehughes]

WRONG ANSWER - Don't you know an unused triode is a Sin?

That was like commandment 11: THOU SHALT NOT WASTE TRIODES.

I wish I could get a spare triode... guarantee you I'd find a use for it quick.

Yeah, I can't think of what I'd use it for.... I find that the standard AC30 preamp stage is pretty darn sweet for my tastes, so I'll just let it sit there for a bit, unused, and tease the rest of you with my unused triode. Either that, or it will piss everyone off that a newbie has an unused triode and hasn't a clue what to do with it...

I disconnected the cathode of the unused triode and connected the used cathode directly to the 3k3 resistor and 25uf cap. This, unfortunately, did not remove the staticy/hissy noise, so I'm back to re-wiring the input jacks on Monday when the parts arrive. Thanks though, Mickey_C, for your input. Much appreciated.

[Mickey_C]

What's your voltage? Was the 3K Rk spec'd for a singular or shared cathode? Remember, a singular triode is going to run twice the current over as a shared triode with the same Rk value. The fix is changing Rk or connecting the other triode's voltage (and now the cathode too).

At least, if that is the trouble to begin with... you didn't mention what it is you're building, or what the voltages are.

Sorry I didn't see this until now... I don't have ampgarage in my normal bbs rounds... need to fix that pronto. This is one of the cooler sites out there!

[unklmickey]

hi Mickey_C,

i believe it was originally designed around using a 1.5k - shared.

the saga goes back to here: https://tubeamparchive.com/viewtopic.php?t=2112

...This is one of the cooler sites out there!

definitely one of, if not the best for this end of the cord. (these guys are just flat-out amazing.) i know of some pretty good sites for the other end too.

[dehughes]

Yeah, this originally just had one triode wired up running a 220k plate resistor and a 1k5 cathode resistor. In analyzing this, I thought that I should double the cathode resistor to get the stage running closer to the standard 12AX7 Vox AC30 values. According to the AC30 schematics I have, each triode sees a 220k on the anode and the cathodes for the first two triodes are sharing a 1k5. SO, in reality, running just one triode, I should have a 2k7 on the cathode...or a 3k3. The 3k3 seems OKAY, but I actually liked the 1k5 better...though that's probably pushing it a bit (biasing it kinda hot....yes?). I figure a 2k7 is closer to what a real AC30 1st triode would see...but still a bit hotter of a bias.

[dehughes]

Well, I figured out what I wanted to do.....

I basically copied the stock AC30 first 12AX7 wiring....shared 25uf and 1k5 on the cathodes, 220k on each plate...BUT, I left the grid on the unused triode disconnected and the plate only wired up to the 220k resistor (not connected to a plate cap. Is this alright? I figured the unused triode would just sit there and idle away....and be available for something in the future. Sharing a 1k5 sure sounds better than running a single triode with a 3k3 or 2k7...for some reason it just didn't work as well. Maybe it has to do with a shared cathode resistor....but I'm too much of a noob to speak to that. Regardless, it SOUNDS and FEELS good as it is now.

I'll attatch a picture, just for clarity's sake...lemme know if I should ground the grid or something...or just leave it as-is.