double triodes interactions

[erwin_ve]

Hi all,

I'm looking for some insight on the effect of interaction on double triode tubes like 12at7, 12ax7.
What I understand is that the triodes have indepedent plates, grids and cathodes, Only heater circuit is shared.
Since they are in close proximity to each other, what kind of interactions effects are to be expected?
Is there a good read available on this subject?

Thanks,

Erwin

[Stevem]

If the tube is physicaly ok It's a non issue !
If you have problems it's due to the circuits and wire layouts around them!

[erwin_ve]

If the tube is physicaly ok It's a non issue !
If you have problems it's due to the circuits and wire layouts around them!

Hi Stevem,

Thanks for your response! I dont have any problems or issues regarding this, I'm looking for possible literature on this subject if any.
If there isnt thats fine, and we could put this question in the brainfart territory

Erwin

[ChopSauce]

Nothing thereoretical about the subject, just one experiment - in case you hadn't noticed it already:

I had a triode feeding an EF86 as pre-amp tube and managed to use the second triode as a cold clipper, after the EF86. That brought too much noise. Did not tried tube swapping, though.

Didn't Phil had some issues pertaining with the order of the triodes within the same tube, too?

[erwin_ve]

Nothing thereoretical about the subject, just one experiment - in case you hadn't noticed it already:

I had a triode feeding an EF86 as pre-amp tube and managed to use the second triode as a cold clipper, after the EF86. That brought too much noise. Did not tried tube swapping, though.

Didn't Phil had some issues pertaining with the order of the triodes within the same tube, too?

Sorry I cant follow your reply, you're trying to tell me something but I dont get it. Reading similar stories in triodes where 1 triode amplifier is followed by a cathode follower triode resulted in hum. This was brand specific.: I'm specifically interested in a double triode in the same tube and possible interactions within the tube itself.

[didit]

Hello -

Perhaps this thread at DIYaudio would help get you started down this rabbit hole - https://www.diyaudio.com/forums/tubes-v ... tubes.html.

Best .. Ian

[ChopSauce]

I'm specifically interested in a double triode in the same tube and possible interactions within the tube itself.

It was just that. One single 12AX7 (most likely a JJ) whose two triodes were used as described.

[pompeiisneaks]

Nothing thereoretical about the subject, just one experiment - in case you hadn't noticed it already:

I had a triode feeding an EF86 as pre-amp tube and managed to use the second triode as a cold clipper, after the EF86. That brought too much noise. Did not tried tube swapping, though.

Didn't Phil had some issues pertaining with the order of the triodes within the same tube, too?

not sure if you're meaning me or some of the other Phil's I had an issue where the leads from the OD stage were too close tot he first triode and caused oscillation/feedback of the positive type. I just swapped the sides of the triode so the first (more sensitive side) was on 1,2,3 and the second was 6,7,8 pins instead, which meant it had more gain before being close to the OD wires. That's not a 'per tube' issue as much as a 'sensitivity of the first gain stage in any amp issue'

again not sure if that's even the same phil or same issue

~Phil

[erwin_ve]

Hello -

Perhaps this thread at DIYaudio would help get you started down this rabbit hole - https://www.diyaudio.com/forums/tubes-v ... tubes.html.

Best .. Ian

Thanks Ian! Rabbit hole doesnt sound promising tho

[ChopSauce]

Nothing thereoretical about the subject, just one experiment - in case you hadn't noticed it already:

I had a triode feeding an EF86 as pre-amp tube and managed to use the second triode as a cold clipper, after the EF86. That brought too much noise. Did not tried tube swapping, though.

Didn't Phil had some issues pertaining with the order of the triodes within the same tube, too?

not sure if you're meaning me or some of the other Phil's I had an issue where the leads from the OD stage were too close tot he first triode and caused oscillation/feedback of the positive type. I just swapped the sides of the triode so the first (more sensitive side) was on 1,2,3 and the second was 6,7,8 pins instead, which meant it had more gain before being close to the OD wires. That's not a 'per tube' issue as much as a 'sensitivity of the first gain stage in any amp issue'

again not sure if that's even the same phil or same issue

~Phil

Sorry: mis-interpretation of a report about your adventures in Dumble land, does it seem ...

[T Wilcox]

I dont know where I read it (probably SLOclone forum) but I have seen discussion regarding VERY high gain amps using only one triode of the V1 tube and or using the 2nd triode for the clean channel input to eliminate possible positive feedback or parasitic between v1a and v1b or the first and second gain stages where this would be most sensitive.
You may also find more info in HiFi tube discussions where they use tubes that come with internal shielding like the 6922
https://frank.pocnet.net/sheets/049/6/6922.pdf

that's all I got

Todd

[erwin_ve]

I dont know where I read it (probably SLOclone forum) but I have seen discussion regarding VERY high gain amps using only one triode of the V1 tube and or using the 2nd triode for the clean channel input to eliminate possible positive feedback or parasitic between v1a and v1b or the first and second gain stages where this would be most sensitive.
You may also find more info in HiFi tube discussions where they use tubes that come with internal shielding like the 6922
https://frank.pocnet.net/sheets/049/6/6922.pdf

that's all I got

Todd

Thanks Todd, gonna catch up soon. The link Ian posted is also very enlighting. Capacitive coupling through grids seems to be the biggest possible source for crosstalk between the triodes. Some tubes are more prone to this than others.
Tube designers must had have their vision on this matter in previous century, the 6922 tube seems to be one that is partially tackling the crosstalk.

Erwin

[katopan]

I built an amp heavily based on a Gibson GA-75 a while ago and was having major problems that ended up being due to cross-talk in the dual triode. My mistake was making it single channel and using the same valve for the first two stages. The GA-75 has an uncommon set up where the volume pot is before the tonestack rather than after it. I was getting major volume coming through even with the knob down at zero. We noval substitutes for the preamp octals, so it was a 12AX7 for the first two stages rather than a 6EU7. After a lot of testing and pulling my hair out, I was able to prove to myself that the sensitivity of cross-talk to the second stage grid was dependent on that grid's effective impedance reference to ground. Because of the tonestack, even with the volume at zero there is significant impedance to ground and the signal was bleeding through effectively jumping the volume pot. Once I'd worked that out, some Spice sims showed me there was no real difference in the tonestack response if I moved the volume pot to after instead of before. This fixed the problem because now with the volume on zero (or even down low) there was a low or zero impedance to ground, which killed the cross-talk. With the volume turned up the direct connected signal was so much more than the cross-talk coupled that you wouldn't know it was there (like most amps).

I had many educated people on my favourite forums tell me it wasn't possible at the level I was experiencing and it had to be layout related coupling. I did extensive testing to prove it was happening and work out a solution. So yes, cross-talk is real and can be a problem in certain circuit topologies (just not the common ones).

[erwin_ve]

Thanks Katopan!

So when grid reference to ground has a high impedance, more crosstalk.
I'm trying to understand what exactly happens. Could it be there is a form of bias excursion?

[katopan]

I think of it in this way.... You know how if you plug your lead into an amp with the volume up, but no guitar connected on the other end you get a heap of hum and noise pickup? Then if you do have a guitar connected, it's acting as a (relatively) low impedance source and isn't as susceptible. This is kinda the same. A floating grid (except for the practicality that it wouldn't bias) would pick up the most noise. A solid grounded grid would pick up none. A higher or lower impedance to ground in between those two extremes would result in a higher or lower noise pick up. Although we usually think of noise from inside the amp (PT or heater hum, rectifier buzz, other parts of the amp signal) or noise from outside the amp (flourescent lights, plug packs, mains wiring), the closest thing to a grid of one half of a dual triode is actually the other triode. With a high impedance to ground it is susceptible to picking up signal from the other triode resulting in bleed of signal or cross talk. In the case of the Gibson amp I was copying as I said, it was all about the order of the volume pot before the tonestack. We don't normally notice the amount of cross talk because the volume is usually after the tonestack. If you're talking about other cascade stages then the actual coupled signal swamps the cross talk so it's not noticed at all.