Crosstalk in a dual triode

[Kit]

Hi everybody.

The title says it all really.

How bad is the crosstalk between the two halfs of a dual triode (noval housing)?

I tried to google it but I havent found any info or numbers on the subject, exept for the internal C values which can be found in the datasheet.

The datasheet for a ecc81/12at7 states the following numbers;

Caa 0,4 pf

Cgg 0.005pf

Cag 0,007pf

Cga 0,04pf

These numbers are to small to matter at audio freq, but.....

Cathode crosstalk?

Thankfull for any replies.

 

[beatpoet]

I've seen this topic a few months ago... Now where is it?

 

[JohnRoberts]

First the caveat that I'm not a tube guy, but I did sleep in a Holiday Inn once. :grin:

It seems like the crosstalk may indeed be influenced by those internal capacitances wrt to actual circuit termination/source impedances.

In an input circuit the source impedance of whatever is plugged in could impact the termination impedance and thus the crosstalk, so a meaningful generic crosstalk spec would be hard to provide without also defining specific terminations.

JR

 

[craigb]

I've done some experimenting with a stereo circuit that shares a single 12ax7, and crosstalk was something I was concerned with. Using RMAA to measure crosstalk, I've typically seen values anywhere from the high 70's to under 50dB. There is quite a bit of variation even from the same manufacturer. It varies with frequency sometimes as well.

 

[NewYorkDave]

Being capacitive, it would definitely vary with frequency.

It's very difficult to generalize a "crosstalk figure" for dual triodes because it depends on the geometry/spacing of electrodes, whether or not there's an internal shield, the voltage gain in a particular circuit, the impedances involved, the circuit topology (e.g., there'd be less crosstalk between two cathode followers than between two common cathode stages, since in the former the plates are at AC ground and acting as shields). Of course, the manner in which the external wiring is executed has a huge impact!

I've seen "inter-section" capacitances listed on a few tube datasheets. But absent that number, you have to do your own measurements to even have a starting point for figuring out on paper how bad the crosstalk might be.

 

[Kit]

Thanks for your input, everyone.

Yeah, i guess the only way to know is to try it out under "real world" condtition.

I still find it odd that theres seems to be so little info avaliable on the subject.........I mean i cant be the first one who have asked this question.....oh well.

Based on the internal C numbers, crosstalk shouldnt be so bad.
Tubes are all about voltage gain and therefore high Z circuitry so a few 10th of pf shouldnt matter.......but only a real test can show this.

But yes, maybe more important, high Z circuitry demands "neat" external wiring.

 

[JohnRoberts]

While again I'm not a tube guy, I suspect the crosstalk performance you measure was considered adequate in the days of vinyl playback phono cartridges with a couple tens of dB separation and multiplex FM stereo also somewhat challenged. It wasn't until the CD medium that we had stereo separation that challenged your tubes and I suspect high end designers could have always just not shared different sources in a single tube.

I recall once talking with a guitar amp designer about crosstalk in high impedance guitar amp circuitry and he said it was one of the things that doesn't show up on schematics that can influence an amp's sound signature. Note: he was not just talking about crosstalk within dual tubes but also between wiring and circuit board layouts. For example since guitar amps often use saturation (hard clipping) of early preamp stages as a useful effect, a little HF leakage into other stages, perhaps of opposite polarity might add some desirable coloration.

Of course there are major philosophical differences between designing guitar amps, which are extensions of musical instruments and creating original sounds, while GP circuitry in trying to accurately process some input without distortion.

JR

 

[craigb]

[quote author="NewYorkDave"]Being capacitive, it would definitely vary with frequency.[/quote]

The interesting thing is that it was quite varied. The crosstalk on some tubes would be fairly flat across the spectrum, while others might have a decrease in either the high or low end, or both. Even of the same date code from the same mfr.

Just one of the charms of a handmade device I guess. IMO, a little bit of crosstalk inside the bottle isn't the worst thing.

 

[aletheian-alex]

Are you absolutely in love with the ecc81? A 6CG7 (not the new EH version... that is actually a 6FQ7...grrr), or 6DJ8 would pretty much eliminate the possibility... you'll get more crosstalk from your layout then those tubes.

Even with the ecc81, I doubt crosstalk will be above the S/N threshold, even at the highest frequencies unless your cathode/heater insulation has a lot of leakage.

 

[NewYorkDave]

Is there actually a difference between 6CG7 and 6FQ7? They were considered equivalent in Ye Olde Days.

 

[ciminosound]

Equiv in performance. Didn't the original 6cg7 have an internal shield that the 6fq7 didn't?

 

[aletheian-alex]

[quote author="ciminosound"]Equiv in performance. Didn't the original 6cg7 have an internal shield that the 6fq7 didn't?[/quote]
As far as i know that is the only diference. I think the 6CG7 have slightly lower plate dissapation ratings but I'd have to look at the sheets.

i have an RCA 6CG7 blazing awain in an amp right next to me. I like them... I just wish the new manufacture EH models had the sheild

 

[pstamler]

There turn out to be quite a few differences when the 6CG7 vs. 6FQ7 are compared. In all of what follows, 6CG7 is listed 1st:

Grid-plate capacitance: 4 pF vs. 3.6/3.8 pF [two sections]
Input capacitance: 2.3pF vs. 2.4pF

Maximum Ratings, Class-A amplifier:
DC plate voltage 300V vs. 330V
Plate dissipation, either plate: 3.5W vs. 4.0W
Plate dissipation, both plates operating: 5.0W vs. 5.7W
DC cathode current: 20mA vs. 22mA

There are similar differences in horizontal and vertical oscillator applications.

Essentially, therefore, the 6FQ7 is about 10-12% more robust, but lacks the internal shield connected to pin 9.

Peace,
Paul