Crosstalk between Triodes in Dual Triode Tubes - diyAudio
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Old 12th August 2014, 06:17 PM   #1
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Default Crosstalk between Triodes in Dual Triode Tubes

I have been searching for definitive data that would explain the crosstalk level between triodes in dual triode tubes. For me, part of this hobby is to understand the physics and not just blindly do what appears acceptable. I have researched books and threads in diyAudio and other websites and could not find concrete information. I would appreciate others to review the test data I have created and comment.

My research suggests that the phono cartridge will dictate the channel separation of -20db at 1kHz as this is the average separation and higher quality cartridges would have -30db at 1kHz separation. Further, it is noted that vintage amplifiers regularly use dual triode tubes (and new designs too); therefore, from the perspective of the designers of these amplifiers, the crosstalk level must be acceptable. This would suggest that a crosstalk between the triodes of at least -30db at 1kHz is acceptable.

I decided to construct a test circuit and measure the crosstalk. The triode amplifier circuits used for all tubes were based on the resistance-coupled amplifier circuits in the RCA Receiving Tube Manual where Ebb=180V and Rp=47kohm. A sine wave generator was used for the input to the driver triode and the test triode was given a 47kohm input impedance. Crosstalk data was recorded for input levels of 50, 100, 500, and 1000 mV rms and at 1, 10 and 20 kilohertz. Separate plate voltage supplies were used for each triode to eliminate crosstalk through the plate power supply and for tests where two separate 6SN7 tubes were used, separate 6.3V DC heater supplies were applied.

6SN7GTB -54dB at 1 kHz, -34dB at 10 kHz, -29 dB at 20 kHz
12AU7A -74dB at 1 kHz, -53dB at 10 kHz, -49 dB at 20 kHz
E88CC -70dB at 1 kHz, -54dB at 10 kHz, -47 dB at 20 kHz
A photo of the test setup and a more detailed test result table is included in the attached files.

It appears that the crosstalk between the triodes within a single tube glass envelop should have minimal affect on the Channel Separation for a phono amplifier.
Attached Images
File Type: jpg crosstalk test setup x.jpg (85.7 KB, 450 views)
File Type: jpg crosstalk test setup 2 x.jpg (80.8 KB, 447 views)
Attached Files
File Type: pdf Crosstalk between triodes Summary of Results.pdf (79.3 KB, 52 views)
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Old 12th August 2014, 07:47 PM   #2
Merlinb is offline Merlinb  United Kingdom
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Interesting contribution! I would also like to see the ECC83/12AX7 added to that list, since it has the highest internal resistance of all the common triodes and might therefore be expected to have the worst crosstalk.

Also, I assume you mean the 50, 100, 500, and 1000 mV test levels were at the input, not the output, right?
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Old 12th August 2014, 08:05 PM   #3
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Very interesting. Is the mechanism most likely to be the capacitance between one plate and the other? Or is there a significant flow of electrons from one cathode to the other plate?

Last edited by Malcolm Irving; 12th August 2014 at 08:22 PM.
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Old 12th August 2014, 09:15 PM   #4
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by Malcolm Irving View Post
Very interesting. Is the mechanism most likely to be the capacitance between one plate and the other? Or is there a significant flow of electrons from one cathode to the other plate?
The fact that it worsens with frequency is the dead giveaway that it is capacitive. If the tests were continued at higher frequencies you should expect the usual 6dB/octave degradation.
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Old 12th August 2014, 09:52 PM   #5
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Increasing the anode load will reduce crosstalk at retrospective frequencies. If you tested the Russian 6h2N, maybe that is why there is a screen between the two anodes.
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Old 12th August 2014, 10:11 PM   #6
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i can do a test of a 12AX7/ECC83 next time i am in the shop, probably tonite.
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Old 12th August 2014, 10:14 PM   #7
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Quote:
Originally Posted by JonSnell Electronic View Post
Increasing the anode load will reduce crosstalk at retrospective frequencies.
could you explain further what you mean. would this mean a smaller Rp? or something else? what physical activity are talking about?
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Old 12th August 2014, 10:46 PM   #8
Merlinb is offline Merlinb  United Kingdom
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I'm more interested in knowing what "retrospective frequencies" are supposed to be!
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Old 13th August 2014, 01:09 AM   #9
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Greetings from FixitLand!

Thanks for this work; very interesting! I would suggest testing the 6CG7 (with center shield), 6FQ7 or 6CG7/6FQ7 (without the shield), 12AT7 (no shield) and 6/12DT8 (virtually same as 12AT7 but has shield). It would be of interest to see how much difference that shield actually makes (for audio use).

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Old 13th August 2014, 02:36 AM   #10
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i could test a whole bunch of different tubes with dual triode construction but i think the results will be roughly the same. note that the E88CC has the internal shield and it improves the crosstalk considerably. i would suggest other tubes with the internal shield will test the same.

the real question i have is what does this mean regarding channel separation. there are amplifiers that offer specifications with absurdly high channel separation. channel separation greater than -30dB at 1kHz appears not required and therefore one should not try to spend a lot of time, money, and effort trying to better -30dB
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