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Old 2nd February 2012, 05:34 AM   #11
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The behaviour around 0V is not a classic 'diode' characteristic from measurements I've seen, with heater-cathode leakage resistance increasing with voltage away (positive or negative polarity) from 0V. This appears to not typically be a symmetric characteristic about 0V, but there is not much in the way of measurements to look at.

When trimming hum to low levels with 12A?7 tubes, the datasheet indicates there may be some benefit in choosing the 2nd half for the lower signal level use - although there doesn't seem to be any difference in the capacitive coupling specs between the halves.
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Old 2nd February 2012, 09:25 PM   #12
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Most interesting - only we are talking about two different things?

Not sure I understand correctly, Trobbins. As said I see the heater-grid insulation problem acting more from an emission point of view than an ohmic one. In that sense it does not matter when the heater is positive (heater acting as anode) to the cathode. It is when the cathode is positive to the heater (cathode acting as anode) that faults in heater insulation can cause electronic emission to the cathode, thus getting 50/60 Hz into the signal line. (After all, one does have 8000 to 16000 mVp ac in the proximity of a few mV signal.)

Regarding tube construction, I visually inspected. For the 12A*7 construction strapping 'grid to pin 2' lies close to the heater center tap, and strapping 'grid to pin 7' close to the heater strapping going to pin 5. In certain models these come quite close, a small fraction of a mm. But in one 12AU7 I have, the grid posts lie on a radius from tube center. That then created different mechanical proximities.

It might be an interesting experiment to hook up both triodes similarly and investigate these effects - have never thought about that myself.
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Old 2nd February 2012, 11:50 PM   #13
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The heater to grid interaction is effectively stray capacitive coupling via the wiring path, which includes the wire geometry within the valve socket and at the lead-ins to the heater/cathode tube (and also under-chassis wiring). That stray coupling can be a different magnitude for one 'end' of the heater, versus the other 'end', which I believe is why a humdinger pot is often 'off-centre' to achieve a hum null from the contributions of the two heater 'ends'.

The other hum mechanisms involving the heater-cathode interaction are a leakage resistance coupling (which is affected by voltage difference between heater and cathode), and a capacitive coupling due to the proximity of heater within cathode.

Measurement of the leakage resistance level does not typically show a 'diode' character - eg. where resistance is hugely different for one polarity compared to the other polarity, or where a 'junction potential' is observed.

Ciao, Tim
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Old 4th February 2012, 08:40 AM   #14
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Quote:
Originally Posted by Johan Potgieter View Post
.....the heater-grid insulation problem acting more from an emission point of view than an ohmic one....
Jeepers Tim - things are a disappearin' round here...

The post where I apologised for the above mistake plus a few more comments 'appears to have disappeared' from the thread, even after I checked for its presence after having clicked the 'submit' button.

Thus to repeat; SORRY! The above should have read "the heater-cathode insulation of course. Hope that did not confuse you.

Quote:
Measurement of the leakage resistance level does not typically show a 'diode' character - eg. where resistance is hugely different for one polarity compared to the other polarity, or where a 'junction potential' is observed.
I respect your examination; I never dit that myself. We did occasionally find, with certain rogue tubes, that hum suddenly (i.e. not gradually) disappeared when the heater-cathode voltage was raised above a certain level. That seemed to confirm some sort of diode effect of a positive cathode being able to collect emission from a (faulty) emitter. This possibility was generally known at the time and we gave it no further thought. (There were not enough examples around to test anyway - think I found this only two - three times.)

Now let me try again ....
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Old 4th February 2012, 12:21 PM   #15
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No probs Johan. The only measurements I have come across are:
http://dalmura.com.au/projects/Heate...erformance.pdf
http://dalmura.com.au/projects/Heate...on%20plots.pdf
I haven't got any new 12AX7 to check for comparison - all those valves are used.
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