Hum problem in ECL82 tune

[JoeAlders]

[FONT=Times New Roman, serif]After building a headphone amplifier with the ECL82 and testing it, I noticed that on one of the outputs hum was added. [/FONT]
[FONT=Times New Roman, serif]The output signal on the other channel was clean. To detect if this was due to the print layout or[/FONT]
[FONT=Times New Roman, serif]the tube used in that channel, I exchanged them and now the hum was also moved to the other channel.[/FONT]
[FONT=Times New Roman, serif]Because I had an extra ECL82 in stock, I replaced the troublesome tube and then I got completely rid of the hum on both channels.[/FONT]
[FONT=Times New Roman, serif]Before the exchanging, I probed both the signal grid input and the cathode signal on the triode section and as a result, they did not show any hum. [/FONT]
[FONT=Times New Roman, serif]Also the power supply was nicely clean, especially the separate decoupling section of the triode section’s power supply. [/FONT]
[FONT=Times New Roman, serif]This is the first time I faced this phenomenon: crosstalk from filament to the plate of the triode section.[/FONT]
[FONT=Times New Roman, serif]I made my designs until now with pentodes only and never used twin tubes.[/FONT]
[FONT=Times New Roman, serif]Then, examining the ECL82 data sheet I noticed this:[/FONT]

[FONT=Times New Roman, serif]Microphony and hum of the triode section[/FONT]

“[FONT=Times New Roman, serif]The triode section can be used without special precautions against microphony and [/FONT]
[FONT=Times New Roman, serif]hum in circuits in which an input voltage Vi ≥ 10mVrms gives an output of 50mW of the output stage. Zg (50 c.s) = 0.25MOhm.”[/FONT]

[FONT=Times New Roman, serif]And so, after increasing the input voltage at the triode to about 50mVrms I got rid of this hum![/FONT]
[FONT=Times New Roman, serif]Is this a known problem in ECL82 tubes and if so, how can I detect this to prevent buying such a tube in future and having the possibility [/FONT]
[FONT=Times New Roman, serif]to send it back after measuring? [/FONT]

 

[Steve Morley]

I've encountered the same thing with this tube. Some are just inexplicably noisy. As to how to test for this ahead of time I haven't a clue.

The only reliable way I've found to test 12AX7s for noise and microphonics is to plug them into the first stage of a phono stage, shorting the inputs then slowly advancing the line stage volume while listening for noise and gently tapping the tube.

Would you care to share the schematic of the amp you built.

Cheers, Steve

 

[disco]

If the origin would be with too low heater-kathode resistance you would have probed AC on the kathode. Although the warning suggests it's a known phenomenon with this type, not all seem affected. Was it a worn tube perhaps? Was the grid leak indeed no more than 250K? I'd say some sort of capacitive coupling to the strongest AC source, probably in the penthode section.

 

[Mark Tillotson]

Filaments are not supposed to be emissive, despite being hot. Indirectly heated cathodes are supposed to emit only from the outside. Coatings are used to enhance emission where its wanted and suppress it where it isn't. I suspect sometimes there are unwanted currents between the hot filament and other electrodes due to imperfections in the coatings or their coverage.

 

[JoeAlders]

Disco's comment made me do some resistance measurements between the signal grids, cathodes and filament pins of that particular tube. Always very high impedances measured with multi meter. So it is not some leakage problem with that tube. Then perhaps Mark's comment is right or Steve's experience with this type of tube. Anyway the problem is gone. At this moment I am working on a decent circuit diagram. Will publish it shortly.

 

[DF96]

You measured the leakage resistances with the cathode hot? If not, not relevant. H-k leakage is always a potential problem but good manufacturing reduces it.

 

[JoeAlders]

You measured the leakage resistances with the cathode hot? If not, not relevant. H-k leakage is always a potential problem but good manufacturing reduces it.

No I did not. So if I only apply the heater supply and then measure leakage is that the way to do it?

 

[DF96]

Yes, for h-k leakage. For other leakages you may need to get the glass hot too.

 

[Osvaldo de Banfield]

This tube has been designed for TV, phonograph and radio audio amplifiers and vertical deflection, where signals are sufficient strong (Any radio or crystal pickup is around a volt or more). So, it is reasonable that some hum and noise appears at those lower levels.

 

[JonSnell Electronic]

As with all valves, the heater cathode can be a weak spot.
Some people get over it by using DC heaters. I always replace the faulty valve for reliabilities sake as I don't want an unhappy customer or re work the same job for nothing.