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[Mark Lemaire]

I recently needed to replace a noisy EF86/ 6267 on one of my U67s. I have a small collection of tubes, most dating back to the 1990s.

NOTE: I never throw away anything away, even tubes that I have taken out of service because of 'failure' in the past. Each tube has a slip of paper explaining its history, etc.

Anyway,  I found three that are as low noise as the one in the other 67 in my pair. I was surprised and happy, as my notes tell me they were 'noisy'  before. I chose one, and then (afterthought) checked the heater trimmer voltage. It was at 6.35 volts, so I adjusted it to 6.15 volts before calling my tube replacement 'complete'.

And then I found my noise level had risen to an unacceptable level. I set it back (this time to 6.31 volts), and the noise level is back in the 'quiet zone'.

What to do? I recall that 6.15 is the 'best' voltage setting. Perhaps this helps insure long life for the tube, but is not optimal for noise level? What suggestions do you folks have?

[klaus]

A couple of thoughts, and one question right up front:
where did you measure the voltage-inside the mic or inside the power supply?
If you measured inside the supply, the voltage drop through the cable would be considerable enough, so that you end up with less than 6 Volts at the tube. That could be a reason for audible noise, especially with a marginal tube.

If the voltage was 6.15VDC, measured at the tube, then there is something wrong, either with your tube or your mic that causes noise. I have never encountered any noise level differences between 6.15VDC (my recommendation, to extend tube life) and 6.3VDC (Neumann's and the EF86 tube's recommended heater voltage), especially considering that in some Neumann schematics the heater voltage range for the NU67 is specified as "5.8 - 6.3VDC".

Were these tubes Russian or East German copies of EF86, by any chance? (Yet another possibility vor noise sensitivity).

Let's explore this. I am sure we will find the cause of the noise, which, I am pretty sure, is unrelated to a 0.15VDC difference in tube heater supply.

[Mark Lemaire]

Thanks for your reply!

The voltage was measured at the microphone.

The tube is so old/ worn that there is little to read on it now- except that it says "England' on it, and "EF 86". Mullard? Let's see if I can successfully attach a photo of it.

I would like to know what causes the noise shift as well. I adjusted it back to 6.32, and the noise level is in the right spot again- very low. Looking forward to your thoughts!

[klaus]

Though I am no tube expert by any means, I don't think that this tube is a Mullard, or that it was made in England, but the construction looks like a German-made Valvo, possibly one of a series made in East Germany.

I am curious: does this noise buildup happen with any other tube as well when you throttle the heater voltage back to 6.15VDC? Or does it happen only with this specific one? Have you tried any NOS and compared noise levels at the two respective supply voltages?

[RadarDoug2]

If a tube has been sitting for a length of time not operating, it may have gas that leaked in. There is a getter in the tube, to hoover up these stray gas particles, but it works best when heated. If fitting this old tube, leave it on for at least 24 hours before making decisions.

[Mark Lemaire]

Thanks for all the advice and thoughts. So....

I have a few other tubes that have tested well, so I removed the tube in question (setting it aside with a note regarding it's history), and plugged in one of the others. NOTE: the trimmer voltage did not affect the noise level in this tube. After setting the voltage, letting it cook overnight, and then readjusting the voltage again (it had risen slightly over the hours), I am ready to use it today at a drum session.

 I believe we are all set, and thanks for your good thoughts on this issue.

[klaus]

To sum it up, then:
* It was a marginal tube, triggered by a slight lowering of heater voltage that caused the noise.
* A healthy tube would not have caused noise as a result of lowering the heater voltage from 6.3 to 6.15VDC.

[uwe ret]

The tube in question probably has marginal emission, and increasing the filament voltage raises the cathode temperature enough to establish sufficient electron flow and reduce the 'shotgun' noise. To prove this theory, verify how the plate voltage changes with the filament voltage. Reducing the filament voltage should result in an increase of the voltage observed at the plate terminal.
Increasing the filament voltage of an 'exhausted' tube can often extend its usable life. If it eventually kills it (unlikely) you have not lost anything... Just make sure to return to standard conditions upon replacing the tube!