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Possible causes of tube noise?

zackthedog

zackthedog

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I know there's a lot of discussion about noisy tubes, but this is a very specific case. A friend bought two matched pairs of GEC KT66s from a reputable dealer in the Netherlands. Previously he was using Gold Lions. After 10 minutes or so, one amp starts making a low-level spitting/crackling noise which comes and goes. He sent me a recording and it's a familiar sound--usually a dirty socket, dirty pins, or maybe a bad resistor. In this case, the pair biases perfectly and balance evenly. This is a Williamson amp I built for him with my PCBs. We just replaced the PCBs three weeks ago with upgraded components and brand-new Belton sockets. My friend cleaned the tube pins with steel wool but that didn't help.

He's going to do some extensive testing today, but so far the old Gold Lions don't make this noise, nor does the other pair of GECs. The seller says he tests the tubes on an Amplitrex and checks for noise, but if he only did this for a few minutes he might not have caught the problem.

My question is, what in a power tetrode could cause this sort of intermittent noise after warm-up? Gas? A misaligned grid? I'm trying to help him get to the bottom of this.
 
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Thanks, Tom, good ideas! Apparently he spent most of yesterday trying to isolate the problem but got frustrated and gave up. A complicating factor is that his system lives in a strange cubby behind his living room that makes the amps difficult to access. That and the intermittent nature of the problem had him going nuts. He's going to replace the Gold Lions, which never seem to have caused any issues, and send me the GECs. I work at home and my amps are out in the open so I can easily pop them in and swap them around all day long to see if I can spot the problem tube.
 
zackthedog said:
Thanks, Tom, good ideas! Apparently he spent most of yesterday trying to isolate the problem but got frustrated and gave up. A complicating factor is that his system lives in a strange cubby behind his living room that makes the amps difficult to access.
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I keep a long pair of interconnects, speaker wires, and power cords behind the units. Unplug the amp and pull it from the shelf. Then connect to the long interconnects, speaker extensions, and power cord on the floor in front of the rest of the system.

If you have banana plugs on the speaker cables, it's easy to rig a temporary extension using male and female end adapters.

It's so much easier than bending over behind a bird's nest of wires.
 
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yeah, pulling things out for access makes it less frustrating. I often resort to a board laying on the floor with cables pulled up to it so I can poke at stuff.
 
zackthedog said:
I know there's a lot of discussion about noisy tubes, but this is a very specific case. A friend bought two matched pairs of GEC KT66s from a reputable dealer in the Netherlands. Previously he was using Gold Lions. After 10 minutes or so, one amp starts making a low-level spitting/crackling noise which comes and goes. He sent me a recording and it's a familiar sound--usually a dirty socket, dirty pins, or maybe a bad resistor. In this case, the pair biases perfectly and balance evenly. This is a Williamson amp I built for him with my PCBs. We just replaced the PCBs three weeks ago with upgraded components and brand-new Belton sockets. My friend cleaned the tube pins with steel wool but that didn't help.

He's going to do some extensive testing today, but so far the old Gold Lions don't make this noise, nor does the other pair of GECs. The seller says he tests the tubes on an Amplitrex and checks for noise, but if he only did this for a few minutes he might not have caught the problem.

My question is, what in a power tetrode could cause this sort of intermittent noise after warm-up? Gas? A misaligned grid? I'm trying to help him get to the bottom of this.
Click to expand...
sometimes when they are just sitting in storage they will oxidize a little and a few hours of running the tubes the oxidization cooks off the electrodes causing a crackling and spitting sound. It should be fine once all the oxidization is cooked off. Which it might have already. Also, when you change out blade style sockets you need to work the pins a couple of times by plugging in and out a tube to seat the spring part of the blade. In both situations, the problem is not going to show up again.
 
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sddave said:
sometimes when they are just sitting in storage they will oxidize a little and a few hours of running the tubes the oxidization cooks off the electrodes causing a crackling and spitting sound. It should be fine once all the oxidization is cooked off. Which it might have already. Also, when you change out blade style sockets you need to work the pins a couple of times by plugging in and out a tube to seat the spring part of the blade. In both situations, the problem is not going to show up again.
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Interesting, thanks! I'll see what I can do with them.
 
gadget73 said:
yeah, pulling things out for access makes it less frustrating. I often resort to a board laying on the floor with cables pulled up to it so I can poke at stuff.
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This is very much a WAF thing. He used to have his gear in a little bookshelf nook next to the fireplace but that was not appreciated. ;-) It's not a bad setup, actually, it's just an odd raised storage nook you access from the powder room behind the living room, originally for linens etc. It's fine as long as he doesn't have to do a lot of tube swapping. ;-)
 
One issue with long unused tubes (not to be confused with cathode poisoning, which is a very similar condition) is the formation of interface boundary layers between the oxide layer and the metal cathode tube. The noise comes about much like the noise present in carbon composition resistors.

When this issue presents, operating the tube at heavier loadings for a reasonably short period of time will often restore acceptable operation.

I could once explain the actual scientific breakdown of this problem off the top of my head, but my studies in inorganic chemistry were completed in another century;)
 
Interesting, arts. You mean at higher current than normal? And what about the idea of putting them in the tube tester and bumping the filament voltage for a few minutes? I've read that this can have a rejuvinating effect. I'm pretty experienced, of course, but these are also $300@! ;-)

I've never heard original KT66s so I'm actually looking forward to trying them. He likes them a lot but is frustrated with this intermittent noise thing. I hope I can resolve it, or at least provide indisputable evidence for the seller.
 
I would increase the plate current via bias adjustment, and let them cook at as close to their rated dissipation as possible. The heat will increase getter activity and the heavy cathode current will break down (hopefully) the layer. I would advise against the heater increase.
 
The heater voltage bump can sometimes burn through cathode poisoning, but it also doesn't do the heater life any favors so its a gamble. On a tube thats basically dead otherwise you have little to lose. Needs to have plate current flowing for that to work though. I had a couple of junk tubes in a Pilot tuner/preamp that I should have tried this with, but they were very common ones I had plenty of so I didn't think to do the experiment. They sat powered for a long time with no plate voltage applied.
 
arts said:
One issue with long unused tubes (not to be confused with cathode poisoning, which is a very similar condition) is the formation of interface boundary layers between the oxide layer and the metal cathode tube. The noise comes about much like the noise present in carbon composition resistors.

When this issue presents, operating the tube at heavier loadings for a reasonably short period of time will often restore acceptable operation.

I could once explain the actual scientific breakdown of this problem off the top of my head, but my studies in inorganic chemistry were completed in another century;)
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Is this the reason some guys used to bake NOS tubes in the oven before use?
 
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Looks like it's probably not the GECs, but a driver or input tube instead. Probably one of the 6SN7s. Which is good, since those are a lot cheaper to replace. I've sent him some fresh ones to test. Apparently just a bad coincidence that something else decided to act up just as he swapped in the new tubes.
 
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