[ksan]

I am one of those who had humming noise with 6SN7 new tubes on Darkvoice 336SE headphone amp. After reading many threads including suggestions for Mod I had hard time to believe Darkvoice would design and manufacture any amp that would not work well with driver tube (6SN7).  I didn't feel like modifying the amp even it's simple to do.  I preferred finding a solution that not requiring modification because I believed that any modification to the amp is going to change the design intend of the product and it could also lead to sound signature change or else.
I contacted Darkvoice and they provided me with a simple suggestion:  Aging the new tube.
 
By following their instruction my 6SN7 tubes are now quiet and sounding really sweet.
The aging process time varies depending on how loud the humming noise you have on individual tube.  One of my 6SN7 took 120 continued burn-in hours to get rid of the humming noise completely. Below is what I suggest you to try before doing any mod to your amp:
 
  1. There are total of two stock tubes in Darkvoice 336SE Headphone AMP (back tube: 6N5P  |  front tube: 6N8P).
  2. Remove back tube-6N5P( leave the socket empty).
  3. Keep 6SN7 tube (the new tube that has humming noise) in its socket.
  4. Turn on the Headphone AMP and leave it on continuously for 48 hours (for burn-in the new 6SN7 tube).
  5. After 48 hours put the back tube 6N5P back to its socket and listen to it.
  6. Continue burn-in process If humming noise is still persist, until it's gone.
 

 

[Lord Soth]

Quote:

I am one of those who had humming noise with 6SN7 new tubes on Darkvoice 336SE .....................
 
I contacted Darkvoice and they provided me with a simple suggestion:  Aging the new tube.
..........

Most of the time, vacuum tubes which have been kept unused in storage have to undergo "tube conditioning".
Residual gases have a way of accumulating inside the unused tube.
These gases have to be "burnt off" over 24hrs before your vacuum tube can settle in.
 
There is a very well written article about tube conditioning at the following link:-
 
http://www.kk5dr.com/Tubes.html
 
I consider this article to be a classic on burn-in w.r.t. vacuum tubes.

 

[tomb]

Quote:

Most of the time, vacuum tubes which have been kept unused in storage have to undergo "tube conditioning".
Residual gases have a way of accumulating inside the unused tube.
These gases have to be "burnt off" over 24hrs before your vacuum tube can settle in.
 
There is a very well written article about tube conditioning at the following link:-
 
http://www.kk5dr.com/Tubes.html
 
I consider this article to be a classic on burn-in w.r.t. vacuum tubes.

That's a great link, but in scanning it I didn't see any mention of tube getters (maybe power grid tubes are different).  Every audio tube was designed and built with getters in the tube for the very reason mentioned - something was needed to "burn off" gas infiltration.  As noted here and elsewhere with NOS tubes that have sat on the shelves for 40-50 years, it takes time to burn off the accumulated gases inside the tube.
 
 

 

[Graphicism]

Another shortcut I've found is to use a high impedance headphones, only sensitive phones (300 and under) produce any audible humming noise.

 

[ksan]

Quote:

Most of the time, vacuum tubes which have been kept unused in storage have to undergo "tube conditioning".
Residual gases have a way of accumulating inside the unused tube.
These gases have to be "burnt off" over 24hrs before your vacuum tube can settle in.
 
There is a very well written article about tube conditioning at the following link:-
 
http://www.kk5dr.com/Tubes.html
 
I consider this article to be a classic on burn-in w.r.t. vacuum tubes.
 
Thanks Lord Soth, the article is very educational material and it helps a lot.

 

 

[Grado77]

-----------

 

[Shasari]

There is a very well written article about tube conditioning at the following link:-
 
http://www.kk5dr.com/Tubes.html
 
I consider this article to be a classic on burn-in w.r.t. vacuum tubes.

 
Does anyone have a copy of this article? The site seems to have gone dark.

 

[Shasari]

Other than generating heat (and the subsequent safety hazards) and possibly shortening the working life of the power tube (it gets shorter every time it's used anyway), does it do any harm to leave both in, and let the unit burn in with an audio stream for a few hours? Can that shorten the length of burn-in to rid the audio tube from the hum?
 
I have a new Electro Harmonix (Russian made) Gold 6SN7 in my Darkvoice 336SE, and an RCA NOS 6AS7 for power. I narrowed the hum to the 6SN7 by swapping in the stock (cheap) audio tube that came with it, and it changed the hum characteristic to a very light hum in the right channel, barely audible. The new 6SN7 has quite the hum out of the box, but I did run it on the burn-in prescribed here overnight, and it did change somewhat, it's down a db or two.
 
How long should it take to fully burn in a 6SN7 so that it's a: full quiet except for music signal and b: have the best audio characteristics?
 
Technical data:
 
Darkvoice 336SE
RCA NOS 6AS7
Electro-Harmonix 6SN7 Gold (plated pins)
Sennheiser HD-650 cans - 300 ohms
Source: iPhone 6S+ (not the best source, but not worst)
 
~Dave

 

[majormojo]

Apologies for reviving an old thread, but here is a link to an archived version of the dead page linked above and the info it contains. Hope this is useful for someone.
 
https://web.archive.org/web/20120605105654/http://www.kk5dr.com/Tubes.html
 

 About vacuum power tubes.​

By Matt Erickson KK5DR​

 ​

The info I am putting forth here comes from engineers at CPI/EIMAC, Rockwell/Collins, and my years of experience with RF power tubes.  The data I am publishing here, is supported in the technical article "Care and feeding of power grid tubes".
 
Fact: There is no such thing as a perfectly sealed vacuum tube. All tubes leak.
Fact: Over time, molecules of air pass through these "imperfect" seals, contaminating the inside of the tube.
Fact: A tube that sits on the shelf collects a relatively large amount of air inside over time.
Fact: The longer the tube sits inactive, the more air is accumulated.
Fact: The larger the tube, the greater the leakage.
Fact: The high level of vacuum, and removal of contaminant molecules is maintained by something known as the, "getter", which is usually a form of "rare earth minerals", that absorb the air, when heated by the filament in the tube.
Fact: Over-heating of tube seals, can compromise the seal, and cause rapid tube failure. Tube seals can also be damaged by high current start-up in-rush, which can crack seals by mechanical stress.
Fact: There is no such thing as, "too much cooling air flow" over the tube. Too little cooling air can be a big problem, and lead to seal over-heating.
Fact: Broadcast radio/TV stations keep all their final amp tubes running with filament current, at all times, for two reasons.
 
1. To keep the tube ready for instant use, should the other tube fail.
2. To keep internal contamination to a minimum.
 
It would be impractical for amateur radio stations to keep our tubes on at all times, an alternative is to "rotate" the final amp tubes on an annual basis.
Many hams have a "spare set" of tubes. I have heard of several cases where a ham had a spare set of tubes stored "safely" away in the closet for years. One day, they decide to place the spares into operation in his aging amp. Little does he know that over the years these tubes sat in the closet, molecules of air have been sneaking into the tube, gathering in a "pile" near the bottom of the tube, which is usually the negative cathode. The ham puts the tubes into the amp, turns it on, and begins to tune up as normal, suddenly, BANG!, the tubes flash-over internally and "self-destruct".
 
Here is what happened inside, the "pile" of air inside the tube, was ignited into a "plasma" (a super hot ionized gas) by the combination of high voltage, heat, and the igniter RF, the plasma is negatively charged, which then travels toward the positively charged anode of the tube. Most of the time, between the anode and cathode, lays the delicate grid, which has a large hole burned through it by the traveling plasma cloud, which is extremely hot (up to 30k degrees).  This rarely happens to small receiver tubes, but it is not completely unknown. I have had a number of 12BY7A RF driver tubes that over time became "gassy". The lower plate voltages in these tubes, was not enough to "ignite" the gas, but strange behavior of the tube made its replacement necessary. A good indication that a tube has become "gassy" is a "blue/purple" color near the top of the tube, or around the plate. A tube that is free of contaminant gas has no color, aside of the filament glow.
 
Solution: Keep your tubes "de-gas-ed", by "rotating" the "spares" with the regular tubes. A good rule of thumb; is to swap out the tubes annually, while removing dust, lubing the fan, etc. This may not be the "best" way, but likely the most practical one.
You may ask, "how long should I allow the tube to burn-in, prior to operation?" On a tube that has an unknown amount of shelf time, 10-12 hrs would do well. For a tube that has been in your closest for known amount of time, I have developed a rule; 2 hrs per year of shelf-time. When you "rotate" your tubes, allow them 2 hrs of burn-in time prior to applying RF drive.  With a new tube fresh from the factory, 6-8 hrs for large glass envelope tubes, 2-6 hrs for metal/ceramic tubes, burn-in time. A little patience, and conservative use, your tubes should see full life.
Another reason to "burn-in" a new tube is that it will finish the vacuum, removing the last remaining gasses and brings the filament/cathode up to full electron emission.
 
Other contaminations
There is another source of internal tube contamination other than the outside atmosphere. When a vacuum tube is driven by excessive RF levels, particles of oxide and metal gases are released into the tube. These metal gases and oxides can cause atmosphere contamination of the tube, which can lead to HV "flash-over" inside the tube. When a flash-over happens it can cause extreme levels of current to flow which cause further damage to grids, cathode, and tube seals, which then causes further flash-over, you can see where this is going.  A tube that has had a history of flash-over can be opened and inspected, metallic powder and or tiny bits of gold or oxide coating will likely be found.
This is just one more reason NOT to over drive your tube(s).
 
Filament voltage?
EIMAC states that; " A 3% increase in filament voltage above the maximum rating will result in a 50% decrease in tube life." For example: a filament with a max rating of 5.00Vac, increased to 5.15Vac (3% above max) can expect a 50% decrease in life span. However, decreasing the filament voltage to a low level can cause a radical drop in electron emissions. Check your filament voltage at the tube socket, using a VOM that reads "True RMS".  If the voltage is to high (many times it is), and your amp has a separate filament transformer, install an adjustable power resistor in the transformer primary, and carefully adjust the secondary voltage till it is with-in the specified range, preferable slightly lower. If your amp uses an "all-in-one" power transformer, you can use a length of small wire to lower the filament voltage, but this will take a great deal of "trial & error" to find the right size and length or wire.
 
Plate voltage?
Exceeding the manufacturers maximum plate/anode voltage can lead to an HV flashover, and possibly destruction of the tube and or damage to the PSU or other parts.
A good rule of thumb; is to not exceed the maximum voltage by more than a 2% margin. Meaning that if your tube has a max plate voltage rating of 3000vdc, the max safe voltage would be 3060vdc. So, if you are using 3200vdc, you are taking a gamble.  The key would be to closely match the plate voltage to the tube in use. For maximum gain, and best safety margin. This rule is most important when installing a new tube, which has a large gain level when compared to the old weak tube. Also, the new tube may have a high level of gas remaining in it, and operating the tube over its max plate voltage rating may well push it "over the edge". 
A tube operating at maximum plate voltage, will reach its maximum RF gain level, and max power output level too. Anything over that max level becomes wasteful, dangerous, and sometimes unstable.  Sure, it looks great on the watt meter, but it might come at a high price.
 
Emission
Every vacuum tube has an element that is known as the electron emission source usually it is either an oxide coating as in indirectly heated cathode tubes, or a treated filament as in directly heated cathodes. When the element is heated, either directly or indirectly, it emits "extra" electrons that are driven by the electro-motive force & current flow within the tube, to the anode, this is how a tube amplifies. This process is continuous, as long as the tube heater/filament is active, electrons flow, even when the tube is in standby/cut-off mode. The supply of electrons is NOT endless, every tube that is active, is on the way to the end of its life span. However, turning the tube "off" and "on" a number times unnecessarily would cause more damage than to leave the tube on for a few hours between uses. Thermal cycling is more harmful than continuous operation, but a balance of the two should be had.
 
For example; If you use you amplifier for a morning schedule, and then again for a noon sked, it would be better to leave the unit on and in standby mode during the interim time. If you operate several times during the day with your amp, it is far better to leave the amp on all day than to turn it on and off several times each day. This minimizes the thermal cycling of the filament/heater elements. Thermal-cycling is far more damaging then loss of emission levels.
 
Emission levels remain nearly constant all the way from new tube, to 10,000 hrs or more of continuous filament operation. When a tube is brand new its emission will be lower than after it has been in operation for a few dozen or more hours. This interval in known as the "burn-in" time, where-in the tubes vacuum is "finished" by the heat applied to the "getter", also the emission element is coming up to full electron flow during this time. After a few hundred hours the tube emission will reach a peak, after this the tube will have a very long slow glide to the end of its life span which is the point where the emission falls too low to be usable, or the filament/heater fails. Usually the end of the tube life is somewhere between 12,000 and 24,000 hours of filament/heater on-time. By amateur radio usage standards this is a very, very long time.  I would not worry too much about your tubes failing in this mode, 90% of all tubes used in ham radio fail due to chronic over-drive conditions, which damages the oxide coatings of indirectly heated tubes, and or the grid structures of directly heated filaments. This is NOT my opinion alone, but info shared with me by the engineers at EIMAC. They have seen it far too many times, for too many years for it to be an accident.
 
{missing graphic}
 
Above is a graphic representing the emission level verses hours, of a typical power tube. The graph is not to scale, the hours side is 12K hours.
Keep the thermal cycles of the tube to a minimum, and RF drive levels to the proper settings as prescribed by the amplifier manufacturer.
Keep your tune-up times down, and extend the life of your tubes.
To read much, much more about RF power tubes, stop by the EIMAC web site, and read the technical article titled "Care and feeding of power grid tubes." Mind you, this booklet is fairly technical and might be above the level of the average ham today, but it is very informative. Please download the PDF, you will learn much more than I can go into here.
 
How does a vacuum tube amplify RF power?
The EIMAC booklet goes into much greater detail, but here is a quick and dirty explanation. The cathode surface emits free electrons, which are propelled by the flow of DC current and RF, driven by the RF voltage and DC voltage combined. These free electrons account for a greater number of electrons arriving at the plate than are accounted for by the DC current alone. Thus we have amplification of any signal, audio or RF, that is imposed on the DC current stream.
 
Tube conditioning:
New tubes should be carefully "burned-in", some will have "flash-over" problems in the very beginning of their life due to residual gases and metal vapors remaining inside the tube.  I have found that this process can take some time, and RF drive should be very carefully applied to the tube during this time. If plate current levels become unstable, the RF drive and or the plate voltage should be reduced. Sometimes the process of "conditioning" the tube could take all day (8-24 hrs). During the process, the tube is run with heater/filament only (no RF drive) for at least the first 4 hrs of operation. Then, a small amount of RF drive is applied, if abnormal operation is noticed, the drive should be removed and the tube should be "burned-in" for a few more hours. Gradually, during the conditioning period the RF drive signal is increased, operating conditions are closely monitored during this time.
 
When full power out is reached, and normal operating conditions are observed, the tube can be considered fully "conditioned" and ready for regular use in the amp.
Used tubes that have been sitting in storage for a number of years should be treated in the same way, but the time of "burn-in" is shorter since the tube has already been conditioned when it was new. The main reason for repeating this process is to "burn-off" any contaminating gases that have reached the inside of the tube. The general "rule of thumb" for the number of hours of conditioning required for a used tube is simple. If the number of years is known that the tube was out of service, add one hour of burn-in for each year. If the number of years is unknown, use 4 hours to begin with, if the tube does not behave well after this, repeat the burn-in period.
 
More difficult cases of HV flash-over can require that the HV be removed from the tube for a number of hours until residual gases are absorbed. A tube that refuses to settle down after more than 24 hours of conditioning may be beyond help. If they do not settle after 48 hours of burn-in, they should be removed and discarded. 
 
Remember; have a spare tube on the self, and swap them out every year.  The set will likely out last you, even with daily use.
 
End of useable life:
Every tube will have an "end of life" point. Directly heated cathode tubes usually drop off emission until they are no longer usable. Some say that the tube has gone "soft" at this point. When the filament opens is an obvious sign, but this can happen at nearly any time during its life span.
Indirectly heated cathode tubes end their lives a little more abruptly with a sustained HV flashover that damages the grids and or cathode surface to the point where the tube is no longer stable at any voltage or drive level, or the heater opens. Emission will have dropped off a great deal by this point.  The cause for the HV flash-over is usually barium contamination of the atmosphere in the tube making the vacuum conductive at all plate voltage levels. The oxide coating of the cathode will have been reduced to the point where areas will super-heat, causing ionization and then HV flash-over and or sparking to the plate. This is not a reversible condition and indicates end-of-life for the tube. The HV arc in this condition can cause enough magnetic field that the heater can be distorted to the point which it opens under such mechanical stress. Either way, it is end of the line for the tube.
 
With careful attention to these details, you can get a full life span from your tubes.
 
Copyright © 2010  M.A. Erickson, KK5DR.  All rights reserved.

 

[Assimilator702]

Thanks for reposting this article. This should be a sticky.

 

[MLGrado]

I am going to assume the amp has AC heaters. I try to stick with amps that have DC heaters. A bit more complicated to do, as you need to rectify not only the main power but low voltage heaters as well. (actually, probably not complicated at all... it just takes extra time and a few more parts, meaning the amp will cost more.

Since going with a DC heater amp, I never hear any hum. Tubes that went in the 'trash' pile for hum have been resurrected and what a shame if I had gone ahead and thrown them out.


That said, when I did have an AC heater headphone amp, it just depended on the tube if it hummed or not. There was no rhyme or reason; a random thing. SOMETIMES, though, just letting them 'break in' with several hours listening and the hum will gradually disappear, or at least get so low in level it is a non factor.

 

[JKDJedi]

Other than generating heat (and the subsequent safety hazards) and possibly shortening the working life of the power tube (it gets shorter every time it's used anyway), does it do any harm to leave both in, and let the unit burn in with an audio stream for a few hours? Can that shorten the length of burn-in to rid the audio tube from the hum?

I have a new Electro Harmonix (Russian made) Gold 6SN7 in my Darkvoice 336SE, and an RCA NOS 6AS7 for power. I narrowed the hum to the 6SN7 by swapping in the stock (cheap) audio tube that came with it, and it changed the hum characteristic to a very light hum in the right channel, barely audible. The new 6SN7 has quite the hum out of the box, but I did run it on the burn-in prescribed here overnight, and it did change somewhat, it's down a db or two.

How long should it take to fully burn in a 6SN7 so that it's a: full quiet except for music signal and b: have the best audio characteristics?

Technical data:

Darkvoice 336SE
RCA NOS 6AS7
Electro-Harmonix 6SN7 Gold (plated pins)
Sennheiser HD-650 cans - 300 ohms
Source: iPhone 6S+ (not the best source, but not worst)

~Dave

Have a DArky myself and am currently burning in a Foton tube that has the loudest HUM to date.... I've read these take up to 100 plus hours of burn in.. so we'll see!

 

[InsanelyConfused]

I know I am very late to the party but I am just wondering if it's safe to even leave an amp running without the power tube in.. is there any risk of damaging anything except for the tubes? I don't have a Darkvoice but a Xduoo TA-26, which is very similiar.