Tube Lab

Last week we talked about Gain, and this week we're going to talk about Emissions.
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[@janno288]

I am not a tube amplifier guy, but do work with tubes since i wind them fascinating and electrically way more tollerant to overload. is there a way to restore vacuum tube emission? I have done a bit of testing in raising filament voltages about 2 times and "baking" the tube a bit at that higher voltage together with drawing anode current. There are a few videos of people doing that. To my knowledge they are basically removing old cathode material exposing new one underneeth. I do enjoy these videos from time to time, I don't really understand when people claim one tube sounds better than the other and they all pretty much sound the same to me, placebo maybe? Good video tho.

[@tubelab194]

Sometimes (as in almost never) a cathode needs a special period of regeneration, that's a process that every tube is designed to do the minute you turn them on. But what happens is a tube is stored for a long time and the cathode becomes contaminated and needs a few minutes to get working properly. The claims you see of someone taking a worn out tube and bringing it back are mostly BS. After you over heat the filament the tube will test better for one or two minutes and then start to slowly fade away again. The reason for this is the filament is worn out and can't rejuvenate the cathode or even fire up the electrons enough to get much output. We throw out hundreds of tubes every year and low/mismatched Gm (mutual conductance) or emissions (mA) are the main reasons they get binned.

[@tubelab194]

If you can't hear the difference between different quality tested and matched vintage tubes, that's a sure indication that your listening system doesn't have enough resolution and needs upgrading and or your hearing is buggered. Basically the resolution of your listening system is lower than your tubes, probably by a significant amount, so you could plug any tube in and they'll all sound the same. I'm lucky to have a family member who is a hearing specialist, she tested my hearing and plotted the frequency response of the test. I wasn't shocked to see that my top end stopped at 11khz, but I didn't like the wee bit of wiggle through the midband, then she showed me her midrange hearing graph and I was grateful that I've always protected my hearing all through the years when I had a commercial woodworking shop and worked in the bush bringing in the firewood for the year.

[@janno288]

​@@tubelab194 I mostly was referring to tubes with the same designation, I dont hear any difference between different ECC83s, no tube sounds "warmer" or something like that. I do see that their internal construction is a tiny bit different in plate structures, but for the tube to be classified as a ECC83 the spacing of the grid to the cathode and the grid shape must be the same between the different versions to cause the tube to be still a ECC83. I am still in my youth and can still hear CRT TVs high pitched whine (and I love it), I doubt that my hearing is the problem. I was building my own "amp" (More of a modulator) and I couldnt hear a real difference between American made 12AX7 (Not General Electric no etch), Siemens ECC83 and an RFT East German ECC83. They sounded completely the same. I dont really take the "sounds warmer" excuse, I would honestly like more scientific testing if there is sigificant difference between tubes of different brands with a large sample size of preferebly Used and New old Stock tubes. and yes rejuvinating tubes will not make them like new again, that is a given. I would be interested how much life you could really squeeze out of an old tube by rejuvinating it. "The reason for this is the filament is worn out and can't rejuvenate the cathode or even fire up the electrons enough to get much output." What do you mean? The Filament doesn't wear out compared to the indirectly heated cathode. The filaments (heater) purpose is to make heat, unless the tubes filament opened it it will do its job just fine. The filament in an indirectly heated tube is removed from the plate circuit, so it doesnt matter in a case of rejuvination other than making more heat than it is supposed to. Could you please explain what you meant by this?

[@tubelab194]

@@janno288 that's exactly what I thought you meant, if you have a well designed and quality built tube preamp or power amp with good cables and speakers, you will most definitely hear a difference between almost any type you listen to, especially a high gain tube like the ecc83. What can easily happen in any audio chain is one (or more) weak links mucks up the signal (tubes, coupling caps, RCA patch cords, RCA jacks, volume pots, circuit design/build etc), even one bad sounding tube or crappy RCA cable and everything afterward is reduced to the sonics of the lowest quality point in the system. When you try and rejuvenate a cathode, in 98% of the cases (give or take a few) it will look good on the tester for one or two minutes and then it reverts all the way back to where it was and worse, because you've fried the filament. Quality tubes normally don't go pop, what happens is over a long period of time the filament wears out, it starts getting weaker the minute you fire up a brand new tube and from that moment forward the clock is ticking (quality tubes typically have a 2,000hr rating on the filament). Eventually the tube will sounds like shite, it's still working but the quality of the sound will be significantly degraded and the gain will be much lower. We throw those tubes out immediately, and make sure they're destroyed or someone at a recycling depot will be selling them on eBay with repo boxes for top dollar!

[@janno288]

@@tubelab194 the filament (heater) does not wear out, unless that is a directly heated tube, like for example a 813 or a GU-81M, since it glows really brightly, tungsten slowly evaporates from it causing it to weaken electrically and physically until it burns out. For indirectly vacuum tubes, the filament heater is a tungsten wire barely glowing, and the oxide insulation ensures little to no tungsten evaporation throughout the tubes life, the filament doesn't wear out electrically over the lifespan of the tube, the indirectly heated cathode does with its emissive oxide layer. frying the filament on indirectly heated tubes is hard since in most tubes it glows very dimmly and you can increase the voltage on it a bit over the ratings, 12.6V to 18V and let it bake and it wont affect the life of the tubes filament in any significant amount. Usually the tube weakens way before the heating filament burns out, unless it suffer rf some sort of voltage spike if its a series string device.