Quick links:

1. Auto Bias Card
2. Adjustable Bias Card (Standard)
3. Tesla Factory Cards

KT88, an unusual tube.

On the one hand, we always are not to mistreat a tube, in order to achieve long lifetime and good reliability. That means as a minimum, heat the tube up in a controlled, way, pack the tubes carefully, do not drive the amplifier in full distortion, and all of these things. Those who followed designer classes, will know why not to run the tubes above 70% of any maximum limit, specially not with heat dissipation. So when a proven and reliable way to use KT88 produces a certain result, this is a fact. Jumping over your shadow, with power electronics can be quite funny to see other people try it, but for yourself you should not do it. The failure rate is not a low number, until you reach the maximum limit, and all products suddenly fail above that. Failure rate is number which belongs to the product and it is always there. It goes up when you begin to approach maximum limits, and it goes up particularly fast if you approach several limits at the same time. What means approaching? The answer is: 70%.

On the other hand, KT88 is a strange exception. Of course so called "quality tubes" are sold for whatever a fool is prepared to pay, but in reality, production cost of KT88 is so low you would not believe it. In the days before transistors, where KT88 was a normal consumer product, it simply was a pentode on steroids. It t was abused, and you just replaced them when they fail. The generic tubes design was never any better, and this is what KT88 today still is. Add to this, there are very little tube factories left, and no factory can buy any more those good materials from the past. Lead containing glass is sure better, but not forbidden. Metals produced and vacuum are the best, but not for sale any more. Etc, etc. You can just not expect any new made KT88 to be as good as the M.O. Valve originals, and like I tried to explain, even those were not extremely perfect.

So designers still have the choice to be gentle to the tubes, and they will last a reasonable time. Those that get "so and so many" Watts out of the poor tube with their "great" amplifier, only produce broken tubes.

Below here, are some of the items that can cause quality problems with KT88

Lets just go through some details:

COST

It begins with materials and manufacturing. The most delicate is the cathode and the first grid. So just by using those few words here, we already can say where the savings will be, and we can try to understand the effects of cost saving at this point. Things that can go wrong here, will be:

1. loss of emission
2. too low initial emission
3. emission depending too much on heater voltage
4. grid current
5. grid rattle
6. Cathode to heater leakage (hum)
7. Cathode to grid short.

Some of these things can be tested nicely, and some can not. The manufacturer can be tolerant or less tolerant with things he can test, but the things he can not test will cause growing trouble too when the tubes get older.

GAS

Apart from new made Russian KT88, which have always been a little bit "so so", vacuum is normally good when tubes are factory packed. Under normal use conditions, the vacuum begins only to lower when the tube's out gassing can no longer be compensated by the getter flash anymore. When the vacuum gets totally lost, the tube stops working of course, but there will be a moment where the vacuum begins to get less good, and the tube is still capable to work. Of course this is the moment to replace the tube, but it is very difficult to say exactly when that will be. At first the tube will begin to show some blue gas cloud inside the anode. You can see this when looking inside the inspection holes of the anode. This is a light-blue haze, evenly distributed, inside the whole anode, and the anode only. It needs a trained eye. Do not confuse this with the more common blue light effects. which is a dark blue fluorescent lighting of glass, some outside parts of the anode, or edges of the inspection holes. This shows the tube has good vacuum, and even so very old tubes don't do this anymore. Though this is no requirement, and two fully good, new tubes of a pair, may have one with fluorescent lighting, and the other doesn't have that. So keep apart dark blue fluorescent lighting spots from light blue overall "cloud" lighting. Only the last stands for a gassy tube.

TESTING

A lot of mess is made here today. Many tube factories run at full capacity, and people but the junk on Ebay like crazy. I have seen many many times, new made Russian tubes, which obviously had gas problems from the day they were made, and I don't think they did not know that. The brand the same tubes differently, and sell them for different prices. When testing, the expensive ones were not better than the cheap ones. Also the vacuum problems with the expensive ones are not any less. When asking for incoming inspection criteria, they laugh at you. They say the tubes are "good", already tested. They sell 95% of the KT88 in the guitar amplifiers, and those 5% of HiFi people make only trouble, and can take it or leave it.

The Chinese have definitely a higher standard with respect to good vacuum, but also not all of them. Yet the problem with Chinese tubes is, they mix the weak tubes with the good ones, and effectively never throw away something. It dpenends who you are, and what you buy, but it the end they always try to find out if you are stupid. So good testing by yourself is very important. I I was send KT88 with the brands stamps of well known "re-branders" not carefully removed, and even writings about the reject cause were not completely removed! Those re-branders do probably a good job with testing, and return what that reject. Then the Chinese test those only quickly, see no problem, and just sell them again.

Czech KT88 I sold for a while, but these were such a disaster, I gave up on that. I later found out, they even re-used the glass bottom of KT88 I rejected.

Link to dedicated page to blue light effects.

Original English Genelex NOS. The mother of all KT88 
Not to be confused with Russian made KT88.

THE GETTER

KT88 will work a little longer, when the getter begins to turn bad. Such tubes can electrically still be ok, but it's a no good sign for lifetime expectation. If tested critically, such will show all specs are not as nice any more as it could be. So just a tiny little bit of gas, but not too much. Transconductance is a little bit low, but not too low, etc. etc.

Yet it can be said, a good getter has no sharp edges all around. So the edges should have a range of some 3...5mm which is transparent, and then they are good. If the transparent edge, fails, the edges become sharp. You can see this transparent area nicely in the above picture. In the main article about KT88 there are many examples of good and bad getters.

THE AUTO BIAS TEST CARD . (Order nr: 380-080-67)

Final inspection in tube factories is really a weakness. The KT88 production line is always automated. Meaning it has high overall costs, which have to be spread over all tubes. So when a batch is low quality, there comes this factory manager with a white coat on, and his function is to see what he must throw away, or perhaps re-test and try to save some. It just means tubes with small issues, will leave the factory still, and there is this excuse saying "we replace them" if there is a problem. However, in real life, the tubes can be on stock with dealers, and/or pass several persons before YOU get them, and in the end the last seller or the buyer, is the looser.

It is also for this reason, inferior brands, like the Russian, refuse to hand out incoming inspection criteria. They just say the tubes are "good" and "well-tested". You may even find data on the KT88 box, saying: IP=44mA, Gm=10.000 and that's it. This is matching data only. Though matching and quality are two fully different things. For this reason, we use the old G.E.C. datasheet for the KT88 which has such data called "Characteristic". This is not the normal user data, because Characteristic is at 250V. Nobody uses KT88 at 250V. This is just a relatively low voltage, because this is for incoming and outgoing inspection. The advantage of 250V is also, you can now use the KT88 at higher plate current as normal, and yet not reach maximum plate dissipation. We all know that at lower voltage, it is hard to pull a lot of current. (And vice versa, low current at high voltage is easy to do). For this reason, KT88 should be tested at 250V (Plate + Screen) like in the G.E.C. datasheet, and then you should be able to pull 143mA as it says there, at a control voltage of approximately 15V. And Transconductance should be 11.5mA/V at 143mA.

To make it easier, you can use the auto bias circuit, so you don't have to worry about the control grid voltage anymore. This is our test card Nr: 80-080-67 for the Russian L3 tester. The idea here is, when a USED tube cannot supply 140mA in Auto Bias anymore, it has probably low emission. Of course, it MAY also be such a tube which by default has a strange bias setting and not low emission at al. (So a problem with internal geometry - See the next point below here, under packaging). However, such a tube can be easily detected, because an internal geometry (drop damage) problem leading to a lower bias value will also lead to a higher Gm value at the same time. So this is really easy to see. If it is like this, you rather have a manufacturing issue. Would it be reversed, so too high bias, and too low Gm, it is either a shipment drop problem, or a gas problem. Whereas a gas problem can be seen from the gas cloud, and also too much grid current. If both are too low, it's rather a low emission tube. If both are too high, is actually unlikely.

An experienced tester will also observe the speed at which a cold tube begins to develop plate current. KT88 has a massive cathode. So when you plug a cold tube in the L3 tester, you will see nothing happen for a time which is much longer as with smaller tubes. This unusual long delay means nothing. It can take some time before the tube is only at 20% of maximum. However, when plate current BEGINS to rise above 20%, it can be said, the faster it rises from now on, the better quality the tube is. This applies until you reach 80% of normal. The range from 80% to 100% takes longer with all tubes. On the other hand, it is also good, when 100% is reached faster. Particularly such tubes with not extremely strong emission, benefit from the higher heat development inside the anode, which back-radiates also on the cathode, so emission improves by greater heat. This takes several minutes. Whereas tubes with extremely strong emission don't need this, and reach 100% level a lot faster. This difference can be amazing, but you need to work yourself into this, and learn to understand it from testing many tubes. I cannot compress this in a few lines easily. Very short: The additional heating up of the cathode, by back radiation of the anode, can be seen on the L3-3 when the KT88 is DC heated. The higher temperature will raise the resistance of the heater wire, and you will see the 6.3 Heater voltage rise a little bit, because of this. Of course you need to re-adjust the 6.3V before final measurement, but it is just this rise which indicates the heat development.

So you see here, the Auto Bias card tells you right away very much about the tube, when looking at the right things. Particularly the analog effect, so the way you see the bias change, you cannot replace that with any digital testing.

While story telling here.... I remember when I was 15 years old, I took my old Radio Tubes to the local radio shop, and the man had a tube tester at the counter. It cost 25 cents per tube, which was the price of portion of French Fries. So you would think twice before testing 20 tubes. He could see at the speed at which the needle went up, if the tubes were weak or strong. Of course we waited until fully hot, but when he said: Oh, that's a good one, just by looking at he speed at which the needle moved, he was always right. So I adopted that method from this experienced old man. There is no absolute number for this, it is also tube type and brand dependant. So just compare some strong and weak KT88 of the same brand, and you will see what I mean.

THE ADJUSTABLE TEST CARD . (Order nr: 380-072-55)

This card is not so much for tube quality testing, but rather for matching. If the tube will auto bias nicely is more which you need to know for an amplifier, but the proof of that is given with the auto bias card. However, the auto bias card will (and should) bias at a fixed position. With the adjustable bias card, you can precisely adjust the bias current, and now the question is, do two tubes have the same Transconductance at the same plate current, and does THIS do, what YOU want to know.

If two tubes have different transconductance at different plate current, this is normal. So also the auto bias card will also tell this.

Normally the KT88 is used around 350...500 Volts, and few tube testers can do that. Besides there is no use to test at higher voltage, because chances this equal the amplifier's setting for voltage and current is low anyway. Unless somebody can say the tubes have to be tested at 350 Volts, 85mA. This is not real life.

In reality, rather somebody says, I need a matched quad for my "Balanced Audio Tubes" BAT-7 amplifier, with best sound there is. So we have to match tubes for an unknown working point anyway. Can this be done at all? The answer is. Yes!

First, you need to make sure the tubes have good emission. This is found out NOT by fiddling around with the adjustable bias card, but by using the auto bias card, or with the special EMIOSSION CARD, which I plan to make later. If a tube fails the emission test, or auto bias card, it's a reject, and the following does not apply.

So provided, the tubes have good emission, you test them for plate current at working point "A". This working point means: fixed plate voltage and fixed Screen (Ug2) voltage. You can test the for instance at 300V 120mA. Then at this working point you find a transconductance (Gm) and a control grid (Ug1) voltage needed. Now provided you have two tubes with the same transconductance, later on these will be used at (say) 390 Volts 90mA in the BAT-7 amplifier. So how will you be sure of matching? First question is: Will the plate current be the same? For this, the transconductance at 300V plays a role. If it is an identical 11mA/V for both tubes, the auto bias circuit of the BAT-7 will provide the same grid (Ug1) volt for both tubes, and so the tubes will bias the same. This is of course a very important thing, and it is good to know this. Next question is, the transconductance at 350V 93mA will it be the same for both tubes. To answer this question, you need to investigate how much the tubes change their transconductance when going from one plate current to another. If that is the same too for both tubes, confidence these tubes will bias perfectly in the BAT-7 is very high.

Tesla Factory Cards

I once got hold of a box with 20 un based TESLA KT88, given to me by the ex-operator of the KT88 test machine of the Tesla Factory in Prague. This person left there 20 years ago, but he still had that box. In there were copies of the original factory test procedure, which is of course extremely valuable information!

WIth this information I made one card for the cut off test. Learn from this, they did that test. The second card is for the transconductance test they were doing. Note, these vintage TESLA KT88 looks identical to the MO VALVE KT88.

CONCLUSION

We rarely know the real operating point in the amplifier. So the requirement is actually: The tubes must show good matching at a RANDOM operating point. This is a very hard requirement. To ensure this, the tubes should show good match at TWO different operating points, and you are usually safe. Suppose we only test for operating point "A". In that case, we can assume good matching in Operating point "B" as well, provided the grid voltage (Ug1) at Point A was the same for both tubes, as in case 1. If you find differences in Ug1, you should do a second operating point "B", and you can get CASE 2 or CASE 3 then. When matching a greater number of tubes, the tubes 3 + 4 should not be paired. For a small number of tubes, you may match them when it appears like in CASE 3.

PACKAGING

What packaging? KT88 is packed with almost no boxing at all. It's crazy. Though grid to cathode short is a known problem with KT88, it is a miracle this doesn't happen even too often. However, there is another thing to mention. You should try this with a KT88 that you want to throw away. If a KT88 is dropped, the plate current will go up and the transconductance will go down. This is cause by deformation of the internal structure. Also, the distance between grid and cathode is small, and the grid is very fine, so small particles can get in between and cause a short there.