Note: Note, this page is also linked from the Emission Labs factory page, and in that case we only refer to such amplifiers that can use the Emission Labs tubes, like 2A3 and 300B, etc. For other cases, this page also gives information about other tubes than Emission Labs, like KT88, 6550, etc.

List of amplifiers that need special attention
(Last updated: 06-Jun-2018 23:35 )

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If a problem happens, most of the time we see tubes with signs of overload, or tubes that stopped working just after a few hours. The root cause of a problem can not be concluded very good by observations, or by comparing with other tubes. Same as a good engineer does not design a product by trial and error.

For instance, somebody has broken tubes in a 220V amplifier, which was connected to 240V mains, and he never had a problem, until he replaced the tubes. Logically he will say, 220V or 240V, was never an issue. When we analyze this, it means he expects tubes to work fine in a 220V amplifier, which he connects to 240V. He feels this should be possible, because it never went wrong. At the same time, he presents how this caused a problem, so it DID go wrong.

In short, for quality we expect two things from the tube, that is LIFETIME and FAILURE FREE operation.

This following is a quote by Jean Claude Verdier. Yes, the Jean Claude verdier, who runs the VERDIER amplifier company in France.

..... Satisfaction is most often based on a simple criterion, explained as follows: "Some years ago, I bought an amplifier, it is good and it works well" -or, unfortunately: "The amplifier I bought is not good, because it always fails." Read the Original article here



What we need to do in problem cases. We must look at all possible factors.

Wrong conclusions: In case you see a particular problem with one tube of a pair or quad, the normal reaction is, you will try this tube in other sockets. If you do so, it may happen the problems shows still with this one tube only. Now you may observe "the problem follows the tube", and you draw the conclusion "it must be the tube". This can be a bad mistake! It can happen, that three tubes of a quad will forgive an amplifier problem, and one tube will not. There is no "must be" that a tube has to forgive amplifier problems. There is a hidden and wrong expectation, if brand "A" forgives an amplifier problem, and brand "B" will not, that in such a case brand "A" must be good, and brand "B" must be bad. If you use the tubes the right way, for a change, perhaps brand "B" might even perform better.

The tube information.

Regard the tube data sheet a fact sheet, and not just an advise. In some cases an amplifier can have extremely high in-rush current during switch on, like four times average current can happen. In-rush current with a cold tube should be minimized. There are ways for that. specially with KT88 it can happen in such a case that a develops a spark inside. Of course, the experiment is repeated by the owner great many times. It will be always that one particular tube, and it is plugged into all the sockets. If he has more amplifiers than one, the tube will be passed around those other amplifiers as well, to see what happens. Of course the problem "follows the tube", but it may well be you are step by step destroying a good tube. After 50 or so times, this tube has lost some some emission, and in addition develops a risk to short the grid to the cathode. The loss of emission, in a push pull stage (typical for KT88) will now cause over load of the other tube of the pair. So after a while it will also have lower emission. Latest by now, the owner gets upset, puts back in the old tubes (that worked fine always) and will refuse to accept his own behavior as a mistake. Perhaps even so, you need to read back this paragraph again, because you also did not recognize the mistake. This becomes more difficult as the amplifier manufacturer will never admit a failure with his design.

The amplifier information.

With this we mean two things: The schematic, and the instructions how to exchange tubes. Schematics are usually kept secret. Please believe me, there was no new schematic invented since the OTL by Julius Futterman in 1942. There is no 250 people, not knowing what to do, and waiting for your "new" circuitry to copy.

However when I look into a problem in great detail, the situation reverses most of the time, and we find a failure often in the schematic. Then again, we have the discussion like, "we are doing this since 30 years... and we never had a problem". However it is a fact, when the 5U4G rectifier datasheet says, the maximum capacitor is 47uF, you should not use a higher value.

When we suspect an error with the amplifier design, specially then, we get send no schematics, and no support. Well, we are used to it. Luckily the internet is full of schematics.

A good diagnosis requires experience. Not tube rolling. My honest judgment is, that with amplifier tubes 50% of the tube problems could have EASILY been avoided by better amplifier design. I think manufacturers should put more effort, into making it difficult for users to do stupid things. With rectifier tubes that percentage is even higher. Reason is, rectifier tubes schematics are so easy, but choosing the right values is so hard. Yet even with wrong values it initially works. Even when a mistake is obvious, like using a capacitor that is too large, they do not admit it, and simply continue making the product that way.

Another risk cause, is mixing tube electronics with semiconductor electronics. Though this is possible, it was never a break through.


When replacing tubes in such an amplifier, it may damage the new tubes if there is an adjustment procedure for the amplifier, and you don't use this.

To find out if there is a procedure, only ask the manufacturer, and nobody else. If such a procedure excists, and you fail to use it, destruction of the new tubes can result. The tubes can short circuit with a bright blue or white spark from the inside, with or without speaker damage. Even so, not all amplifiers are short circuit proof, and need a repair afterwards.

If the amplifier has hand control of the bias, for each tube, we sometimes see the following mistake is made:

MISTAKE: The user replaces all tubes, switches the amplifier on, and starts to adjust the tubes one by one. When such an amplifier has 6 or even more tubes per channel, you may be too slow adjusting, and one of the tubes will damage before you have a change to adjust it. So at least, you need to set the bias for the tubes to a lower value, before you begin, and use the old tubes for this, to see how it works. Some amplifers can damage when you set all regulators to minimum, because the internal voltage may become too high. So this is no advice how to to the settings.

GOOD ADVICE: Please get the official service procedure from the manufacturer, and do not experiment without it. Be cautious when the procedure is secret. Really an amplifier can be made such that you can exchange tubes yourself, but this seems not always wanted. So any amplifier with a secret schematic, and a secret procedure to exchange tubes, we advise you not to buy such an item.

Heater Voltage.

It is amazing, and sometimes shocking, to see how private people but just as well very well known companies, throw advice of tube manufacturers in the wind. The most common mistake is invent some self made stories, about why heater voltage is not so important. Whereas every SINGLE percent deviation will impair lifetime a lot. Also some theories about over heating or under heating, this is for the birds, I do not even read it.

AC Heated amplifiers.

These all have a problem with the mains voltage, because today the quality of the mains voltage is terrible. The electricity companies increase the mains voltage slowly over the years. The higher the mains voltage the more power you use, and the higher the profit. Also, the higher the voltage, the less loss they have in the power lines, which means also more profit. Whatever the reason, they LOVE to increase your mains voltage. So 220V was officially and unofficially increased to 230V, and 240V is coming up soon. In my house I had 243 Volts already at several moments, and I had good Hewlett Packard equipment failing from this with blown out protection capacitors, as they were internally set for 220V.

The problem is, tube heaters can take maximum 5% voltage tolerance. So your good old 220V apparatus, at 243 Volts is overheated by 10% which makes it a real tube destructor. Better set it to 240Volts at the back. If it is too old, and doesn't have that option, buy an adapter transformer from 240 to 220V. Or, exchange tubes 2...3x more often. (Yes that much). These are the only options.

DC Heated amplifiers unstabilized.

These have the same problem, when the heating is unstabilized.

DC Heated amplifiers stabilized.

The the best way is really to have electronically stabilized heaters inside the amplifier. The episode where everybody was shouting that AC heated tubes sound "better" is forgotten now. Instead of that we hear now similar shouting that "Current source" heating sounds better. But I wonder what is the sound of a current source like. Main thing is, you keep the heater VOLTAGE constant, regardless the mains voltage.


Application Note AN07. AN07-How-to-bias-DHT-tubes-without-mistakes


The following table, is some personal experience, how to best deal with this model amplifier. We write it here, to make sure you use the products in the best possible way.

The amplifiers are placed here in alphabetical order:


A problem is, the schematics were not available. I was given a hand made schematic by a person, who helped himself. Imagine, he is not the only one with that problem, but perhaps the only one that draws the schematic before he begins. Later I was given this Schematic here

From this schematic it gets clearer what you need to look at. There is a phase reversal stage in here, with two ECC82, and it has an adjustment potentiometer. Adjusting this stage is "overall" so it also affects the EL34 stage, which is drives. I would say, the best way is adjust this with an oscilloscope, so the El34 output stage is driven nicely symmetrical at full power. You need to do so, after bias adjustment. Now comes, what I think is important to know. These ECC82 tubes are Electro Harmonics tubes with the Air Tight logo stamped on it as well. Generally speaking, Electro Harmonics tubes have larger parameter variation than other tubes, and their ECC82 seems to draw less current than an average ECC82. So I would say, EHX is not an ideal tube for this. However it was chosen, and since the amplifier is adjustable it doesn't matter any more. Since the adjustment of the amplifier is an adjustment for the individual set of tubes, the best way is to have the amplifier re-adjusted after a tube change. Furthermore, since two random ECC82 can be quite different, you should never exchange those amongst each other by mistake. Needless to say, this can happen quickly! So the recommendation is, when you have those ECC82 replaced, you should always take what is called a balanced, matched pair.

Something similar applies for the EL34. Each channel ( Left and Right) has it's own balancing potentiometer, so in the end you'll end up with different settings for those two potentiometers. Once that is the case, you can NOT exchange the EL34 amongst each other. So the moment you pull them all out, you should mark them carefully 1....4.

This said, it is obvious, you can not just replace the ECC82 and EL34 by "some" tubes. The EL34 (Left and Right Channel, are controlled by the SAME bias voltage, so for that reason you can only put a matched QUAD in this amplifers, and not two pairs.

Overall this is a really very good schematic, and once adjusted correctly, the EL34 stages should work ultimately good. Apart from what others say or do, my opinion is, best results will only come from a proper adjustment by using an oscilloscope.

Disclaimer: This schematic was hand drawn from an ATM-1 and it may have mistakes in it. So no guarantee for the drawing being correct.


The issue is, sometimes a tube can violently spark inside the bulb.

This amplifier is DC coupled. With such concepts, you always need to understand which tubes warms up first, and what this does to the bias of the next tube. A good designer make a worst case design. Like try things with a very weak, but "just ok" 12AT7, and a 2A3 of which he knows it is a very strong one. That will sure "spark" a surprise sometimes, since the bias voltage of the 2A3 depends on the plate current of the 12AT7. What happens at warm up, the 12AT7 tube heats up slower than the 2A3, this can not be changed. The result is, the grid voltage of the 2A3 for a few seconds higher than normal, totally undefined, and even POSITIVE in many cases. So you are only lucky if your 2A3 is not working yet at that moment. Most of the time though, the 2A3 will already be able to pull current. The better quality and the newer the tube is, the more current. Some tubes will draw so excessively much current at POSITIVE grid voltage, you blow off a chip from the filament, and the tube will "fire" with a white spark.

1) Amplifier Version with without the soft start.

  • A white spark is always (no exception) an amplifier issue. So the problem we see here, is not related to "EML tubes only" though we heard some amateurs say this, because "El Cheapo" tubes seldom fire. Even so, tubes that do not fire, still a suffer a lot. I would say a NOS Western Electric probably will fire as well. Even some vintage Osram and an RCA data sheets mention the "firing" as a amplifier problem. If you fire a Western Electric 300B, it is just as bad for this tube as for an EML.
  • A positive note, this firing is only bad when you repeat it too often with directly heated tubes.

2) Version with soft start option. This basically solves the spark issue with this amplifier, and this is a good, and mature product. However if you want to switch the amplifier off an on again, you need to wait 20 minutes in between (or DAMAGE will occur). Don't forget this! Actually this is normal with many products. The only thing is, here you need to wait 20 minutes which is a long time, when you are waiting.

3) Heater voltage of 2A3 tubes. It seems the heater voltage of the paramount is too low. 2A3 must run on 2.5 V heater, tolerance on that is 5% for all brands 2A3. The heater voltage was reported 2.32V on the chassis inside, by one of our customers. This is by itself almost 8% off, and already below minimum. Moreover because of 50mV voltage drop over each heater pin contact, there is another 0.1V drop. So with 2.32V at the socket solder connections, there is only 2.22V on the tube pins itself. It is recommended to have 2.6 Volts at the tube socket solder connections. To correct this, you can add an electrolytic capacitor, directly on the tube socket heater pins. Here is the text from Bottlehead company: "This capacitor should be rated at 6.3V or more, and sized around 4,700uF to 10,000uF" . The recommendation from Emission labs is, use 10.000 uF.


  • Guarantee on EML tubes if used in the Paramount without soft start option:
    You need the upgrade kit for this, to add the soft start. (Also read the next line).
  • Guarantee on EML tubes if used in the Paramount with the heater voltage below tolerance:
    No. (see above text). In case of a tube problem due to under heating, please contact us by email. If EML, chances to repair them are high.
  • Guarantee on EML tubes if used in the Paramount with soft start option + correct heater voltage: :
    Yes. (Thanks Paul Joppa from Bottlehead for the good support)

Note from EML: We recommend to retro fit all paramount with the Soft start kit, and also adjust the heater voltage of the 2A3 such that you have 2.6V DC on the solder connections of the tube socket, for the tube filament.

General link to EML factory page about what causes a white spark: Read more here and here

CARY. Very good and much recommended amplifiers. They push the 300B or 2A3 tubes close to the limits, which is of course possible, but you need to be aware what it does to the tubes, and lifetime in general.

We recommend following:

  • Use EML 300B-XLS for the amplifiers using 300BUse EML 2A3-S for for the e amplifiers using 2A3
  • If you want to use EML 2A3-mesh, you have to use those that are selected by us for a grid voltage not more negative than -47V.(So a tube of -48V should not be used). This is the voltage that is written on the outside of the boxes. Please ask for such a tube, and you are fine with the CARY. We supply this service to you at no extra cost.

Kondo (Japan): Model KSL KEGON The issue is, the Parallel Single Ended output Tubes are hard wired in parallel, without a way to check and adjust the bias for each tube individually. This is very unwise to do, it assumes things about tubes which are not realistic.

This amplifier hard-wires two 300B tubes in parallel. (see schematic) So, without any precautions whatsoever, the terminals of the 300B tubes are connected in parallel, and that's it. This is dangerous with random tubes. Since this method was used here, we need to see how we deal with this. It means, this amplifier will only work safe on a pair 300B tubes with special selection methods, and definitely can NOT work on unselected 300B tubes, and also not on just any matched pair. What you need here is EML 300B tubes with factory tested grid voltage from -56V.... -60V, by the Emission Labs test method.

So how is the Adjustment done? Well don't ask us, ask the manufacturer! From hear-say we expect you could measure the DC voltage over the Output transformer primary, and derive the plate current from that.

Click here for a large picture. In the picture are the instructions. No guarantee on anything. So here is only how I would do it myself if I would have a KEGON here.

We give only guarantee when you use EML tubes like this. Another method, or another brand, it means guarantee will void. So please take care of the above, and you will love the results and the wonderful sound of this amplifier!

A SERIOUS WARNING I have seen a 1991 circuit diagram without bleeder resistors on the High Voltage Caps. (!!!!!). This is extremely dangerous when you open the amp, even when "off" the power supply caps can be charged still. This is unlawful in Europe, and against "CE" regulations.

All together this amplifier is regarded dangerous, and a potential problem maker. It should be serviced only by experienced people, and it needs special help tools to check the bias of each tube INDIVIDUALLY. We talk here about the JAPANESE KONDO, older version.

Kondo (Japan): Model NERO

This amplifier hard-wires two tubes in parallel. It is generally the wrong to do this. We need to deal with some difficulties. It means, this amplifier will only work safe on a pair 2A3 tubes with special selection methods, and definitely can NOT work on unselected 2A3 tubes, and also not on just any selected quad, of which the seller says they are matched very tight, but in reality he did not measure grid voltage at the NERO settings. So ask him for these values of the table below here, and if he can not give you this, these tubes are not what you want. IN THE NERO, you need a quad of matched EML 2A3 tubes with specially tested grid voltage. First, you need to measure the gird voltage of the tubes, with a voltmeter. Voltage is measured against ground. This is in the working amplifier, with the old tubes inserted, presuming they still work. If so, do the measurement and let us know. For legal reasons we must say: Do such a measurement only when you are sure you are qualified for it. If not, let a good technician do this for you.

We use the following table:

Tube grid voltage tested with DC Heating.
Amplifier setting
40,75... 41,75
41,25... 42,25
41,75 ... 42,75
42,25 ... 43,25

42,75 ... 43,75

43,25 ... 44,25
43,75 ... 44,75
43,75 ... 45,25
44,75 ... 45,75
45,25 ... 46,25

There are a few strategies to replace your tubes: 1) The wrong way is: Just replace the tubes, and check nothing else. This may work well by coincidence, or damage the new tubes. This will particularly go wrong, when the previous owner increased the tube settings, in order to use partially worn out tubes, or use badly selected tubes in this amplifier. The new tubes will be overloaded by the increased settings, sound good for a while, but soon wear out, and you are back to where you started. So this amplifier is NOT Plug & Play when you replace tubes.

2) You measure the amplifier grid voltage, so we know the grid voltage you have, and we can send just the right tubes. This is the best way. You don't need to change the amplifier settings, and also you can swap with your old tubes too if you like. 3) Suppose the amplifier settings are outside of the of range of this table, it means somebody adjusted it for a curious set of tubes. In this case you can order a quad of normal, good tubes with values from this table, and you have to re-adjust the amplifier grid voltage for these values. After the adjustment, you can not use your old tubes any more, only the new tubes.4) You want to adjust the amplifier for unknown tubes. This requires a measurement of the plate current, which is not provided in this amplifier in a logical and normal way. It can be done, but it is no do it yourself job. After the adjustment, you can not use your old tubes any more, only the new tubes.5) If you don't have the technical skills, or just trying to learn this, you should not practice this on your own. Bring the amplifier to a tube technician, and give him this procedure. For the NERO we recommend EML2A3-S because this tube is very forgiving with accidental overload. If you want to replace the tubes by NOS or by EML 2A3-Mesh, you need to make sure you understand the above procedure very well. We give only guarantee when you use tubes under the above conditions. Otherwise, guarantee will void.

So please take care of the above, and you will love the results and the wonderful sound of this amplifier!

Manley 300B Preamplifier "NEO-CLASSIC"

this is not a complaint or a problem about this very nice pre amplifier. It is just we want to point out here, you can not operate this amplifier with EML 300B-Mesh tubes. The mesh tubes are recommended to bias at Maximum 28 Watt, and it seems this amplifier runs the tubes above. It is a bit high for a pre-amp, but fair enough this is Manleys choice. So we recommend normal EML 300B, and since this seems an pre amplifier on "steroids" also the 300B-XLS is a good candidate.

Mastersound 300B-SE

this is one of the best 300B amplifiers I know. They are build around a decades old, very mature schematic, and the output transformers are great quality. The later types have electronic regulation for the heater voltage, making this amplifier very universal. This deals better with mains voltage variation, and allows other tubes than classical 300B only. However, they do need tube adjustment when you replace tubes. Failing to do the adjustment, may cause problems. You can check here, for some more information, about tube exchange.

Mingda Meixing

this amplifier loads the 274B tube with an insane value of 100uF capacitor. This is absolutely not allowed by any tube data sheet. The 274B data sheet says maximum 4uF. Then, after the choke comes ANOTHER 470uf, and after this via a 2k resistor ANOTHER 470uF. So you see we put a big question mark behind this, Sure the second one charges via a choke, but it DOES charge also, and also the third one charges via the resistor. The current adds up one way or another, and cold start current of the 274B is heavily exceeded. This can cause the tube to spark, and may cause tube defects sooner or later. You can check the amplifier's schematic at, this is an external link.


Issue: Some of their amplifiers use these metal rings which are positioned too high. We have seen socket contact problems result from this, as tubes do not go inside as deep as they they should.

"Just not" glass to metal contact
Here you see glass to metal contact

Also the glass contact to the chassis is a potential problem. The customer who owns this amplifier contacted the manufacturer, and the reply was, there is no support. Reason is, as they say, there is only one reference tube, and that is Western Electric. This answer was typical, and is not accepted, because WE never defined the diameter of the glass, where it comes out of the base. When you let the glass disappear into the amplifier chassis, we say this is wrong to do, for power tubes. Even when they fit in, this can also cause failures with WE tube. It heats up the socket from the inside, and it can cause problems.

So Silbatone offered no solution. On my advise, the customer solved it himself in less than five minutes. For this you can remove the 5mm distance holder, that you will find at the inside of the amplifier. This will bring up the whole tube socket 5mm higher. The ring can stay, and the problem was gone. It even looked nicer afterwards.

GLASS CONTACT to the chassis is definitely a problem. This leads to mechanical stress in the weakest spot of the glass bulb, which is where it goes into the tube base. There is a temperature difference there, also the glass is sharply curved there. This is normal with all tubes, but if we do see a crack in the glass, it us usually here. So particularly chassis contact to the glass must be avoided.

VAC 30-30

Good amplifier, but it does put quite some stress on the 300B tubes.

We are in contact with the manufacturer to see if the 1.5 Ampere 300B-XLS tubes can be used here. Probably the answer is going to be yes. We are waiting for a confirmation. Please ask them yourself if you need to know.

VTL 185

Adjustment range is not very high, and (good) new KT88 tubes may draw not enough current. When replacing with Svetlana S-logo, Sovtek or EHX, it is recommended to use KT88 that show test values much above 60mA, by the factory test method. For other brand tubes we don't know the limits, but you need the higher current ones here as well.

VTL. Model: ST 150

Adjustment range is not very high, and (good) new 6550 tubes may draw not enough current. When ordering Tung-Sol 6550, it appears you need tubes that are tested above 60mA, by the factory methods. The same values apply for EHX 6550.

Welborne Apollo

First let us say here, this is an excellent amplifier, much recommended! The issue we see often, users replace want to replace the original KR842 tube, by another type. Please take good care when doing so, because different tube numbers always indicate the tubes are indeed different. So by ignoring this you can have problems with the tubes or the amplifier.

1) Running EML520B-V3: (not EML520B-V2) This amplifier can not run on EML520B-V3 without changes, or at least verify the result. You need to take care of the following:

  1. This tube draws comparable filament current to KR842. So the filament circuit needs no changes.
  2. When replacing by EML520B-V3 you need to verify if the voltage over R20 is 100V +/-5Volts. You need to change R20, such that you get this value.

2) Running EML320B-XLS: This amplifier can not run on EML320B-XLSwithout changes, or at least verify the result. You need to take care of the following

  1. The filament current of EML320B-XLS is closer to KR842 than EML300B-XLS. Though still not exactly the same. So the one can only replace the other, if the filament voltage is at least checked by the user, and adapted if needed. In the Apollo, the filament circuit is unregulated, which is fine, but it means the EML-320B-XLS would perhaps run at too high filament voltage. This should be adapted to exactly 5V by changing Resistor TR1
  2. When replacing by EML320B-XLS you need to verify if the voltage over R20 is 85 Volts +/-5Volts. You need to change R20, such that you get this value.

3) Running EML300B-XLS:

This amplifier can not run on EML300B-XL without changes, or at least verify the result. You need to take care of the following

  1. KR300B-XLS and EML300B-XLS have another filament current, so the one can only replace the other, if the filament voltage is at least checked by the user, and adapted if needed. In the Apollo, the filament circuit is unregulated, which is fine, but it means the EML300B-XLS would perhaps run at too high filament voltage. This should be adapted to exactly 5V by changing Resistor TR1
  2. When replacing by EML300B-XLS you need to verify if the voltage over R20 is 85 Volts +/-5Volts. You need to change R20, such that you get this value.

Important Note:

In the first place, you need to use the EML tubes within the specified data sheet limits. Also the amplifier technical limits may not be exceeded. If the above raises any questions to you, please don't go for this. In that case have it done by someone who has more experience.


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