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

If a problem does happen, most of the time we see tubes with signs of overload. After this comes crack noise or hum, and contact problem. The root cause of the problem is not per definition related to what you see and hear. Even more difficult, you see clearly "A" but the root cause is "B". For instance replacement tubes have crack noises, and the old tubes were fine. Some are too quickly blaming the new tubes. Then after a while it appears we have a 220V amplifier connected to 240V mains. If people will ask, 220V, 240V, is that so important? This means they expect tubes which work fine in a 220V amplifier connected to 240V.

Would you expect from your own dentist, just do something quickly that makes pain go away for the moment, and then tell you this was a professional treatment? Same with tube problems! Just do "something" unprofessional that make you not hear the problem any more, doesn't mean it is gone. Since tubes are devices with a limited lifetime.

Pease read these statements carefully. It is to understand under which view this article is written. Amplifier manufacturers find Statement #1 very important, and tube manufacturers find statement #2 very important.

  1. A tube which works not in all amplifiers is not an ideal tube.
  2. An amplifier which works not with all tubes is not an ideal amplifier.

So what we need to do in problem cases, look at all possible factors. Make sure you do not forget an important one, and do not focus on something small, and unimportant.

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, that if brand "A" forgives an amplifier problem, and brand "B" will not, that in such a case brand "A" is a better brand. Specially if NOS is compared with new production, people LOVE this erroneous conclusion.

EML tubes and all tubes are designed to last longest if you use them the right way. The opposite is, a tube will punish high surge current at start up by giving less lifetime. To treat this careless, is the most common mistake with amplifiers, great brand names just as well as the low level ones.

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 must be avoided under all circumstances. Specially with KT88 it can happen in such a case that one tube out of a quad develops a spark inside. And of course, the experiment is repeated by the owner many times, under careful observation. It will be always that one particular tube, and it is plugged in all the sockets, and of course the problem "follows the tube", but... the problem is the amplifier. After 20...30 times, this tube will short circuit, self-destroy, and by this destroy the other tube of the pair as well. So the owner by default gets very upset, puts back in the old tubes (that worked fine always) and will refuse to understand the mistake with the amplifier. Specially since the manufacturers logically speaking can not admits the mistake. However when I look into a problem in great detail, the situation reverses often.

So to prevent us to understand if an error is made in the amplifier design, we get send no schematics, an no support. Well. we are used to it. Luckily the internet is full of schematics, it is hard to hide those.

Another situation can be, a quad or an octet works fine for some weeks, then the first tube develops some problem, then another comes with the same problem, and so on. Also here, the wrong conclusion "it must be the tubes" is the most obvious one. Well, not always problems have only obvious explanations.

So all in all, a good this diagnosis requires some experience.

My honest judgment is, that with amplifier tubes 50% of the tube problems are in fact caused by careless amplifier design, and with rectifier tubes that number is close to 100%. Reason is that rectifier tubes schematics are so easy, but choosing the right values is not. Even when a mistake is obvious, like using a capacitor that is 5x too large, they do not admit it, and simply continue making the product that way. Another classical generator of problems is mixing tube electronics with semiconductor electronics.

The following table, is some personal experience, how to best deal with this model amplifier, and we write it here, to make sure you use the products in the best possible way. The Amplifiers are in alphabetical order:

BOTTLEHEAD PARAMOUNT

Version with without the soft start. This amplifier is DC coupled, meaning the bias voltage of the 2A3 stage depends on the plate current of the 12AT7 driver tube. So what happens at warm up, a 12AT7 tube heats up slower than the 2A3, as we all know. The result is, the grid voltage of the 2A3 for a few seconds is undefined, and not negative enough during that few seconds. Perhaps even the 2A3 tube sees positive grid voltage for a short moment, since a white spark has been reported at start up. A white spark is always (no exceptions) an amplifier issue. Also an Osram and an RCA data sheets mention this.

Version with soft start option. This basically solves the spark issue with this amplifier. However if you want to switch the amplifier off an on again, you need to wait 20 minutes in between If you do not do so, you can damage the tubes, since the soft start circuit is not working otherwise. Also a short power off at your house, can result in broken tubes that way.

  • Guarantee on EML tubes if used in the Paramount without soft start option:
    No.     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 soft start option:
    Under discussion in 2011 with the Bottlehead company.

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

CARY

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 of what that does to tube lifetime in general.

We recommend following:

  • Use EML 300B-XLS for the amplifiers using 300B
  • Use 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

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!

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. Today the "CE" qualification does not allow this any more.

Kondo (Japan): Model NERO

This amplifier hard-wires two tubes in parallel. So, without any precautions whatsoever, the four terminals of the 2A3 tubes are connected in parallel, and that's it. This can create severe problems. 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 2A3 tubes with special selection methods, and definitely can NOT work on unselected 2A3 tubes, and also not on just any selected pair, which someone's says are " selected very tight", but it is not targeted for what we need here.

Note, that there are two methods to match tubes.

Method1) take a fixed grid voltage, and measure the plate current. This is most commonly used, because it works fast. With the 2A3, you would use 250V Plate, apply -45V grid, and then measure a plate current, of 40....80mA depending on the tubes, and write that value on the boxes. After that match the tubes in pairs. So the test set up is static. You plug in the tube, wait for the warm-up, and then measure the plate current.

Method2) Take 250V PLate voltage, apply a variable grid voltage and hand-adjust it until you get the required plate current for this tube, which is 60mA for a 2A3. So you get a grid voltage of say -40....-50Volt and we write that on the tube box. After that match the tubes in pairs.

Method1 is the common method, because it works fast. It is done by most manufacturers, like EHX, Sovtek and by the Chinese. At EML we prefer method2, and by coincidence this is just what you need here to get a safe matching for your NERO.

IN THE NERO, you need EML 2A3 tubes with factory tested grid voltage from -43 to -47V.

Then use following table:

Factory test data
Amplifier setting
-43V....-44V (values for one pair)
-44,5V
-44V....-45V (values for one pair)
-45,5V

-45V....-46V (values for one pair)

-46,5V
-46V....-47V (values for one pair)
-47,5V
-47V....-48V (values for one pair)
-48,5V

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!

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 www.4tubes.com, this is an external link.

SILBATONE Acoustics, KOREA

Some of their amplifiers use these metal rings which are positioned too high. We have seen contact problems result from this, as tube do not go inside as they should. Also the glass contact to the chassis is a potential problem. We want to say here that tubes that fit mechanically into every Western Electric amplifier do not fit into every SILBATONE. To solve this problem, you can remove the 5mm distance holder, that you will find at the inside. This will bring up the tube socket 5mm higher, and the issue is gone

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.