This document gives some information you may need when replacing 52B tubes by 520B tubes.

There has been no other tube number like 52B that has been subject of so many changes, and variations. The original, genuine specification of the 52B was made by AVVT, and we at EML do not understand why another manufacturer simply changes the specifications, and bring a modified tube on the market. To make it worse, also AVVT has later on changed their own specifications, and so the whole "52B" specifications became a mess. For this reason we took our EML52 part number off the market, and we decided not to offer any more a tube under this part number. Reason is, we have no idea on what kind of amplifier the tubes are plugged into.

Also, we changed to the more rugged construction of the 520B, which was later on improved to make it withstand accidental overheating if not too long.

Note: Previous EML versions of 520B, all 52B, and all AVVT products were permanently damaged at accidental overheating even if just a few seconds. These would suffer immediate damage, and instable bias is the result. Though once overheated tubes appeared to work, these would fail later. We do not like to specify how much you can overheat the new versions 520B, since that should not be done anyway. It is just it can happen by accident during amplifier service, and you have just a fair chance this doesn't do damage if overheating is short only, and noted when it happens.

So now, wIth the part number "52B" being gone, this gives more sensibility with the end user, since it is obvious that 520B-V3 or 520B-V2 is another part number.

To say it again, in most cases you can not just replace a 52B tube in the older amplifiers of KR, VAIC and Mastersound. (These older types KR and VAIC were all made at Mastersound). The problem for end users is, that official tube replacement instructions do not always exists, and schematics are generally not handed out.

However the day will come when your 52B's are gone, and you may not know what to do. Still, a real "how to replace." guideline we can not give you here, but we can tell you as much as we know, and in all cases you need an experienced tube doctor to do the replacement.

 

Some small help table
ITEM
What does this mean?
Remarks
Are you a experienced with this? - Then read table below.
Experience means you did it before.
52B
Part number first used by Vaic-Valve in 1995. Later by KR. EML used to make it too, but we obsoleted the part number, nor the tube. This was to prevent further confusement.
52B is Subject to many changes in filament voltage an current. Great confusion under users.
EML520B-V2
6.5 Volt version of 52B, but all parameters are slightly different
Replacement used in older amplifiers
EML520B-V3
5 Volt version of 52B, but all parameters are slightly different
Recommended for new designs
EML1605
5 Volt tube. Larger than 520B, and a possible candidate to replace a 52B, BUT.... it is not the same tube. 1605 needs a the plate-to-filament voltage to be 500V or higher.
Not directly intended at 52B or 520B replacement, but in several cases this is possible still.
Replace a 52B.

 

ONLY FOR EXPERIENCED TECHNICIANS:

Disclaimer: These are minimum requirements that need to be done, regardless of all other things a qualified technician will have to do. (such as always make sure not to overload the new tubes, not even for a very short time) . We do not guarantee in any way that this process is free of errors. Please understand this not intended as a correct guideline for amplifiers we have never seen, not build our self, and we do not have the schematics of. It is just how we probably would do it our self in our own workshop.

You need to find out some things about the amplifier:

  1. Mark tubes old and new tubes LEFT and RIGHT, on the tubes.
  2. You should not work on the amplifier with the tubes vertically. This means, you need to make a construction to put the amplifier upside down, with the tubes inserted still.
  3. Some amplifers have digital ampere meters inside. These easily damage by over-current. Here is the standard modification, to prevent damage. Before you start, solder a fuse of 100mA inside one of the leads to the Ampere meter. A good place is, simply with one end soldered on PCB. So remove the lead from the PCB, and re connect it via that fuse.
  4. Connect a short circuited RCA connector to the inputs, and short circuit the speaker outputs with a piece of wire. This protects the output transformers against mistakes. For hum test, connect a head phone to the outputs instead, and put back in the short when you are done. This must be a low impedance head phone. If not available, connect an 8 Ohms resistor in parallel. Tube amplifers are short circuit proof when there is no signal input. However they can damage quickly, when you have no speaker connected. Never leave the outputs open, when you are servicing this amplifier.
  5. Make sure you do all these things at the one and only correct mains voltage. That is the one it will be connected to at the end user. That voltage may be another one as in your workshop. When you have +5% higher voltage, and the end-user has -5% lower voltage, we talk about 10% which is a not acceptable. So take care with this, and use a Variac.
  6. Check the filament voltage with the OLD tubes in the amplifier. If they are not glowing, you have defective electronics, or a broken heater. If the heater voltage on the left and right tube are not identical, they may have been swapped by the owner. If swapping does not result in the same voltage, these are replacements put in, without adjustments done. Be very careful when you notice this, as you have a wrong adjusted amplifier in front of you, and other mistakes are possible too.
  7. Do you know how to adjust the bias current? If yes, reduce the bias to minimum. If no, find this out first.
  8. If all is ok, with the previous points, you find the heater voltage to be 5V, 6V, 6,3V or 6.5V. Choose EML520B-V3 for 5V filaments, and EML520B-V2 for 6.5V filaments. Adjust any 6V or 6.3V heaters to 6.5V or 5V. Try what is possible, and prefer 5V, so you can use the standard tube, which is EML-520B-V3. If you can adjust only to 6.5V, use EML 520B-V2. If you can not adjust to either 5V or 6.5V, you need to change the electronics amplifier., so you do get the one or the other voltage. Do all of this with the old tubes, so you take no risk with the new ones. The replacements come in later, and you need to re-adjust the voltages with the new tubes inserted. First continue now with the next point.
  9. Does the amplifier have stabilized filament voltage, and a voltage change is needed, this is possible by exchanging parts. Does this amplifier have adjustable filament voltage, but it is unstabilized? Such as with many older VAIC, KR and MASTERSOUND? This requires special attention. First reduce bias current to minimum, with the old tubes still inserted. Then try one new tube at a time, and test filament voltage right at switch on. Switch off immediately when it is more than 10% off. Re-adjust and try again. When ok do the other tube.
  10. Now set the bias. If you don't know the correct value, you have a problem, but a good technician has ways to work around this anyway, and set the amp correctly still.
  11. Now set the hum adjustment pots. These are often well hidden, for instance behind little printed circuit boards which are upside down in the amplifier. So all you see is a tiny hole in a printed circuit board. On the other side is then the pot meter, invisible. Check these pots for good function, they can be broken. If it hums in any position set them in the middle for the time being, but you do need to get a situation where the amp responds to the setting. Final setting is done after burn in
  12. When you have the bias set, check again the filament voltage, because these often interact with each other.
  13. When you are ready, let the amplifier run without signal and monitor the bias carefully for 30 minutes. If it goes up or down a little, that can be caused by mains voltage changes. Re-adjust only when more than 5% off.
  14. Check bias now every 30 minutes for half a day. Re-adjust bias when more than 5% off.
  15. Check bias now every 2 hours during one day. Never let the amp run without supervision. Re-adjust bias when more than 5% off.
  16. Now comes 50 hours burn in during which time the amplifier should be stabile. Do not get mislead by any changes caused by mains voltage changing. This is why you should use a Variac. Check as needed, and do final bias adjustment, and final hum pot adjustment.
  17. Now comes a hearing test. If ok, the work is finished.
  18. Questions? Only when we can answer it quickly. The amplifier manufacturer is your help desk.

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