520B-V2 and 520B-V3 Data sheet. Emission Labs.

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? (Read table below)	It means you have replaced a 520B or similar tube before.	Read the large table, below.

52B	Part number first used by Vaic-Valve in 1995. Later by KR. EML used to make it too, but we obsoleted it.	Subject to many changes in filament voltage an current. Great confusion under users.
EML520B-V3	5 Volt version of 52B, but all parameters are slightly different	Recommended for new designs
EML520B-V2	6.5 Volt version of 52B, but all parameters are slightly different
EML1605	5 Volt tube. Larger than 520B, and a possible candidate to replace a 52B, BUT.... it is not the same tube. 1605 need plate to filament voltage of 500V or higher.	Not directly intended at 52B or 520B replacement, but in some 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, or a correct guideline for amplifiers we have not seen, do not know and have no schematics of. You need to be able to find out some things of the amplifier. These things are: Mark tubes old and new tubes LEFT and RIGHT, on the tubes. Make sure you do all these things at the one and only correct mains voltage. That is the one it will be connected to. That can be another one as in your workshop. When you have +5% off and the end-user has -5% we talk about 10% which is a unacceptable. So take care with this. Check the filament voltage with the OLD tubes in. A typical defect is defective regulators, and filaments are broken. Or left and right tube are not identical, and have been swapped unknowing by previous owner(s). Choose EML520B-V3 for 5V filaments, and EML520B-V2 for 6.5V filaments. Do you know how to adjust the bias current? If yes, reduce the bias to minimum. 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. Does it have stabilized filament voltage? 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 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 When you have the bias set, check again the filament voltage, these may interact with each other 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 bias when more than 5% off. Check bias now every 30 minutes for some hours. Re-adjust bias when more than 5% off. Check bias now every 2 hours during the day. Don't let the amp run without supervision. Re-adjust bias when more than 5% off. After 50...100 hours the burn in is finished. Do final bias adjustment. Do final hum pot adjustment Questions? Only when brief, to us. The amplifier manufacturer is your help desk.