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.
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