Noen rør har fått status over alle andre.
Det betyr usmakelige priser og store muligheter for å bli lurt.
GEC KT88 er i denne kategorien. Det finnes knapt "nye", det vil si ubrukte å oppdrive.
Det florerer av beskrivelser som "New", "NOS", "Used, but measure strong", osv.
Noen oppgir Ia og gm verdier og en del av disse har oppfatningen "mer er bedre".
Som nevnt i informasjonen jeg har sakset er lekkasje mellom glødetråd og katode et problem ved disse rørene.
For høy lekkasje (fra mindre til større) gjør røret ubrukelig. Denne type måling har jeg aldri sett oppgitt i annonser.
Gass i røret ser jeg heller aldri nevnt.
Eller for å snu litt på de to siste: Spør selgeren om han har testet for lekkasje og gass.
Jeg har valgt å starte med konklusjonen.
Conclusion summary
Rule number one,
When considering purchasing a set of GEC KT88s make sure they have been thoroughly tested, this is equally as important for new and used versions,remember “appearance is no guarantee that the valve is any good” If the seller has no means of testing the valve then I hope that what I have written here will help you to be aware of what you may be getting. Test results for cathode insulation on GEC KT88s is very important, and can tell you a lot about the condition of the valve and most importantly its expected longevity and reliability aspects.
Last but not least, brown getters, badly worn logos, etc are no indication of a KT88s true electrical condition, thorough test results are the only sure way to determine whether the valve is any good, and “remember” nice clean appearance is not an automatic guarantee that the valve is any better [electrically] than one that has “battle scars” from many hours of use!
There is [in my experience] no modern direct equivalent KT88 that combines the sonic qualities and the impeccable reliability of the original GEC!!!
Hints and tips on buying GEC KT88
There is a lot of mis-information being written on various sites thru-out the internet about used GEC KT88 at the moment, so much so that I have decided to share with everyone interested some of the knowledge I have gained over the many years I have been associated with this exceptional audio beam tetrode, especially about what to look for when thinking of parting with your hard earned cash! First of all appearances can be deceptive in judging whether a valve is any good or not, this is especially true of the GEC KT88.
Brown Getters!!!
The main reason most people are put off buying used GEC KT88 is because of what I refer to as “brown getter syndrome” this is where the getters [originally silver] have discoloured, and in severe cases disappeared all together! Although this is an indication that the valve has had many hours of use, it is by no means the “be all and end all” sure way of knowing if the valve is any good. On the contrary, I have had many GEC KT88’s over the years that fit this description, especially ones that have come from industrial and communication facilities, that said I have been surprised time after time with how good a valve in this sorry state has tested. Many although having little or no getter left, upon testing still had full emission!
Shorts and heater to cathode insulation
Testing for shorts and heater to cathode insulation will reveal much more about a valves condition; this is especially true when considering the KT88.
One of the main faults a GEC KT88 can suffer from is low heater to cathode insulation; for e.g., when a KT88 is in good condition it will generally have a relatively high heater to cathode insulation, typically in excess of 10 Meg ohm, [10 million ohms] after high usage this insulation can start to break down, and in severe cases cause total cathode to heater shorts, resulting in catastrophic failure of the valve, therefore testing the heater to cathode insulation will result in a much better understanding of how good the valve is, rather than just relying on physical appearances. Note, in many cases I have had GEC KT88 of new appearance that upon testing have revealed very low heater to cathode insulation.
Warning! be wary of fake GEC KT88,s they do exist! if your not sure of what you may be buying, make sure you look carefully at the internal structures, and compare them with known genuine items, if your still not sure then seek profesional advice before you part with your hard earned cash!