I’m curious as to how this GE5670 is offered as compatible with 6922 types. As you know these are quite different, the tubes are not the same?
Answer: Yes, they have different pin-outs. However, the actual electrical behaviour is closely similar and well within normal observable variation of the 6922 family. The 2C51 family was the premium version of the 6922 family, but many customers just see the same 9-pin layout and assume they are interchangeable. You know what is involved but again, we do not advise separating the tube from the adapter. Plus the adapter has circuitry to reduce noise which also helps that little bit more.
Further question: Even the heater current is slightly different.
Answer: Actually, all ratings are with an implied tolerance according to the standards of the time – some tube datasheets reference the precise EIA standard. Implied for items like heater current is a 20% tolerance. So 350mA vs 365mA is well within normal variations.
Additionally, nominally interchangeable 6DJ8 family versions from different (or even the same) manufacturers show a surprising variance. Our R&D once matched up pairs of NOS Siemens E88CC out of around 50pcs tested (as it so happens) on an Amplitrex. No two tubes had even remotely matching sections and it was impossible to make decent pairs. Many are much further out than a 5670 tested as 6DJ8.
Further question: And the operating point and trans-conductance are quite different in comparing the datasheets.
Answer: Operating points are examples. They do not mean “This is the only way the tube must be used!”
Yes, 6DJ8/6922 Datasheets use a different operating point to specify performance compared to the 5670. Hence the results are different. In practice, the 5670 works well in almost any circuit designed for 6DJ8 Type valves. We have tested.
