Re: 6DJ8 substitute idea - reverse engineering



we run a small tube factory in Prague. We build no 6DJ8, but here is what I know about it.

When you use a worn out 6DJ8 only at 2 mA, and high impedance load, the circuit may work fine, even when that tube fails completely in a 15mA circuit, and with more load on it. Reason for this is, when a triode gets weak, Gm goes down, and Rp goes up. Initially with the same rate. Unloaded Gain however is the product of Gm and Rp, and will hardly change, even at some wear out. Not talking about loaded gain. When such a tube is loaded, gain of the circuit will drop, because Rp has gone up. So for an unloaded circuit, Rp going up will help to keep the gain unchanged. But for a loaded circuit, Rp going up will do the opposite. This is why sometimes worn out tubes work well in a circuit with low bias current, and with small or no load, and why sometimes tubes which are different, can replace them.

6DJ8 is often used in mu stage, cascode circuit, because that improves gain and linearity. The tube which gives the gain, comes closer to the data sheet specified mu, and the current source tube is only a high impedance, and requirements to that tube are not much.

The drawback is the high cathode to heater DC voltage of the current source tube, stressing the limits of the tube. The closer you get to that limit, the more noise or leakage problems you get from it when the tube ages. Because not only dynamic parameters age, but also isolation gets less. It's a good idea when interchanging 6DJ8, 6922 and 6N23 amongst each other in a cascaded circuit, to check the actual cathode to heater voltage of the specific ciruit, vs the specifications of the tube you use as a replacement. Because that has quite some differences. And when you have the tube tester for it, measure the resistance of a cathode to heater, while the tube is at full plate dissipation. Leakage will go up drastically above 80% of maximum anode dissipation, and even a leaky cathode will measure fine at only 20% of anode dissipation. So not compare to the data sheet, but to the real tube you use. Since this resistor is "connected" to the cathode, that gives sputter or click noise, the closer you come to (or exceed!) the maximum isolation voltage.

6922 and 6DJ8 is not quite the same. Heater current is different, and with replacement types, that useually results in differences, not just with heater current. So when the isolation layer between heater and is made different, that will affect heater current and isolation voltages too. 6DJ8 has larger bias differences between the two triodes, and maximum heater to cathode voltage is another. It is curious how tubes which are quite different can sometimes substitute one another, but only when the circuit designer says so. Moreover 6DJ8 is a frame grid tube, with the many virtues belonging to it, and 6BQ7 is not.


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