One of the reasons the 4-1000-A became so popular was the easy and low-cost
availability of "pulls." Broadcast transmitters used the tube in the 50s and
60s (some still do) and there were many which were working fine but which
were retired after a certain number of hours in service.
That is how I obtained the first five 4-1000's I had.
On the other hand, the 3-1000-Z was used in far fewer commercial amps and if
you wanted one you had to shell out over $400 to get one.
There may be other reasons, but these are the first that come to mind.
73
Mike
W0YR
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