BTW, I had never heard of the 3CX1500D7 until recently. It's a fast
warm-up (Thoriated Tungsten Cathode) variation of the 3CX1500A7 with a 50
watt grid dissipation rating. It seems preferable to the relatively
fragile 3CX1500A7 with its onerous 3 minute warm-up requirement. I wonder
why manufacturers haven't switched over?
This topic was discussed here about a week or two ago. First, the D7 wants
Ep greater than
4KV under load, preferably 5KV+. IMD is driven down with lower grid current
requirements and at 4KV (3.7KV under load), 3rd-order IMD may be marginal.
It's pretty hard to accept higher IMD products for the convenience of an
instant-on tube. The 8877/3CX1500A7 has a special focus cathode design that
ensures
very low grid current, and IMD is low as a result. Moreover, the D7 is a
drive-hungry
tube because of its internal design.
Cost per D7 is quite high and adding all thee issues together,
as well as no second-source supplier, I doubt any amp manufacturer will step
up with a D7 design.
Paul, W9AC
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