[Amps] Sad News for Eimac Users

[Paul Christensen]

> 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