[Amps] 5V @ 30A Filament Transformer Wanted

[Paul Christensen]

> What a good source for an all original 77 filter cap Paul?  I got one 
> coming in after I fix the customers HF-2500.

Carl,

Condenser Products in Brooksville, FL can make oil caps in most any size you 
want.  They just happen to be the OEM for Alpha going back to the original 
ca. 1970 PA-70 series.  Last year, several AMPS members, myself  included, 
completed a group buy of oil caps for our Alpha 77 series amps.  The new 
caps are 35uF/5KV.

Today, I've got a different perspective on the 3CX1500D7 change.  Before 
ordering the D7 and socket, I contacted Reid Brandon at CPII/Eimac.  He was 
involved with the D7's development.  In short, 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 has a special focus cathode design that ensures 
very low grid current, and IMD is low as a result. The D7 is a drive-hungry 
tube because of its internal design.

The ARRL Handbook D7 amp as designed by Jerry, K8RA has a very robust supply 
and the ability to tap the HV transformer to produce the needed 4-5KV.   But 
to supply what the D7 needs in a 77Dx, really requires an outboard supply 
like Jerry's where's there's sufficient cabinet volume.  For one thing, the 
existing "K logic" step-start needs to handle 10A more start-up current. 
That requires beefing up the already large vitreous enamel power resistor 
pair.

Then there's the issue of the plate load Z difference between the 8877 and 
D7 and without layout change, the plate tank may not be reliable.  So, once 
again I've come full circle and will likely leave the 77Dx as-is.  Since an 
external supply is best for use with the D7, that opens up the possibility 
of using the smaller 77 (non D) amp cabinet, and gutting out the entire 
power supply half.  That would make for a lightweight amp deck, much easier 
to service.

Paul, W9AC