[ETO_Alpha] Fw: Converting an Alpha 76 or 78 to 2 x 3CX800A7

[George (K8GG) & Marijke Guerin]

----- Original Message ----- 
From: George (K8GG) & Marijke Guerin
To: ETO Alpha
Sent: Sunday, January 22, 2006 8:16 AM
Subject: Converting an Alpha 76 or 78 to 2 x 3CX800A7


Some time ago there were questions about matching the cathode impedance of a 
pair of 3CX800A7 tubes (about 25 ohms) with a torroidal inductor.

Alpha and after market conversions used a 2:1, actually 9:4 or 2.25:1 UnUn 
in the 77S for the same purpose.

I would get some Amidon core material such as:

FT-114-77 0.748 1.142 0.295 2.25

 or
FT-114-F 0.748 1.142 0.295 3.75


Where the numbers are the
Catalog No.,  ID,  OD,  Height(thickness) and cost each.

Both are rated from below 1 MHz to at least 30 MHz.

See Amidon web pages
http://www.amidoncorp.com/pricing/ferritetoroid.php
and
http://www.amidoncorp.com/aai_ferritecores.htm

Then wind three 18gauge enamelled wires twisted together and make maybe 7 
turns around the core and connect the three windings in series.
The bottom of the first winding goes to ground.
The top of the 1st winding connection to the bottom of the 2nd winding 
connection is soldered and insulated.
The top of the 2nd winding is soldered to the bottom of the 3rd winding and 
is the 25 ohm output to the cathodes of the pair of 3CX800A7 tubes.
The top of the 3rd winding is the 50 ohm input.

If the match is not perfect, experiment by removing one turn from the top of 
the third winding and feeding 50 ohm input at that point as it changes the 
ratio from 2.2:1 to about 2:1

On the 77S, there is no compensating capacitor anywhere.  Hopefully one will 
not be needed across the UnUn for a 76 or 78 conversion to a pair of 
3CX800A7 tubes.

I hope this helps whoever was considering the project.

Another suggestion is to try to get another wafer on the bandswitch and use 
an Ameritron input network board to switch Pi-net matching circuits for each 
band.  That is a challenge as the whole bandswitch has to be pushed back and 
may require some rewiring!

Good luck        73

George    K8GG