[Drake] Drake L4B/8877 6m Retrofit

[w2cqm@juno.com]

	Work has begun on a retrofitted  Drake L4B/8877  dedicated for  high
power 6 meter operation. At the risk of being presumptuous, I thought I�d
share some of the details  with those Drake owners  who may be interested
in the project .  Perhaps it will provide  you with some food for thought
for similar undertakings. I personally  find supercharging the vintage
rigs both  challenging and enjoyable. Perhaps you�ll agree! 

	Based on previous experiences with the Eimac 8877, I  anticipated that
this tube,  powered by a custom designed, small footprint  Hipersil
transformer developing 4000VDC @ 1-1/2amps, will produce a minimum of
2000Watts output for the 6m  DX/Contester.  The tube is ideal for this
project since it  has a full power rating up to 250mHz.  The project
involves a considerable amount of circuit alteration to accommodate the
tube�s substantial plate voltage requirements. Consequently, if you�re
reluctant to undertake major �surgery� on your amplifier and unwilling to
completely build up a new power supply, then read the following material
solely for its entertainment and informational value.  

	To begin, the RF deck has to be wired and fused for 115VAC primaries
with an appropriate 3  prong plug.  When powering up with the front panel
switch (poles pinned for tandem operation), voltage is supplied
simultaneously to the high speed capacitor start fan, the  filament
transformer, and  the small accessory antenna relay power supply. At the
same time,  a 115VAC control voltage is passed  down to the remote power
supply via the interconnect cable to activate the 3 minute  solid state
delay timer. A dedicated 220VAC line is wired directly to the custom
built supply and used solely to power that unit. After the delay, to warm
the directly heated  8877 cathode, the primary contactors engage. A fully
adjustable delay timer provides the  control for the step start
circuitry.  The supply has its own voltage and current reading meters. 

	In order to prepare the Drake RF  deck to accommodate the new tube, 
many other circuit changes were made. A heavy duty antenna relay was used
as a direct replacement for the Drake unit. A new plate and parasitic
choke assembly was custom designed and installed for the higher
frequency.  New low value plate and antenna tune  capacitors, along with
a broad banded coil, were installed. To minimize  the possibility of
spurious harmonics, careful attention was paid to the placement of
components in close proximity to each other. The 10-80m band switch
assembly was removed and a  front panel fully shielded  adjustable tune
input circuit was installed in its place. An 8877 air system socket with
grid to ground tabs was fitted after blocking off the holes previously
occupied by the 3-500�s. Heat generation is a serious problem at the
higher  frequencies. To address that issue, a small DC muffin fan was
positioned on the back panel to blow across the tank circuit.  In
addition, a new hole was cut into the chassis and a  considerably larger 
high speed fan was mounted to pressurize the compartment below the tube
socket. Getting the big fan set in place was an undertaking. The pilot
lamps were rewired. One glows on start up and the other lights on
transmit  via a small auxiliary relay that keys with the antenna relay. A
biasing zener diode circuit was installed in the cathode circuit.  If you
take a look at the two digitals on the site listed below, and click  on
the >Drake  8877 6M RETROFIT PROJECT<,   you�ll notice that there is no
filament transformer installed as yet. Filament overvoltage is the unseen
�killer� of 8877 tubes. Unfortunately, the Drake 5VCT @ 30amps filament
transformer exhibits too high a voltage with the considerably less 10 amp
heater  requirement of the 8877. Of course it�s possible to wire in some
series resistance on the primaries and to  keep the Drake unit in place.
I elected to purchase a correctly rated transformer using the �WANTED�
option on QTH.COM.  It�s on the way and hopefully it will arrive shortly.
In the interim, I sat down at the computer and wrote up this preliminary
report of progress. My plan is to keep you informed as progres is made.
Hopefully, you'll find the project  interesting.  Regards, Ron W2CQm/3 


Epilogue: visit  www.QRZ.COM/W2CQM for a listing of the  most recent
articles authored by  W2CQM. For an update on the most recent Drake
L4B/8877 retrofit project(s) see HTTP://Briefcase.Yahoo.com/W2CQMron