[Drake] Closing comments on the Drake L4B/8877 retrofit

[Ronald Lumachi]

	For those who expressed serious interest in the progress of the Drake
L4B/8877 retrofit, I�ve compiled all the information that is relevant to
what can only be described euphemistically as a labor of love. The
retubing  project has been finally completed and almost all the technical
 obstacles have been overcome. Although the results are gratifying, in
good conscience, I�m not certain if I could recommend the task to anyone
only because of the amount of work involved. In all honesty, it was
considerably more involved than I had initially  anticipated.   There are
readily available, easier to modify  3-500 amplifiers  out there that
accept the Eimac 8877 with a considerably less amount of work. 
	The newly  modified Drake L4B amplifier loafs along at 1700 WATTS output
 with only 55 watts of drive. I probably could squeeze out a bit more
with some increased levels of excitation but the rig  seems comfortable
with  the (4000VDC) plate voltage at these settings.   I  had anticipated
problems with arcing in the plate tune capacitor. Fortunately, at these
power  levels they did not materialize. I did have to  grind  down the
mounting screws heads  at the rear of the air capacitor for some
additional clearance between the movable and fixed plates.  That  helped 
when it started to arc in that area.   I was hesitant to make any changes
to the exterior of the Drake especially since the only option was to
install a vacuum variable and counter. As some of you might remember, I
suffered the slings and arrows of some Drake diehards simply based on the
previously reported fact that I internally altered some components on the
Drake amp. for what it's worth, even the strictest historical
preservation groups allow interior work to structures as long as the
exterior remains pristine. That aside, what drove me nuts for a while was
the fact that when tuning 80m with full power, the antenna tune capacitor
was not meshed at all. No problem in the  half power/voltage position. 
It never occurred to me to check out the  value of the dedicated 
capacitor  that switches in on  that band. Dumb and dumber!  Apparently,
the previous owner clandestinely installed a 1000 pFd mica that proved to
be the culprit. Replaced it with a 500 pFd doorknob (Drake calls for
700pFd)  and off  I  went on my merry way. 
	 The last of the dilemmas, that probably caused me the greatest level of
consternation, was the very elusive parasitic that continually reared it
ugly head. The arcing across the plate capacitor generated a pretty array
of colors but it was a bit disconcerting. Usually, the 8877 does not
require any suppression. However, in this case an oddball  �U� shaped
parasitic choke  assembly (silvered) solved the problem. It�s  about ��
wide thin wall flat stock copper flashing  about 3� overall and formed
into a hairpin shape. Four (4) odd value 91ohm 2W resistors were soldered
in parallel  across the hairpin shaped plate parasitic near its base.   
Along with that modification, I wound an additional small �� diameter 
close spaced  coil  and placed  it in the HV line. That was wired  in
series with the homebrew heavy duty HV plate choke (1� porcelain rod 6�
overall length; #20 gauge enameled wire; 5� of coil).  I borrowed  that
additional space wound  mini-inductance idea from an article appearing a
recent edition of the ARRL Handbook. I found it  as a result of
researching solutions to parasitic problems. Then there was the shorting
to ground across the teflon chimney that claimed  the life of two Weston
Model 301  3" round kilovolt meters. Apparently some metal filings
clinging to the  inside of the chimney formed a path to ground. Scared
the hell out of me!  There's a lesson to be learned here. Incidentally I
need a replacement meter (any voltage scale)  if someone has one laying
around.  I need the shallow back of the Model 301 because  of the minimum
clearance behind the panel.    What follows is a brief  narrative of the 
modifications that were made in the event someone would like to duplicate
the project.

	The Drake L4 supply was not used. My  anticipated power needs exceeded 
the output of the stock supply.  It was sold to an L7 owner who responded
to a Drake Reflector posting. It was replaced with a custom built
4200VDC@1-1/2amp supply utilizing a solid state 3 minute delay timer
circuit working in conjunction with a step start relay system. Provisions
were made to interrupt the cycle in order to tune for resonance  at  half
power.  A 220VAC power cord was fitted to the PS enclosure. Rectifier
board downsizing techniques were utilized resulting in a  circuit PC 
board, including filter capacitors, diode modules  and bleeder resistors,
that  measures about 6�x8�x3� The entire power supply enclosure  is only
8� deep with  typical rack mount dimensions. As an aside, an article
describing in depth this downsizing strategy has been submitted to QST.
Awaiting their response!    
	The RF deck was rewired and fitted with a power cord and RFI/EMI input
filter to power up the filament transformer and control voltage to the
delay timer and primary voltage contactors in the power supply. Although
the supply and deck are connected via a smaller socket receptacle and
matching plug,  each component  is wired (and functions  independently)
except for the delay  timer voltage originating in the RF deck. That 
controls the main contactors. When the switch on the deck is shut, power
is removed from the delay relay via the interconnecting cable  which in
turn shuts down  the other relays. Sounds complicated but easier to wire
up  than to  explain.  A small  shaft mounted variac was installed on the
rear wall of the deck to control filament voltage. The stock Drake
filament transformer supplies too high a voltage (5.5VAC)  since there is
no current draw down with the single 8877 (only 10amps).  Minor filament
voltage adjustments are made off the back panel via the variac.   Pin
outs were installed  to monitor absolutely critical filament voltage on
the 8877. A small 24vdc rectifier circuit was installed to power the
newly replaced  antenna changeover relay to ensure compatibility with the
solid state exciters. The ALC board  and control switch/rheostat were
removed and replaced with a DPDT  rotary switch in the same front panel
position.  In the on position, one pole controls the standby and the
other powers the amber front panel lamp indicating the amp is online. The
red lamp is wired in parallel with the filament transformer and blower. 
The two front panel rocker  switches were epoxied together to control
both poles of the 110VAC input. One of the 3-500 socket holes  was
completely   blocked off. Both sockets and chimney mounts  were removed
and replaced with an 8877 air system socket with grid to ground tabs. A
heavy wall Teflon chimney was fabricated on the lathe for the 8877. The
stock Drake blower was removed and replaced with a higher volume unit
salvaged from my son�s discarded  fireplace insert. The Millen HV socket
was repositioned to clear the blower. A new  50W  zener diode circuit was
installed to accommodate the ceramic tube. The only problem remaining is
to fine tune the input circuit. I dry tuned it using the MFJ Analyzer and
it worked nicely with the vintage FT101E. However, it appears that it
needs some additional work to articulate with the finicky auto tune
circuits of the solid state rigs. Right now I�m a bit  tired of working
on the project but I�ll get to it some snowy afternoon. Incidentally, my
intention was to post the digitals of the  project on my photo  Yahoo
site but my browser and Juno are not cooperating of late. Not certain
when I'll get them speaking to each other. In the interim, don't hesitate
to drop me a note and I�ll send a photo along. Truly, a picture is worth
a thousand words! Best regards and good luck on the project if you
consider it worthwhile. Conversely,  those with narrow minds and 
derogatory comments need not respond!  Ron  W2CQM/3