[SFDXA] [Drake] Drake L4B/8877 6m Retrofit

[Bill Marx]

For the Six meter aficionados...-Bill W2CQ


----- Original Message -----
From: <w2cqm@juno.com>
To: <Drake@mailman.QTH.net>
Sent: Saturday, February 15, 2003 10:55 PM
Subject: [Drake] Drake L4B/8877 6m Retrofit


> 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