Drake L-4B Amplifier Modifications by K4PWO

The Drake L-4B amplifier is a dual 3-500Z amplifier that covers the basic 80 through 10 meter bands without specific WARC band (60, 30, 17, and 12 meters) coverage.  It is a real workhorse design that can stand modifications to improve performance and stability.  Its late 1960's design can also use some modernization.

The modifications that follow are based on ideas and designs from numerous Amateur radio operators.  There is little original content and my hearty thanks are offered to the fellow Hams' works on which I have based my work.

NOTE: These modifications require the amplifier be used with a 3 or 4 wire (L1, L2, and N, plus ground for 4 wire) 208 to 250 volt AC service.  Trying to run this amp on a 117 volts AC after modification will result in operational failure and is a fools errand for this class of amplifier in any event.

Amplifier Deck Modifications

The easiest place to start is with the grid modification.  Direct grounding of the grids eliminates the grid inductor blow out problem, increases gain, and some claim it improves stability.  I first removed all components from the grid pins of the tube sockets. Be sure to save the screws for re-use. I then cut 1/4" wide copper strap and soldered it from the three grid pins to chassis ground where it was attached with the left over screws.

Rear Socket Grid Detail
FIG 1: Rear Socket Detail
Front Socket Grid Detail
FIG 2: Front Socket Detail
Cathode Compartment Overview
FIG 3: Cathode Compartment Overview

The grid modification does run one risk... if a tube "arcs" it can now draw very large amounts of current possibility destroying the power supply, metering, and other components. To prevent such events, you need to install a "arc fault" circuit which is basically a 1 amp fuse with a 20 ohm series current limiting resistor.  The preferred location is in the power supply enclosure but for convenience, I installed it on the back panel near the HV connector. The components used here are NOT ideal! The fuse should be a HV rated type and the "glitch" resistor should be at least 50 watts and surge rated.

HV Fault Protector
FIG 4: HV Fault Protector
Close-up of HV Fault Protector
FIG 5: Close-up of HV Fault Protector

The L-4B has one area that is somewhat failure prone, the power ON/OFF switch.  The switches contacts have to handle the full surge current of the power supply on turn-on.  The custom dual switch assembly is very difficult to locate for repair nearing "unobtainium" status. One way to prevent failure is to off-load the current to a Solid State Relay (SSR). I used a 40 amp, 480 volt AC rated SSR I got from Marlin P Jones Associates along with a silicon rubber thermal pad to aid heat transfer.  I used the 115 V tap on the transformer, available in the L-4B, to derive a low voltage DC source to trigger the SSR.  The ON/OFF switch contacts now only have to handle the 10 to 20 ma of low voltage current used for the SSR trigger. The AC is picked up on the screw terminal strip (orange wire in FIG 9) at the connection by the "115 V" label. The AC wires that were on the switch are relocated to the load terminals of the SSR.

Low Voltage DC Supply
FIG 6: Low Voltage DC Supply
Schematic for LV DC Supply
FIG 7: LV DC Supply Schematic
Right Side View
FIG 8: Right Side View
Overview of SSR View
FIG 9: Overview (Note: Orange wire for LV supply near "115 V" label)

The next modification is to update the keying circuit to be compatible with modern transceivers.  I used the Harbach SK-401 "Soft Switch Kit" available from Harbach Electronics.  It was installed near the ON/OFF switch assembly and can be seen in the lower right corner of Fig 9 above.  The only "down side" is the keying connector is now "polarity" sensitive and which side the ground return is on is important.  The worst that can happen is the amp keys when you plug in the key line.  You just flip the plug to correct that condition.

The last modification involves changing the method of cathode bias to lower the power supply and amp deck heat load and to, hopefully, improve operation.  I eliminate the 100 volt cathode cutoff voltage in the power supply and used simple resistor cutoff instead.  This lowers the un-keyed idle current (receive state) to near zero and removes a large wattage heat source in the power supply.  I also used Electronic Bias Control (EBC) to supply the idle operational current limit.  Now you have only 80 ma. of idle current which lets the amp deck run cooler. I have received solicited and unsolicited reports that my signal is "clean" with no distortion or splatter. This is not  to say that some is not present as ECB is somewhat controversial, especially a simple circuit like I use. The only way to properly evaluate requires a spectrum analyzer which I don't have. The keying relay contacts (marked "K1" in FIG 12) formally used to switch the 100 volts bias were repurposed to short the 30 K ohm bias resistor out of the circuit.

ECB Board
FIG 10: ECB PC Board
Cutoff Bias Resistor
FIG 11: Cutoff Bias Resistor
ECB Schematic
FIG 12: ECB Schematic

The only other modification on the amplifier deck was the addition of protection diodes to the metering circuit which I haven't detailed here.

Power Supply Modifications

The power supply modifications and repairs were as follows:

  1. Replace filter/rectifier boards with Harbach PM-400 Power Supply Module.  The board combines the two original boards into a single board.  The extra space created allows better HV PS cooling.

  2. Remove the two 50 K ohm, 50 watt resistors and the 5 K ohm, 7 watt used as a filter bank bleeder and voltage divider formally used to derive the 100 volts cutoff bias.  The new power supply module includes higher value, lower wattage, bleeder resistors.  The resulting "heat load" is much less allowing for cooler operation.

  3. Installation of the Harbach SS-100 Step Start Module.  The module helps to lower the "turn-on" surge offering some filament protection as well as limiting the capacitor bank charge current.

  4. Replace the original power cord with a 10/4 cord terminated by a 4-pole "twist-lock" plug.  I like the Hubble style twist lock outlets and plugs but you need to be aware there are several classes of these plugs.  Some of the 4-pole plugs are actually designed for 3-phase operation, do not use them.  The 4-pole 240 volt "2-phase" plug and outlet is in the NEMA L14-20 series.  The "20" is the current rating.  There is a 30 amp series but that is overkill for the L-4B.  The safety ground may not be required but I always include it.  The new cord set required some modification of the HV PS sheet metal to mount the "seal tight" strain relief I used.

Pictures to be added