Jeff Weinberg, W8CQ

Your World-Wide Source For Amateur Radio Amplifier Parts & Upgrade Kits

Your World-Wide Source For Amateur Radio Amplifier Parts & Upgrade Kits
12 July 2021: Due to a flood at my home/shop, Harbach Electronics is closed until I get this mess cleaned up and all of the damage repaired. Effective immediately I will be closed for 30-60 days or how ever long it takes to clean up and repair the damage. All pending orders will be canceled and refunded via the method of payment. I am sorry for the closure but I have to get things repaired and cleaned up. I will be back but it will be a little while. If you wish to order Dean Jacobs racing gear, please email me at and send me your phone number and I will contact you and process your order.
You can still view products, but no orders are being taken at this time. Sorry for the inconvenience!

ES4L4-B Soft-Start/Soft-Key/Bias Module


SKU: ES4L4B Category:


The ES4L4-B is a combination soft-start/soft-key/bias board for the L4-B amplifier.  This all-in-one board limits inrush current during amplifier start-up by placing a small resistive load in series with the AC mains.  The load resistor is switched out of the circuit by a relay approximately 2.5 seconds after power-on.  It also features a opto-isolated soft-key circuit that reduces the voltage on the keying line to 12 VDC at less than 10 mA current that is also interlocked with the soft-start.  The kit also utilizes a string of 10 1N5408 diodes and a 100KΩ resistor to provide DC cutoff bias and user-adjustable idle current biasing.  There is also a ready light output on the board for those who wish to use it.  The unit mounts on the underside of the RF deck using existing holes in the chassis; no extra holes or modifications to the chassis are required.  The board is assembled and comes with all necessary mounting hardware, miscellaneous parts and wiring.  NOTE:  The ES4L4-B is configured for use in L-4B amplifiers operating from 240VAC mains voltage.  If the L-4B is operated from 120VAC mains, resistor R2 must be changed from 22Ω 25W to 12Ω 25W.

Additional information

Weight 0.55 lbs
Dimensions 4 × 4 × 1.5 in