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Electronic Bias Switching For The Drake Amplifier

Written by:  Garry Drummond / K4OR

E-Mail:  g.drummond@verizon.net

I became interested in bias circuitry for linear amplifiers in the early 1980's after reading an article written for QST in 1974 by J.A. Bryant, W4UX.   The article described an electronic bias circuit the author had seen used in the Alpha 77 amplifier and felt it could be adapted to other amplifiers, which he proceeded to design and use with success.    Some operators had trouble with the circuit producing distortion on SSB signals.   Marv Gonsior, W6FR (then W6VFR) revised this circuit in an article that appeared in Ham Radio magazine in March 1975, to alleviate the distortion problem.    I built several of these electronic bias switches, (EBS) for friends using SB-220's TL922's and one for myself for a Drake L-7 Amplifier.   Having been off the air from 1990 until 2006, I was curious to see what the current feeling was on electronic bias switching in today's environment.   I found the trend is toward fixed lower level idling current rather than a completely cut-off condition the EBS provided.

Seeing this, I built a small circuit on a piece of Vectorboard using just 4 components, a 1-watt 8.2 volt zener diode, a 47 ohm 2-watt resistor, a 2N3055 pass transistor and a 1 amp fuse.   I mounted the circuit board in the bottom of the RF deck of my Drake L-4B.   As you can see, I mounted the zener and resistor on the bottom of the board with the pass transistor and fuse on the top.   This easily fits near the filament transformer and wiring terminal strip.   With this configuration, I have idle current of 40 MA in CW mode and 100 MA on SSB.    I also placed a switch in the circuit to bypass it if I need to.   This may not make any earth shattering difference but I felt if I could gain even a small amount of efficiency, it was worth the effort.   I also did not use the +120 volts from the bleeder resistor bank for bias.   I am picking up that from B- with a 100K ohm placed across the TR relay which provides enough bias to cut off the tubes during receive and is shorted out during transmit

For reference, if you want more information on the original design or see the circuit diagram. The original circuit is shown in Bill Orr's Radio Handbook 23rd Edition, page 12-27.

73,  Garry Drummond - K4OR