[ARC5] Vibrator Power Supplies

[Robert Nickels]

I had an interesting experience with a similar vibrator power supply 
last night.   I like to restore and converte old "boat radios" (the AM 
transmitter-receivers for the old 2-3 Mhz marine band).   This 
particular one used a Mallory 552 Vibrapack, which is a 30 watt vibrator 
power supply that includes the self-rectifying vibrator and transformer 
in a bolt-on module - an early example of what would later come to be 
known as a DC-DC converter

I couldn't get the vibrator to start, so in order to check out the BC 
receiver function, I just temporarily jumpered in my bench B+ supply.   
The receiver worked very well and after a few minutes of the Grand Old 
Opry, I turned off the external B+, but to my surprise the receiver kept 
right on playing!  I hadn't discovered free energy - - evidently the 
vibrator came to life by itself and was now happily humming away.

Incidentally, having just re-read some discussion in the old ham mobile 
publications on the topic, the consensus back then was that 30 watts - 
typically 300 volts at 100 ma -  was about the practical limit for a 
vibrator-powered transmitter.  And even then it took careful power 
budgeting to stay with in that current limit with a transmitter having 
10-15 watts input power:

Oscillator - 20 ma
Power Amplifier - 45 ma
Speech Amplifier - 3 ma
Modulator resting current - 20ma

This totals 88 ma without modulation.  A carefully-designed class B 
modulator would rise to about 45 ma on voice peaks for an instantaneous 
current draw of 113 ma - a tolerable intermittent level with good-sized 
filter caps.     This type of transmitter was achievable with simple 
designs and affordable tubes using the vibrator power supply.   To gain 
3 dB or one-half S unit of signal strength,   the power input needs to 
increase to around 55 watts, which required a plate voltage in the  
400-500 volt range, and that meant using a dynamotor in most cases.

73, Bob W9RAN