[AMRadio] Power Levels

[Donald Chester]

>From: "James M. Walker" <chejmw at acsu.buffalo.edu>

>I have (2) BC-610 transmitters one (E) and one (I), both run 2400 VDC
>on the plate of the 250TH. According to the Technical Manual for the
>transmitter the plate current should be 390 milliamps for the system
>to work correctly, RF stage and Modulator stage.

It sounds like someone made  the "high power" modification.  You will note 
the plate transformer has two taps on the primary... one is marked "2000" 
and the other "2500", which should be the DC output voltage of the power 
supply if the correct a.c. line voltage (115 volts, I believe) is applied 
between each of said terminals and the common terminal.  The tube is 
supposed to run 2000 volts an AM and 2500 volts on CW.  I suspect the 390 
mills is the total drain from the power supply, final plate current plus 
modulator plate current, since both  run off the same power supply.  
According to the tube manual, in plate modulated service, the 250TH is rated 
2000 wolts at 250 mills, to give 335 watts carrier out with 60 mills grid 
drive.

The stock BC-610 switches the 115 volts ac from the 2000 volt tap to the 
2500 volt tap when you switch from AM mode to CW mode.  The earliest version 
of  the 610 (610-B, I believe), as well as the pre-WWII HT-4, used a 
transformer that only had one ratio that gave 2000 volts DC out of the power 
supply.

No doubt the 250TH will function at the higher voltage and current, but at 
the expense of tube life.  After WWII, it was not uncommon for hams to put 
both wires to the transformer on the 2500 volt tap, to leave the plate 
voltage at 2500 wolts on AM.  I doubt if that made much if any difference on 
the receive end, but it did represent an increase in power.  But those were 
the days when 250TH's and 100TH's were still plentiful and inexpensive.

Don K4KYV


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