[AMRadio] potentiometers


John Coleman jc at pctechref.com
Mon Jun 11 15:36:34 EDT 2007


This Mod is a very good idea.  It might be noted however that most
horizontal output transistors have very high voltage ratings but do not have
a lot of current gain (BETA).  BETA is the output current divided by the
Base Current.   Transistors with higher current gain are available; just
make sure that their voltage rating is higher than the supply voltage.  The
greater the BETA of a transistor in this circuit, the better the output
voltage will follow the way it did before the mod.  A lot of horizontal
output transistors only have a BETA of less that 10.  With that type of
transistor the values for the pot and the series resistor with it would need
to be a lot lower ohms in order to get enough current in the output for the
screen of the 5763.   

A good transistor that might be considered is the ECG198 or equivilent.  It
has a BETA of 30 to 150 while the output current is less that 300 ma.  I
don't think the screen of a 5763 will ever pull 300 ma and still have glass
around it. HIHI--- This transistor is a TO-220 style instead of a TO-3 so it
is also easier to work with.

Some examples of what I'm talking about are as follows.  And note these or
just examples and the value of currents is strictly a relative thing and the
actual current may be much less. 

Let's say you need .005 amps (5ma) of output current.
If the BETA is 10 then the Base current would need to be .0005 amps (500
microamps)

The 470K ohm resistor would drop .0005 X 470000 = 235 Volts

This might be more voltage than is actually available after the main
dropping resistor.  So the output voltage may not be able to go as high as
it did before. And dropping down on the variable resistor would cause a very
quick drop in output voltage.

Were As:

If the transistor had a BETA of 100 then the needed base current would be
only .00005 amps (50 microamps)

.00005 X 470000 = 23.5 Volts

With less voltage drop the circuit will more closely follow the feel of the
original drive control and is linearity for voltage control.

It will also mean less heating of the new pot and its series dropping
resistor.

John
WA5BXO




-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of A.R.S. - W5AMI
Sent: Monday, June 11, 2007 11:54 AM
To: Discussion of AM Radio in the Amateur Service
Subject: Re: [AMRadio] potentiometers

On 6/11/07, kk4tr <kk4tr at hotmail.com> wrote:
> I was wondering if anyone out there knows of a place that I can get
> Potentiometers for my Viking Valiant.  Yhe Grid drive control took a dump
> yesterday morning and I have been having difficulty finding a place to buy
> one either nos stock or even a brand new one.  The one that I need is a
25K,
> 4 watt wire wound unit.  I think I replaced this one once before so a
> regular carbon will work .  The last one was and I think it lasted about
10
> years!!!!! Any help would be greatly appriciated
>

Joe, you might consider this mod for your Valiant:

http://amwindow.org/tech/htm/drivepot.htm

Eliminates the need to get a BIG pot and takes all that current off
the new little one.  I've done it, and it works well.

Brian / wa5am




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