SB104 Driver transistors

[Ray Mack]

As I promised, I looked at the manual for the 104A last night.  Ron,
AB5WG, got information that a MRF314 would be a proper replacement for
the drivers.  I respectfully disagree.  I believe the MRF 412 is a
much more appropriate choice and only about half the cost.  The 412 is
designed for linear service @12VDC and 3-30 MHz.  The 314 is designed
for Class C service @12VDC 30-220MHz.

One thing that is *very* strange in the specifications for the 104A is
that it has 1W PEP out for low power and 100W PEP for high power
(according to the manual).  In order for that to be true, the finals
would need in excess of 20 dB of gain.  Not very likely.  It seems
more likely that the output in low power mode is 5 to 10 W PEP.

I don't remember if the driver transitors are stud mount or flange
mount.  If they are flange mount, an alternate is the MRF 406.  There
is *no* advantage of the 406 over the 412.  The price difference is
not enough to worry about.  It just gives us 2 different types we can
use as replacements that are available from reasonable sources.

When you look at the schematic for the 104A, you will see that you
have to select the value of the dropping resistor depending on which
of the original transistor types was installed.  My suggestion is to
select this resistor such that the idling current in the transistors
is 1A.  This essentially puts the drivers in Class A.  The reason for
this is that it makes most of the IMD products come from the finals
rather than the combination of the 2 stages.  This also makes the need
for matching less important.  Of course the *best* solution is still
to use a matched set.

I hope this information helps.


Ray Mack
WD5IFS
mack at mails.imed.com
Friendswood (Houston), TX

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