SB-104(A) Finals Feedback Circuit

[Kechkaylo, David]

This is in response to John King's request:

The orginal finals for the SB-104 (either the TRWs or CTCs) are no
longer manufactured. But RF Parts, and others, provide a suitable
replacement set of HFE matched finals.

In the SB-104, no attempt was made to flatten the RF frequency reponse
from 3 to 30 MHz utilizing negative feedback; instead, a frequency
compensation network was used at the input to reduce the RF level at the
low end, and slowly remove this RF shunt effect towards the high end
where the devices' gain drops (towards 10 meters). With this, the specs
of the final amplifier in the SB-104 or in the (A) version for that
matter as the PA board is the same, is flat to within +/- 2.5 dB from 3
to 30 MHz. Utilizing negative feedback would have flattened this
response even more, but requires a more complicated circuit. The ALC
circuit derived from the ALC/Lowpass Filter circuit board will try to
compensate for the lower foward power as you go up in frequency as well
(assuming the reflected power component was negligible).

The finals provided today, say from RF Parts, et.al., are not direct
replacements, and they recommend an additional negative feedback
compensation network. This may be required in order to provide an
additional gain AND impedence compensation in order for these finals to
work properly on that PA board design in the SB-104. Be aware that you
will have to do some modifications to your PA board in order for these
new RF transistors to fit (I've already contacted them about this some
time ago), and add the feedback compensation networks as well. The
original SB-104 design in concert with the ALC & input level
compensation really didn't require the negative feedback design and was
fairly flat. The use of the negative feedback in the orginal SB-104
would not have increased substantially its ruggedness, as this problem
was device ruggedness-shortfall related.

Be aware, though, that this compensation to flatten both the impedance
and gain variations over this decade bandwidth using negative feedback
will never take the place of making your finals more rugged.....it may
help to stabilize the amplifier under various load conditions, however,
but you still must be careful when operating them with a watchful eye on
the duty cycle.....there is no power level cutback based on heat sink
temperature as in the SS-9000 or in today's new rigs.

The original finals used in the SB-104 were not a rugged as those used
today, and therefore couldn't tolerate unwieldly load conditions. Today,
though, you may enjoy a more rugged PA with those replacements as you
can take advantage of a more rugged, modern RF transistor design which
could tolerate a higher load VSWR and with lower junction to case
thermal resistance, all of which are more beneficial in keeping your
finals alive.

As a final note:

There was instability noted in some SB-104 PAs on 10 meters when used
with an external antenna tuner. With this, the (A) version added shunt
360pF dipped micas on all four transistor's collector to ground. You may
have to investigate this aspect if you plan on changing your finals to
the new design. Those new PAs, though, may not have this problem, but
you should be aware of this.

If you don't choose to buy the RF Parts version, there are some original
SB-104 PAs out there floating around. I think I've seen a few offered
for sale at a reasonable price. You can post a request on this
reflector, and you may find some originals. This may be easier.

Regards,

Dave K.

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