The Fender 'silverface' amps just didn't sound as good as the 'blackface' era for 6 basic reasons:
1) The rectifier tube was changed from a 5AR4/GZ34 to a 5U4. There are a lot of people, myself
included that think the GZ34 is more efficient, has less tube sag (voltage drop), and better sounding... however, because it was so much more expensive to make there were not as many made, and a NOS GZ34 Mullard, for instance, may run you anywhere from $80 and $150 ea. The Chinese and the Russian Co. Sovtek make 'copies' but, fall a little short.... JJ is working on one, and should be out by the end of 2001.
2) Most of the SF Fender amps had small value caps on the power tubes which grounded out high
end harmonics and made the amp cold sounding. Those should be cut out of the circuit.
3) The output tube BIAS control was a BIAS adjust in the BF amps and a BIAS balance in the SF
amps - making it impossible to have a correctly biased pair of tubes on the output thus sounding a bit
thinner (not as full).
4) Smaller plate load resistors were used on the phase inverter stage (the small 'driver' tube just
before the output tubes), and this caused a decrease in gain, dynamic response, and sustain.
5) The BF amps utilized a 'fixed BIAS' whereas the SF amps had, sometimes, a combination of fixed
and cathode bias circuits... there is no way to make these sound right except to change it back to a
fixed BIAS circuit.
6) The resistors in the power supply filter cap section were changed so the B+ voltage was dropped in
the SF amps... it's easy enough to change these to boost the voltage to drive the amp correctly.
The main problem an old amp has is that the electrolytic caps, in both the power supply filter
section, and the cathode bypass (preamp tubes) caps are pretty much dried up, and it's just a matter
of time before they (literally) smoke...
Taken from: http://www.unclespot.com/SF2BF.html