On CW it puts out a good solid
100Watts.. But on SSB it only puts out about 10W unless I whistly loudly
into the mic.

Jeremy-

I had two SB-102s and an HW-100 about 25 years ago. Neither had low audio,
although there were numerous other problems.

From the wording of your message, I assume the radio did have good audio, but
now doesn't, using the same microphone et cetera. As with most tube rigs, the
first thing to do was to check the tubes. Your tester would probably have
shown up a problem that would have caused your low audio, even if it wasn't a
conductance-type tester.

The microphone must be high impedance. If you are using a low impedance
microphone, you just might not have noticed the low output before.

Is the microphone from Heath? They supplied a hand microphone with a high
impedance ceramic element, similar to the Turner 350-C. If you happen to have
a non-Heath microphone with a crystal element, it is possible the element is
going bad. A Rochelle Salts crystal element must be kept at just the right
humidity level. If it gets too humid, the crystal absorbs moisture and
dissolves. If it gets to dry, its "waters of crystalization" evaporate and the
crystal turns to powder!

When you replaced resistors, did you refer to the manual for the correct value?
Some of the old resistors can be misread. Mistaking red or yellow for orange
in the third band, would make a ten-fold error.

Beyond tubes, microphones and resistors, there may be an electrolytic capacitor
that has either developed leakage or has dried out. Most of the lower value
capacitors are probably OK, but no guarantee.

If you have the manual, there are probably voltages marked on the circuit
diagram. It might help isolate the problem if you checked them with a high
impedance meter.

73 & Good Luck
Fred, K4DII