I have two D-104s that came with two rigs I bought at hamfests. The one pictured to the left is a newer amplified model (T-UG 9 base) that came with a Yaesu FT-101E. The one on the right is an unamplified older model (Model G base) that came with a Johnson Vikimg Valiant.
Shortly after I got the newer one I started having problems with RF feedback with it. I was able to get rid of most of the feedback but wasn't satisfied with the tonal quality of the mic (little low freq response). The pre-amp was also generating noise on it's own which I wasn't happy with.
Searching the internet presented several pre-amp designs to choose from. I was looking for a design that would provide some equalization and decided on this one, the W2AEW pre-amp. It is an op-amp design with bass and treble controls. The design has no gain control but I'm sure one could be easily added. The mic gain control on the FT-101E works well in place of it. The design also takes power (+ 8 VDC) from a newer rig's mic connector instead of a 9V battery. As there is no voltage for this on the mic connector on the FT-101E so I used a 9V battery to power the circuit. There is a video on You Tube decribing the pre-amp design that can be viewed here. The schematic in the video shows two 2.2 Meg ohm resistors connected to each side of the mic element to ground which are not shown in the schematic on the website. I added the resistors and found that the mic sounds a little better with less noise pick up. I used a TL072 op-amp to avoid working with surface mount components.