Jimmy -
It depends on which Hiwatt you have. Ideally, you need a model that provides a line input to the power amp section. In which case, you'd simply connect the output from the F-2B to the Line In on the Hiwatt.
For Hiwatts that don't provide that insertion point, you have two choices:
1> Have a Line In jack installed. It's not difficult; any decent amp technician will know where to inject the signal and how to protect the input from any power supply voltages. But it does mean drilling a hole in the chassis, somewhere, so add the jack. So it is invasive surgery, in the sense that it might hurt the vintage value of the amp slightly. (I wouldn't worry about it unless the amp Blue Books for over about $2500.)
2> Run the F-2B into the lowest gain input jack available on the Hiwatt, then keep ALL the volume controls in the signal path (both F-2B and Hiwatt) fairly low. Except the last volume control on the Hiwatt, which will set your overall volume. The idea is to prevent the F-2B from overdriving the front end of the Hiwatt, and then to prevent the Hiwatt's first preamp stage from overdriving the second. By carefully setting volume controls, you should be able to get the maximum clean signal through to the Hiwatt's power amp driver stage, which is where the Line In jack mentioned above would be connected.
Now, it may not be possible to keep the F-2B from overdriving the Hiwatt. Most of the older all-tube Hiwatts have pretty sensitive first-stage preamps that overdrive fairly easily. Which is why guitarists like them so much, of course. I modified one by installing an Alembic Stratoblaster FET preamp directly at the input (powered off the tube heater supply) and brought the gain trimpot out on a front-panel Overdrive control. It was the best sounding overdrive I've ever heard (and the waveform presented to the 3rd amp stage was incredible: all even-ordered harmonics; zero odd-ordered harmonics, and a great stair-stepped waveform that simply wouldn't clip), but it was the result of the Hiwatt's sensitivity. Boosting the guitar signal through the 'Blaster by even 2db overdrove the first tube stage. An F-2B provides somewhat more gain that 2db, unless you really keep it under control.
If you simply can't keep the F-2B from overdriving the Hiwatt, you might need to add an in-line volume control in between the two, to act as a Master Volume for the F-2B. That was a fairly common trick we'd use whenever we had to stack preamp stages. Like, when you run the output from one channel of the F-2B into the input of the second. Or when running the F-2B into a guitar-level pedal effect.
Making an in-line volume control is easy: use a decent quality 1Meg pot, connect one end to an input jack, the other to ground, and the wiper to an output jack. Mount the works in a small steel box. Plug the F-2B into the input; plug the output into the Hiwatt.
Set the Hiwatt for the cleanest sound you can get, then use the new In-Line Volume control to turn down the output from the F-2B until it no longer overdrives the Hiwatt.
nic
