Convert a Dynaco ST-70 into a guitar amp?

It'll work great as a guitar amp . . . but whether or not it's the sound you're looking for is anybody's guess. Believe it or not, there were actually some early Sunn amps that were made from the Dynaco MKIII kits. I've also known more than one jazz (electric) bass player that used a McIntosh MC75.

You will need to use it with a proper guitar preamp, however . . . you can build one from an old Fender or Marshall schematic pretty easily, and even power it from the Stereo 70's "preamp power" socket. Or if you're not so inclined, there are many rack-mount guitar preamps on the market for not too much money. The pedal you're describing may or may not be such a device - most guitar volume pedals are purely passive devices, and most effects pedals are designed to go into the preamp input on a guitar, not a power amp. No hurt in trying, however.

Now as far as the driver itself goes . . . the KEF woofers aren't a good idea; they'll probably sound pretty bad and blow up really easily. A much better choice would be a single JBL D130 or E130, with the ST-70 strapped in mono. With a decent guitar preamp . . . that would be a HUGE sound.

I've been playing around converting old tube gear to guitar amps for a little over a year and am hardly an authority, but here's my 2 cents anyway. 1) I think you'll be disappointed in the sound using your KEF drivers unless they truely produce the bandwidth guitar amps are engineered for. 2) It would be a mistake IMHO to do anything to the Dynaco to get a "guitar" amp sound. It's overkill and would destroy a great Hi Fi amp. However, if you simply intend to leave it as-is and plug a pedal into it using a 1/4" to RCA adapter cable, what do you have to lose?

I've found that old Tube PA amps are much easier to mod for guitar use than hi-fi amps. If you want to build a guitar amp from old Tube hardware, there's an abundance of old Tube PA amps out there cheap. They are easy to convert for guitar use and take pedals very nicely. One that I've had good success with is a Bogen CHB-50 - they can be found for $50-$75 on ebay depending on condition. 50 watts output with 2-6l6 tubes. They usually need a recap job and 1/4" jacks, and may or may not need new tubes, and they're ready to go. In this stock condition they tend to be voiced fairly neutral - near zero breakup. The Bogen CHB-35 is also easy to find, and easier to convert, but uses 7868 tubes which are less common. Still, it puts out 35 watts, and is otherwise the same as the CHB-50.

I should point out that I have MANY other guitar amps including a Mesa/Boogie Mark IV, an Egnater Rebel 20 stack(variable between 1 & 20 watts w/ 6V6 and EL84s) & a Vox so I'm kinda set for dirty sounds :-)

Secondly, I believe Verlaine(guitarist/singer from Television) uses some sort of tube hi-fi/pa amp as his guitar amp on this album and it sounds great. The tone is not distorted but more akin to Santo & Johnny-Sleepwalking or Peter Green's-Albatross.

Remember, the purpose of a hifi amp is to sound clean and uncolored. On the other hand, coloration and distortion are at the heart of guitar amp sound. I think the ST70 uses 4 EL34s for about 35 watts per channel. You'll need to push it pretty hard to get overdrive and it will be loud!

I don't know Tom Verlaine, but I think a pair of 8s will sound rather thin. Amps like the Fender Champ got away with this, but they had highly overdriven output stages (single ended class A!) to fatten up the sound. Even so response on the last 5 frets of the low E string was weak. Also KEFs are way too clean. Speaker distortion is part of the sound. Consider 10 or 12 inch guitar speakers.

On the plus side you can use stereo effects. In fact you could mate it with a stereo effects processor (there are plenty out there) and get floorboard control of volume to boot.

I don't know that I gig with such a rig - home stereo gear might not survive road rigors well. It might make an interesting studio rig - or not. It is an easy experiment to try it out - let us know how it turns out.