BEWARE of units showing rust--it really ate those chassis quickly, and rust is usually a sign of abuse or neglect. Such amps almost always need new filter capacitors, and often new power tubes. Once rebuilt they are as good as new amplifiers in performance. Ebay can be a dangerous place to look for old tube amps, so be aware of this. If you win a bid, tell the owner that you won't accept an amp that has a bad transformer, lots of rust, or other major problems. If they act squirrely, CANCEL the bid.

Dave

The power trans on the ST-70 runs very hot and fails occasionally.
And in 1990 I counted 6 different Stereo-70 driver boards on the market. Most of those companies have since dropped out of sight.

Magnequest makes some Dyna replacement transformers. Rather costly at ~$150 each.

Curcio Audio still makes new Stereo-70 driver boards and repair kits for Dynas. Curcio now calls himself Dynaco Doctor (http://www.dynaco-doctor.com/).

After some looking around, I found that Triode Electronics (Ned Carlson) now carries new driver boards and transformers for Dynas. Their prices are more reasonable too.
http://store.yahoo.com/triodeel/dynacoupgrades.html
(I should warn you that Ned is very opinionated and also friendly with some of the more antisocial elements on Audio Asylum and rec.audio.tubes.)

Welborne Labs has new driver boards for ST-70s and Mark IIIs.
http://www.welbornelabs.com/st70.htm
There's also the Van Alstine stuff.
http://www.avahifi.com/root/equipment/dyna_rebuilds.htm

I should note that if you replace one bad output trans on an old Dyna stereo amp with one of these new replacements, the channel with the new trans might sound slightly different from the channel with the old trans. Usually unavoidable. You could always try a vintage hifi repairer like VTV (which I do occasional work for). VTV actually has a stockpile of genuine old Dyna transformers. (Not cheap!)

If you go looking on search engines or other websites for Dyna info, be aware that they tend to be out of date and / or contain errors or misinformation. For example:
http://www.tnt-audio.com/ampli/dynaco1e.html
It's a pretty good article, though I take great exception to his statement that Dyna output transformers were "among the best ever made". Not really true, and you can even get new ones (Plitrons, for one) that make Dyna transformers look sick. Plus, that article is from 1998, and some of the advice is outdated.