Eldartford is correct. The original ST70 amp contained front panel sockets that you could plug the preamp into. It also had one of the socket holes marked on each side to show where to insert a voltage meter probe to check the bias of the output tubes.
Here is a link to a picture where you can see the original configuration: http://www.tnt-audio.com/ampli/dynaco2e.html
Here is a link to a picture where you can see the original configuration: http://www.tnt-audio.com/ampli/dynaco2e.html