I have seen one of these in a second hand shop. Are they any good? What are they worth? [URL]http://www.gramophone.net/Issue/Page/November%201969/150/739656/Ferrograph+F307+Stereo+Amplifier.+Price%3A+56.+Manufactured+by+The+Ferrograph+Co.+Ltd.,+Mercury+House,+195+Knightsbridge,+London,+SW7[/URL]