As an original Dynaco tube amp gets older it is a good idea to change out the main coupling caps and possibly the interstage cap as well. In an original ST-70 you have a .05 uF interstage cap and two .10 uF main coupling caps per channel - so 6 caps altogether. After many years these caps can start to leak a little and degrade the sound. You can put in some other film caps or upgrade the caps. The four main coupling caps are important because all four are directly in the signal path and are responsible for feeding each of the four output tubes with the audio signal.
The top photo shows a Russian Teflon cap. These may work well in some applications but in general Teflon caps are just too large to fit into most Dynaco tube amps. Yes - I am sure you could squeeze them in somehow but IMHO they are impractical because of their size.
The next two caps are Russian PIO caps. The upper one is a K40-Y and the next one down, the green cap, is a K42-Y. Both of these are small enough to fit in either a stock driver board or a VTA driver board. The K40-Y is an earlier series and a little larger while the K42 is a little smaller. Some favor the K40-Y but after extensive listening to TWO Dynaco ST-35's - one with the K40-Y's and one with the K42-Y's, I have to say there is really little to choose between them. Both give a very large and deep soundstage with a very smooth and detailed sound. The K42's are physically smaller and can fit into areas that the K40's would have trouble.
The Auricap is a reasonably priced (about $15 a cap) small film capacitor that is very small and will fit almost anywhere. These sound good but IMHO not as good as either of the two Russian PIO caps. They don't seem to have a "full" sound as the Russian caps. They maybe sound just a little thin at least in the applications I have used them in.
Have you changed your coupling caps in your original ST-70 or any ST-70 variant or any Dynaco tube amp ? Any comments or conclusions?
Bob
Or maybe a custom paint job!!
