Fully Restored Dynaco ST-70 All Tube Stereo 35WPC Power Amplifier - Fat Bottle USA 6CA7 & 5AR4!

For sale is a fully restored Dynaco ST-70 stereo tube power amplifier capable of 35WPC in stereo mode or 70WPC in mono mode. After the restoration, the sound is classic hi-fi/audiophile quality EL34 performance... Huge punchy bass, perfect balance and clear/detailed in the highs. Details of the restoration are below.

Please study the pics carefully. The metal cage still has great paint overall, no dings etc and presents nicely. The chrome is in nice overall condition and cleaned up well. All controls and sockets were thoroughly cleaned for dropout free operation.

Electronically the entire amp was rebuilt as we would do any vintage tube power amp. Also, since this was a kit build, we went over the builders work meticulously to ensure everything was rock solid. The power supply was rebuilt from the ground up internally while leaving the original can cap installed but disconnected internally for an all original appearance! The bias supply was then rebuilt as well and the rectifier tube socket got safety diodes installed that help protect the power transformer if the rectifier ever fails. The two safety diodes that are recommended for the 5AR4 tube socket to prevent tube arcing are also added (a well known safety mod for the ST-70). A thermistor was also added to reduce modern wall voltages and to slow start the amp. This really helps save the power transformers on these! Next the preamp board was rebuilt with new film caps in the critical positions and all solder joints were touched up as well. The four critical bias sensitive resistors on the EL34 input grids was upgraded to 1% parts that are more in-line with what any EL34 should be ran with for a fixed bias amp. Both preamp tube sockets were cleaned and pins retensioned. Everything else tested in spec and the amp sounds fantastic with no hiss, hum or noises until the amp is making music!

We bias these amps at 1.25V instead of the factory 1.56V to lower the plate dissipation of the output tubes will no discernable sonic side effects. This dramatically increases the output tube life and runs the output tubes much closer to where one wants to bias any class AB push pull fixed bias output stage. The plate voltage does not increase dramatically but the power transformer does run cooler as do the output tubes and everything else. The negative feedback circuit corrects for any potentially introduced crossover distortion etc!

The USA made vintage Sylvania "fat bottle" output tubes were matched on our trusty TV-7 tube tester and then double checked once in the amp for stability. One is labeled RCA but it's a Sylvania like the rest. All four output tubes run at a lower 1.25V bias setting and are rock steady there (we like this bias point for extended tube life without a hit in performance). We run the amp for many hours to verify this. The bias is set on both sides and the tubes will come labeled so the amp will be plug and play ready to go! The rectifier was upgraded by the original owner to a nice vintage RCA labeled but Sylvania-made "fat bottle" 5AR4 (also made in USA) that we tested in our tester for matching/performance and again in the amp for proper voltages/ripple etc throughout. Lastly the 7199 preamp tubes are a beautiful RCA-made black plate set that both tested and sounded great! Like we said...she's ready to rock with these great vintage USA-made tubes!

The amp will be packaged and shipped very carefully with the tubes separated and labeled for easy install.