DYNACO TUBE AMPLIFIERS
There are hundreds of thousands of them out there, and they are much more than tube amplifiers:
They are just like your first high school girlfriend who was the bridge between your before and after. In the mid-1950s and again in the early1970s Dynaco amps opened my heart.
They are art/trade schools, and every man needs a higher education, and there is no better place to enroll because there is so much collective wisdom and tweaks available for these amps.
Are the best value for the newbie to tubes. Even if they are in non-working condition, if their transformers are in good shape, they can be the greatest value.
All Dynaco amps are pentode push/pull. There are four important models: a mono amp that uses two EL 34s: Mk IV, the Stereo 70 which also uses EL34s, the mono MkIII, which uses two 6550s, and mono MK V1 which uses four 8417.
The reason these amps are so valuable is because they used very high quality output transformers because David Hafler was, like his contemporaries in a world where there were no transistorized, very competing with OPTs. That's right at one time the consumer was hip to OPT designers so companies competed with their OPT innovation.
David's was called ULTRALINEAR, and it was based on using a "40%" feedback winding from the OPT connected to the output tube's screen. Pentode's by their nature need their screen energized, and there are lots of different ways of doing this because the proportion of voltage between the plates and the screens is a major determinate of linearity. Ultralinear worked very well.
Those who have read my book THE SEARCH FOR MUSICAL ECSTASY, and see the pictures of the PAOLI 60M amps which are hot rodded MKIIIs that re-introduced me to the glory of tube amps in the early 1970s and relight my tube amplifier flame that started in high school.
Soon I was buying Dynaco 70s, MK IIIs and MkIV by the truck load because they were selling for between $30-75 because, in the early 1970s everyone was into sand amps. I bought a pair of Mk IV in kit form for $250. Not only was there an abundance of suppliers for "new and improved" input circuit boards, but Audio Research made a complete "redo" kit for the Stereo 70s. When I started NYAL we offered a "constant current diff-amp" circuit board, and a solid state regulator kit for these amps.
There is nothing wrong with using these amp in their stock condition, though it is a good idea to make sure that the power supply capacitors are in good shape. Do not install solid state rectifiers.
MOD LEVEL ONE: Just change the coupling capacitors to modern film varieties, and be sure that the power supply capacitors are in good shape. New tubes should be installed. The pot which adjusts bias may have to replaced. Add modern speaker terminals and replace RCA inputs with quality units.
MOD LEVEL TWO: There is a variety of new input circuits that can be installed. This is a big improvement because the ultra-simple used in this amps was never designed for optimal performance.
MOD LEVEL THREE: Add individual bias controls to each output tube. You have the option of connecting the output tubes in triode, which will half the power, and lower the gain.
MOD LEVEL FOUR: This becomes complicated because you can use a separate chassis for created a much more sophisticated power supply that can either benefit from high quality chokes or regulation. This is a picture of how I did this in the 1970s
TUBE PLAY: The amps designed for the EL 34s can be used with 6L6, and that means you literally have more choices than you can shake stick at, especially since you can connect any of these tubes in triode mode with a ten cent resistor. The MK IIIs can be used with any brand of 6550, or KT 88. The MK VI needs to be converted to 65550/KT88 operation, and that requires a complete different bias supply and driver stage, but if you need 100 Watt mono tube amps these are worth the trouble.
WHERE DYNACO AMPS FIT IN THE OPTIMAL TUBE AURAL MATRIX STRATEGY
Use the Dynaco MkIIIs on the woofers, and either the Mk IVs or Stereo 70s on the tweeters. If you need tons of power use the MK VIs on the woofers and smaller amps on the tweeters
LINKS and REFERENCES