Super 70i Vacuum Tube Amplifier Rebuild

Price  Wired: no longer available Kit: no longer available Plans and bare PCB: $39 Circuits Super 70i Chassis Your Dyna St-70 Options Jack set kit $65

The AVA enhanced pure vacuum tube amplifier rebuild kit (or factory wired) for your original Dyna St-70 chassis.

Over the years, we have gotten many calls and letters asking for service and upgrade help with the long-out-of-production Dynaco St-70 vacuum tube power amplifier. A thorough discussion of the basic operational parameters of vacuum tube power amplifiers in general and the Dynaco St-70 amplifier in particular was presented in our July, 1982, issue of Audio Basics. That issue also included directions to correct some of the amp’s worst problems within the context of its original circuit board. We still have that issue available for $5 or download it for free from the AVA website (http://www.avahifi.com).

Now we have designed a complete new motherboard kit for the St-70 incorporating modern parts, new 6GH8A driver tubes replacing the out-of-production and unavailable 7199 tubes, an improved layout, new screen grid biasing to give much better performance with less-than-perfect output tubes, and our input and decoupling circuits. The new Super 70i has solid bass, pure and liquid highs, and a marvelous sound stage. Many people tell us it is the nicest small amp they have ever heard. The kit includes our new circuit card, all new audio circuit parts, new power supply parts, new bias supply parts, and a complete step-by-step construction manual. Instructions for bias adjustments and basic troubleshooting are included in the manual.

You need not have a good-sounding or totally perfect St-70 to start with, but it should work well enough for you to determine that the transformers are OK. Our kit replaces the quad filter capacitor, replaces the bias supply capacitors and resistors, and replaces the entire audio circuit so most old problems will be thrown away along with the old circuits. Instructions for bias adjustments and basic troubleshooting are included in the manual. The rebuild plans and bare PC card are available alone for those who want to furnish their own parts.

The optional AVA Input-Output Jack Set provides heavy-duty 5-way binding posts for your speaker connections. The new gold-plated input jacks are spaced further apart than the originals. This allows modern cable ends to fit better.

We can rebuild your St-70 for you. The Super 70i rebuild is available factory-installed, along with the AVA jack set kit and all new tubes for a complete factory-wired package price.

Remember that the St-70 runs hotter than a solid-state amp and will need ongoing maintenance—periodic bias adjustments and re-tubing. This is because power tubes (unlike the small signal tubes in our preamp and Fet Valve products) work hard, operate at high voltage and high current, and thus don’t last as long. Our small signal tubes run cool and tend to last for years and years.

Why the bother? Because a good working Super 70i with a good tube set, properly adjusted, is a great little amplifier! Because it is faithful to the spirit of the music, absolutely pleasant, and rewarding to listen to on any rationally efficient loudspeaker. Because you will find this carefully engineered AVA project is simply fun to do.

For outrageous musicality and gobs of vacuum tube power consider two Super 70i amplifiers run bridged mono with our OmegaStar bridge. You will make over 75 watts per channel at a price much lower than any vacuum tube competition and also have the advantage of the much lower operating cost of our durable design. Note that Dyna's original mono mode did not work. It simply tied the two channels in parallel with no provision for the inversion of one channel and summing the outputs. All you get is more heat and fuzzy sound. You must have an external bridge to make the St-70 actually perform as a bridged-mode high power amplifier.

The AVA-enhanced pure vacuum tube amplifier rebuild for your original Dyna St-70 chassis – available as a kit or factory-wired.

Front	Circuit
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