Evolution Series ST-70 Tube Amplifier

Evolution Series ST-70 Tube Amplifier "Special Order"

Our "special order" amplifiers require a 3-4 week production period. Each amp is custom built. We need time to order the components and of course time for assembling and testing the unit. Often there will be a few orders to complete before we can get to your build. The components, varieties, brands and features we offer are proven top performers.

Tubes are not included

You are not required to purchase the tubes from us, but we'd appreciate it. Your amplifier will still be fully tested, but it will ship without tubes, unless you purchase the tubes from us. We sell Apex matched tubes.

An article about tube selection can be viewed here

The Evolution Series 70 uses the tubes and quantities listed below.

• (4) EL34 Power Tubes 
• (1) GZ34 / 5AR4 Rectifier Tube
• (2) 6CG7 driver tubes
• (2) 6922 driver tubes

We offer three capacitor options for this amplifier. Please select one of these using the link below.

Options for the Evolution Series 70

This amplifier is not another attempt to improve or modify the original Dynaco 70 circuit. It is a phenomenal modern tube circuit, specifically designed around the Dynaco transformers and chassis layout, for economical reasons. 

The Evolution Series 4T driver employs a "from the ground up" custom designed circuit featuring modern techniques, superior components and audio specific driver tubes. It was cleverly engineered specifically for use with the amazingly successful EL34 push-pull design of the Dynaco 70, taking advantage of the readily available chassis and transformer sets. The results are an amplifier that retains the classic ST-70 look and economy but provides a performance and sonic presentation one would compare to amplifiers costing considerably more.

This amp uses a circuit topology similar to that found in modern solid-state amplifiers, adapted to vacuum tubes. Utilizing the capabilities of 4 dual triodes, this amplifier will amaze you with true audiophile level performance. Using modern advancements in circuit design, engineering and development, this driver circuit utilizes two 6CG7 and two 6922 tubes with an LM334 constant current source for each tube. The 6CG7 is the miniaturized 9 pin version of the legendary 6SN7. The (4) twin triodes are configured as differential amplifiers in both the input and driver stages. Each stage uses a temperature compensated constant-current sink to stabilize its operating point for both linearity and long-term reliability, it ensures that the current through each tube remains at its optimal level under all operating conditions and temperatures.

This is not a design for "tube rollers." It requires the 6922 or equivalent for the input stage, and the 6CG7 for the driver stage. This is due to the specific operating points established by the CCS in each stage. Using tubes other than the ones for which the circuit was designed will likely result in operating points that are unsuitable for this circuit. Also, the 6922 and 6CG7 tubes have a shield between their two triode sections, connected to pin 9, and this pin is connected to the circuit board ground plane to provide electrostatic shielding. Using tubes that designate pin 9 for a different purpose will damage either the tube or the circuit board, or both. Finally, this driver is designed to be as "plug and play" as possible, requiring no user adjustments other than setting output tube bias.

This design includes a low-pass filter at the amplifier input. This prevents high frequency noise from entering the amplifier and creating intermodulation distortion. Also, an input blocking capacitor, which was absent in the original ST-70 design, has been included to ensure that any DC voltage present on the source signal cannot upset the operation of the differential input stage.

One of the most important aspects of an amplifier is stability. Feedback amplifiers such as this require careful compensation to ensure that they do not break into oscillation with complex speaker loads. The component values required for proper frequency compensation are highly dependent upon the characteristics of the output transformer and output tubes. SPICE modeling was used to arrive at approximate compensation component values but there is no substitute for building and testing an actual working amplifier. This design underwent extensive bench testing in collaboration with Master Technician Herb Ward to ensure amplifier stability under all load conditions, optimizing the design for the Dynaco A-470 output transformer and EL34 output tube. Response to a 10 KHz square wave is virtually the same with both a pure resistive load and a typical speaker load, with no sign of overshoot or ringing.

The driver is capable of swinging over 100 volts peak-to-peak at low distortion, far more than required to drive the output stage to its full-rated power of 35 watts per channel. This ensures that the output stage will clip before the driver. This results in a more predictable waveform reaching the speaker under clipping conditions.

We now have 17 of these amplifiers in service with overwhelming satisfaction. I can personally testify as to its incredible sound quality, it's truly amazing.

The photos shown are of previous units to provide visual examples. Generally speaking, most units look very similar when completed. 

Powered by a genuine Dynakit PA-060 power transformer that is made in the USA and wound for modern day voltages. These modern Dynakit transformers operate cooler, produce less distortion and easily outperform vintage transformers in every category. These new Dynakit transformers were reverse engineered from the original designs using modern technologies and superior insulations. The increased lamination stack allows it to run cooler and provide better regulation too. These are super high quality transformers that meet and exceed every aspect of the originals. In any comparison, these new transformers are better than those old originals. Modern advancements in materials, combined with traditional methods and better technologies used in the production of these transformers has resulted in absolutely superb transformers that produce improved sonics because of their cleaner power. They operate quieter and run cooler than the originals, they provide less distortion and increased reliability, they're just better.

The new genuine Dynakit A470 output transformers are also greatly improved, like the power transformer. These modern version output transformers are interleaved/layer-wound and incorporate the high quality M-6 grain oriented laminations like the original design, because that is simply a good design. These use better quality wire with better insulation, the primary is wound the same as the original A470 using the same number of turns on the same number of sections. These are made using the same patented Dynaco winding method, and made on the same core size and material type as the originals. These transformers are in one word, exceptional.

The classic EL34 push-pull design is a proven success and is the basis of this ST-70 design which operates in the traditional ultralinear mode. The amp retains its original 470K input impedance so virtually any preamplifier will be suitable whether you prefer tubes or solid state. Output is set up for 4 and/or 8 ohm speakers with heavy duty gold plated banana jack binding post terminals on the back. The 16 Ohm outputs have been intentionally eliminated in the Series 70 4T amplifier.

The filter capacitor is a Dynakit  30/20/20/20uF. The resistors are comprised mostly of Dale and Vishay metal films ranging in tolerances from .25% up to 1%. The 8 box style capacitors are .22 @ 630 volts and there's 6 MLCC caps with C0G NP0 dielectric. The two electrolytic caps are Nichicon.

The stock ST-70 used a simple bias circuit which set the bias for each channels pair of output tubes. This amplifier employs a more advanced bias circuit which adds two more bias adjusters located on the PCB which will also, additionally, balance the current between each power tube for each channel. This reduces distortion and helps extend tube life. Current for the bias supply is diode rectified for excellent stability and maximum performance. The rectifier tube is protected by two additional safety diodes installed onto its socket. The bias circuit features additional safety resistors to protect the tubes and transformers should a bias adjuster failure occur. 100uF / 100V capacitors with metal film resistors are used in the bias supply for ultra quiet and stable reliability. The caps are low ESR. The bias supply rests on a PCB rather than the old tag strip.

The RCA inputs are gold plated pieces, spaced slightly wider apart for better compatibility with today's larger connectors.

The trophy quality chassis is a new Dynakit made with mirror polished stainless steel and is just a work of art. The traditional steel cover is included. The amp sits on 4 oversized rubber feet for improved chassis ventilation and a great looking stance.

This is a high quality custom built, hand assembled amplifier you'll be proud to own, enjoy listening to and never have to make an excuse for.

The amp operates in stereo only, the switch is eliminated and the opening is blocked off. This amplifier cannot be operated in mono and can not operate in a Monoblock configuration due the design of the 4T circuit.

Before using the amplifier and again about every 100 hours, you will need to check and adjust the bias. You will need a voltmeter capable of reading DC Volts / DC Millivolts and a small slot/flat head screwdriver. Full instructions with diagrams are of course included as well as email and phone support as necessary. This is a very simple and straightforward procedure and part of owning virtually any tube amplifier. AC voltage can actually vary from outlet to outlet so set the bias using the outlet you intend on operating the unit on / from. Anytime you relocate the amplifier or change tubes you need to check the bias and adjust it for a correct setting.

This amp sounds absolutely stunning and will be a great centerpiece for your audio system.

Manuals, biasing instructions and initial set up printouts / diagrams all included. Email and phone support provided if needed. 

• ST-70 Dimensions:
• 13" Wide X 11" Deep X 6.5" Tall (depth includes binding post)
• Height may be slightly higher (+1/2") with larger feet installed
• Weight (unboxed) is about 38 lbs.