04.15.07
Dynaco Stereo 70 (ST-70)
The Dynaco Stereo 70 is ‘’the'’ classic Dynaco stereo amplifier. The tube complement is four EL34s, two 7199s and a GZ34 rectifier tube. The early models are prized for their very high quality American-made output transformers with cloth leads. These are the only models that Tube Dreams restores. If you’re interested, you’ll find the specifications below.
Currently available ST-70s
I currently have one Audiophile restored ST-70 available for $849, includes new JJ/Tesla GZ34 rectifier, new matched quad SED (Winged C) EL34, NOS GE 7199 tubes. Alternatively, the amp is $799 with 6GH8A tubes and adapters.
Tube Dreams restorations
There are several possible levels of restoration that can be done to make the Stereo 70 sing, along with several optional improvements. These prices include the amp itself. If you have an amp you would like restored, please call to discuss. Typically a basic restoration costs about $200 plus tubes. The audiophile restoration costs another $100 or more depending on precisely what is done.
Basic restoration
The basic restoration replaces the bare minimum of parts to ensure that the power supply is stable and that the original sound quality is restored. The multi-section can capacitor (the silver cylinder) is replaced with a new American-made replacement, the selenium rectifier is replaced with a modern diode, the bias capacitors are replaced with new Sprague Atoms, and the 1W resistors in the power supply are replaced with modern 3W metal film resistors for a quieter power supply.
To restore the sound quality, the six coupling capacitors on the circuit board are replaced with new Solen Fast Capacitors, a superior high purity metallized polypropylene film capacitor.
Optionally, the two-conductor line cord is replaced with a modern three-conductor cord, and the four-screw speaker terminals are replaced with modern binding posts wired for your speaker’s impedance. Another optional modification is to use a higher capacitance multi-section can which increases the bass response.
The restored Stereo 70 is then retubed with new SED (formerly Svetlana) EL34 tubes, Sovtek or JAN Philips 7199 tubes, and a JJ (formerly Tesla) or vintage Mullard GZ34 rectifier. The amp is then biased, broken in, and then rebiased for optimal sound and tube life.
Audiophile restoration
The audiophile restoration builds on the basic restoration by also replacing the whole circuit board with a new board stuffed with modern components. The resistors are primarily Vishay/Dale metal film resistors, one of the most respected resistors in the audio industry. Some of the plate resistors are increased from 1 watt to 3 watts. Higher power capacity resistors will run cooler—and less heat means less noise. Metal film resistors are quieter than vintage carbon composite resistors, so the resulting sound is much cleaner with more detail.
Full restoration
For the full restoration, the chassis is completely stripped and rechromed. The original transformers are remounted and the amplifier is completely rebuilt using new tube sockets and all new components. This includes all the upgrades described in the basic restoration and audiophile restoration sections.
Audiophile boards and rebuild kits
We also sell replacement boards and rebuild kits if you’d like to do-it-yourself.
Specifications
*Power Output: 35 watts continuous, 80 watts peak each channel.
*Frequency Response: ±.5 dB from 10 cps to 40 kc.
*Power Response: 20 cps to 20 kc within 1 dB of 35 watts at less than 1% distortion.
*Intermodulation Distortion: Less than 1% at 35 watts (normally about .5%). Less than .05% at 1 watt.
*Hum and Noise: Inaudible; better than 90 dB below rated power.
*Sensitivity: 1.3 volts rms input for 35 watts out.
*Output Impedances: 4, 8, and 16 ohms each channel.
*Damping Factor: 15.
*Minimum Channel Separation: 55 dB
*Power Consumption: 190 watts 50/60 cps
*Preamp Provision: Two power takeoffs with independent heater windings to eliminate interaction. Each 6.3 volts 1 amp, 350 volts dc 10 ma. Dynakit preamplifiers plug in directly without modification.
*Tube Complement: EL-34 (4), 7199 (2), GZ-34.
*Size: 13″ by 9-1/2″ by 6-1/2″ high.
*Weight: 32 pounds.
*Finish: Chassis and bottom plate bright nickel plated. Cover finished in charcoal brown vinyl paint.
*Designed by: Ed Laurent, Bob Tucker
*Year Introduced: 1959