K&K Audio Dynaco Stereo-70 Amplifier Upgrade Kit

"Jillions" of Dynaco Stereo-70 amplifiers were made and sold in the 60s and 70s, both assembled and as kits. David Hafler worked a bit of magic in the design of this amplifier. The output transformers are relatively complex in winding structure, the chassis solid, if undistinguished looking, and the original tubes supplied superb (Mullard EL-34s for much of the 60s), all for $99 as a kit!

A used Dynaco Stereo-70 was my first tube power amplifier (way back in 1982). I made the usual capacitor substitutions and other "improvements" and used it until my EICO HF89 came along a few years later. The HF89 sounded better primarily, in my view, because the input stage circuit was better. In 1990, I was fortunate to participate in a listening panel of several ST-70 mods compared to an original (see the article by David Davenport in the first issue of Glass Audio magazine). This opened my ears to the potential that the basic components (transformers and chassis) in the amplifier have for the DIYer.

This upgrade is radically different from the others that have been and are available, in that the signal phase splitting is performed in an input transformer rather than by a tube stage. While much has been made of the band limiting and distortion characteristics of transformers in the audio press over the last 50 years, it is my experience that the sound quality that results from phase splitting in an input or interstage transformer easily exceeds that achieved by even the highest tech tube phase splitting circuit. The performance of the transformer within its bandwidth is very consistent, rendering very accurate phase splitting from well below 20Hz to well above 20KHz. In addition, the input transformer provides galvanic isolation between the source and the amplifier, removing the possibility of ground loop hum involving the power amplifier, a common problem in audio systems. The input transformer is followed by a 6N23P (6922) differential stage that drives the output tubes through coupling capacitors. This stage, often referred to as a "long-tailed pair driver" is made as "linear" as possible by "standing" it on a high impedance common cathode current source to provide for true differential action. The 6N23P input stage provides relatively high gain and very symmetrical drive to the output stage at a moderately low impedance. The output stage is configured as a differential push-pull stage by connecting both of the cathodes of the EL34s to a pair of paralleled current sources. EL34 bias is adjusted by means of setting the current sources for the bias current of your choice, so is independent of tube brand, characteristics, or age. Operating the output stage differentially also means that the output tubes "track" each other dynamically (as well as their initial matching allows). This lowers distortion and dramatically improves sound quality. The output stage can be set up to operate either in the partial triode ("ultralinear") or pure triode mode, depending on whether the screen grid is connected to the plate or the output transformer primary tap. A special EL34 screen grid network is used to improve sound quality in either case. No global feedback is used, or required, for great sound.

As a result of using a true differential output stage, the output stage operates in Class A1. This results in a very robust sonic presentation that belies the 10 watts per channel (triode connected) test bench power rating. In fact, the kit modified ST-70 sounds far more powerful than a stock unit does. Lest you forget, the "70" in the model number denotes 70 watts output for both channels. Test bench numbers clearly don't tell the whole story! A K&K Audio Kit modified ST-70 provides very dynamic performance in a moderate size room with 89dB-91dB speakers and can credibly drive lower sensitivity speakers, as well.

The other consequence of using current sources to set the output stage tube bias is the ability to "drop-in" other output tubes with the same basing configuration as the EL34 without having to readjust bias. The current sources control the tube bias current rather than the tube. Other tubes that can be used with the kit include the 6550, KT-88, KT-90, KT-100, 6L6GC, and KT-77.

The power supply is made simple by virtue of the good power supply rejection provided by differential amplifiers. The original GZ-34 is reused as a time delay turn-on device, while the main rectification is accomplished by fast high current diodes feeding minimal capacitance. A minus voltage supply is also included for the input stage current sources and uses the original's transformer bias tap. The PC board has been very carefully laid out with a "star" grounding system for the best sound.

The kit includes all of the parts necessary to stuff a single circuit board that replaces the original Dyna circuit board, the large power supply capacitor, and bias potentiometer. You will need to supply an original Dyna ST-70 with functional output and power transformers, or obtain a replica chassis from DynaKitParts and replica transformers from either Magnequest or Triode Electronics. The only parts that are reused from an original ST-70 in the upgrade are as follows:

• Power switch
• Speaker screw terminal strips (unless you replace them with conventional binding posts (discussed below)
• RCA jacks (unless you replace them with better RCA jacks as discussed later)
• Fuse holder
• Line cord grommet
• Output transformers (2)
• Power transformer
• EL34 tube sockets (4)
• EL34s (4)
• GZ34/5AR4
• Chassis parts, including hardware for attaching bottom and cage

The rest of the original parts are removed and "recycled".

An assembled circuit board ready for installation is shown below:

The circuit board (5.1" (13cm) x 7.6" (19.3cm) attaches to the chassis using original screw holes and supplied hardware.

Kit Configurations

The Upgrade Kit is available in various parts configurations to meet various budget and performance needs. The Basic Kit features a very good Lundahl mu-metal core transformer (LL1591) to do the phase-splitting duties and KimberKaps for signal coupling. The Basic Kit is a dramatic improvement over a stock ST-70 input stage.

The Premium Kit uses the excellent cobalt amorphous core LL1544A for phase-splitting and is available with either Cardas coupling capacitors for ultimate transparency and resolution or Jensen copper foil/paper in oil coupling capacitors for a warmer, yet still detailed sound.

Either kit can be purchased without coupling capacitors (0.1uF 400v-600v) for those of you who have other preferences based either on past experience or current fads. (:=))

In addition to the kits, there are several optional items available to make your modified kit more functional in a modern audio system. Both the RCA jacks and the screw-type speaker terminal strip are not really very useful with today's interconnects and speaker cables. We have engineered a PC board to fit in place of each of the two octal sockets mounted on the front of the chassis. The bias check points are no longer required, so the sockets are discarded and these holes are reused as new RCA jack mounting holes with the aid of the PC boards. We have also designed another small PC board that fits in the rectangular cutout covered by the screw-type speaker terminal strip that has three holes in it to locate three binding posts for each channel (normally used for the common, 4 ohm and 8 ohm output transformer taps) with the industry standard 0.75" spacing. You may also want to replace the EL34 sockets with new ceramic base sockets.

Show and Tell

For an idea of what can be accomplished cosmetically with this kit and a Stereo-70, look at the unit constructed by Trent Weaver of Cary, NC. He had the chassis powder coated with a matte black finish and spray painted the output transformer end covers with "Jaguar red" paint. After moving the RCA jacks to the back panel and the power switch to the front panel, Trent cut a thin piece of makore ("African cherry") to fit the front and attached it with screws from the inside for a finished look.

Printable page