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We have combined our two most popular Stereo-70 modules  - the Driver Upgrade Module and the Quad Cap replacement Module. While we have offered them individually, most of you have purchased them in pairs and also as part of our Stereo-70 restoration package to completely renovate your amplifier. Used together, they deliver a completely new amplifier including complete power supply.

The System on Board (SoB) Upgrade module combines both and allows us to include other features to provide a complete Stereo-70 amplifier system on a board (thereby our name SoB). It eliminates all wiring but to the transformers and output tubes making assembly both easy and foolproof. Just as important, it allows us to control all ground and signal paths. This means that both objective and subjective performance is virtually guaranteed. Additionally, when not subject to wiring variances, we can control the amplifier gain and phase characteristics perfectly and be assured that stability is unconditional.

Assembly is actually easier than building the original Dynaco Stereo-70 kit. Simply build the board and connect the transformers and output tubes and you're done.

For you, the bottom line is an amplifier that is easy to build and is guaranteed to provide consistent peak performance.

We've packaged the system with two options: Recommended and Useful. Although we encourage you to use the recommended options, we kept them separate since some of you may have these items already on hand. The Useful options are available if you want to replaces original hardware or components or need items to complete the full restoration.

Stereo 70 SoB Assembly & Installation Manual

Stereo 70 Repair Guide

Stereo 70 Owners Manual (original)

Stereo-70 Restoration Guide


S7-SoB-U Amplifier System

Schematic Diagram

  • All Triode Full Direct Coupled  Circuit Design Provides Ultra High Stability and Highest Sonic Purity
  • Complete System on a Board (SoB) - Just Drop into Your ST-70
  • All Critical Parameters Controlled (on board) Assuring Performance
  • Accepts 6922, 7308, 6DJ8, or 6CG7
  • Instrumentation Style "STAR" Grounding Assures Signal Purity and Freedom from Hum & Noise
  • High Input Impedance is Ideal Match for Passive Preamps
  • Extremely Easy to Install - Just Connect Output Tubes, Transformers & Input Connectors & Your Ready to Go
  • Can be Operated With or Without Feedback
  • Optional AC Balance for perfect phase inversion
  • Optional DC Bias Balance to match Output Tube Parameters
  • Optional Polypropylene Capacitors
  • Power Supply Increases Both the Capacitance and Voltage Rating (to 600 VDC)
  • High Reliability Capacitors used with Resistive Dividers to Protect Capacitors
  • Includes Complete ST-70 Bias Supply Circuit
  • Includes Original Filter Power Resistors
  • Can be Configured for Either Tube or Solid State Rectifiers

The amplifier design in our SoB-UG is based on our 3U Upgrade driver module. Our goal for that design was quite difficult - to improve the spectral balance and harmonic purity of the original and preserve the musicality into the highest frequencies. The stock Stereo-70 highs are rolled off - pleasant but nonetheless dull to most. Another problem with the stock Stereo-70 is the bass performance - fat and loose that users just accept in exchange for that glorious midrange.

Extending that midrange was no easy task - we evaluated many designs both classic and modern and after many hours of auditioning, settled on our Cascode-SLPI (Cascode - Split Load Phase Inverter). This is an all triode design (unlike other pentode-triode circuits) - that we have refined over the years. It is completely direct coupled so the audio signal never sees a capacitor until the output tubes.

It not only met our goals but dramatically improved the imaging and spectral extension over the original. And the bass - well users need not accept poor bass - using all triodes and supplementing it with a robust power supply provides bass performance equal to the best modern tube amps. Its design includes a careful selection of all components in concert with our circuit topology to provide consistent balance across the entire audible spectrum. The board layout has been designed with instrumentation grounding / signal transmission consideration to minimize distortion and noise and deliver outstanding objective performance.

And, over the years since its introduction our customers have consistently told us that this design delivers the purity and  musicality of the stock amp's midrange and extends it across the entire audible spectrum. Another common comment is their surprise with the level of bass performance that can be had from this tiny tube amp.

While we were at it we included separate current resistors for all four output tubes, so that, with our DC Balance option, you can set each tube's current independently (no need to buy dual or quad matched output tubes). We also included an AC Balance option to minimize the system IM Distortion (you will need an IM Distortion analyzer).

The Bias Supply is of course included and has new Bias-Set Potentiometers positioned at the front of the amplifier (as well as an easily accessible Test Point so Bias monitoring is easy and simple. Although you won't need to keep a constant eye on the bias setting, you can if it makes you feel better.

Another problem with the Stereo-70 (and actually all Dynaco power amps) is their use of the 4 section "Quad" electrolytic capacitor (looks like a 1.25" tall cylindrical can). The design of this cap was cost based (not performance) and the result was low reliability (really poorly made) and high effective series resistance ratings (ESR- one reason for the poor bass). To make matters worse, they were operated at or near their max voltage rating (and sometimes exceeded under some line voltage conditions). The bottom line - horrible reliability and poor sonic performance. It is worth noting that these issues are not resolved with even new manufacture replacements - the time to failure clock is simply re-set.

The solution is to take advantage of the dramatic improvement in capacitor design technology since the 1950's and along with solid engineering operating margins provide a capacitor "bank" that not only performs substantially better but dramatically increases system reliability. The power supply on our Sob-UG module include very hi quality capacitors with low ESR and operated with sufficient margin to tolerate any anticipated line voltage

Installation of our SoB-UG system is perfectly straightforward. The PC Board contains everything but the output tubes, input connector, transformers and choke. it mounts in existing holes slightly below chassis to the tubes and power supply caps extend through the original rectangular opening.  You simply remove the old board de-solder the connections from the original board, drop in the upgrade board, and reconnect the wires to the same pads. So, within an hour you will be enjoying your ST-70 like never before. Feel free to download our comprehensive instructions to see for yourself just how easy it is. You may purchase the board blank or fully assembled. You may also purchase the board and parts as a kit. 

·        Blank Board (P/N PWB-S7-SoB-U) @ $165

·        Blank Board w/ parts (P/N KIT-S7-SoB-U) @ $260 (w/o Tubes)

·        Assembled & Tested Board (P/N ASM-S7-SoB-U) @ $325 (w/o Tubes)

Recommended Options

·        Hi-Rel Ceramic Output Tube Sockets, Set of 4 (P/N S7-SKT-80-4) @ $24/ set

·        Teflon Rewire Kit (4X 10ft Red, Black, Wht, Grn) (P/N TWK) @ $24/ set

·        Polypropylene Capacitor Option (OPT-PPC-3U) = Add $21

·        Premium Dual RCA Input Assembly (Gold ) - (P/N D-RCA-G) = Add $28

·        DC Bal Adjust Option (P/N OPT-S7-SoB-U-DCB) = Add $12

Useful Options

·        Driver Tube Option - (P/N OPT-V-3) = Add $48

·        Hi-Rel Ceramic 9Pin Miniature Tube Sockets - (P/N OPT-9PC-3) = Add $21/set

·        Output Tube Quad Set - (P/N OPT-VO-4) = Add $100

·        AC Balance Option (P/N OPT-S7-SoB-U-ACB) = Add $12

·        Replacement Power Switch - (P/N SLSW-2P) = Add $7

·        Replacement Fuse Holder - (P/N S7-FH) = Add $8

copyright Curcio Audio 2008